tor-browser

gl.rs (174884B)

---

/* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, You can obtain one at http://mozilla.org/MPL/2.0/. */

use super::super::shader_source::{OPTIMIZED_SHADERS, UNOPTIMIZED_SHADERS};
use api::{ImageDescriptor, ImageFormat, Parameter, BoolParameter, IntParameter, ImageRendering};
use api::{MixBlendMode, ImageBufferKind, VoidPtrToSizeFn};
use api::{CrashAnnotator, CrashAnnotation, CrashAnnotatorGuard};
use api::units::*;
use euclid::default::Transform3D;
use gleam::gl;
use crate::render_api::MemoryReport;
use crate::internal_types::{FastHashMap, RenderTargetInfo, Swizzle, SwizzleSettings};
use crate::util::round_up_to_multiple;
use crate::profiler;
use log::Level;
use smallvec::SmallVec;
use std::{
   borrow::Cow,
   cell::{Cell, RefCell},
   cmp,
   collections::hash_map::Entry,
   marker::PhantomData,
   mem,
   num::NonZeroUsize,
   os::raw::c_void,
   ops::Add,
   path::PathBuf,
   ptr,
   rc::Rc,
   slice,
   sync::Arc,
   thread,
   time::Duration,
};
use webrender_build::shader::{
   ProgramSourceDigest, ShaderKind, ShaderVersion, build_shader_main_string,
   build_shader_prefix_string, do_build_shader_string, shader_source_from_file,
};
use malloc_size_of::MallocSizeOfOps;

/// Sequence number for frames, as tracked by the device layer.
#[derive(Debug, Copy, Clone, PartialEq, Ord, Eq, PartialOrd)]
#[cfg_attr(feature = "capture", derive(Serialize))]
#[cfg_attr(feature = "replay", derive(Deserialize))]
pub struct GpuFrameId(usize);

impl GpuFrameId {
   pub fn new(value: usize) -> Self {
       GpuFrameId(value)
   }
}

impl Add<usize> for GpuFrameId {
   type Output = GpuFrameId;

   fn add(self, other: usize) -> GpuFrameId {
       GpuFrameId(self.0 + other)
   }
}

pub struct TextureSlot(pub usize);

// In some places we need to temporarily bind a texture to any slot.
const DEFAULT_TEXTURE: TextureSlot = TextureSlot(0);

#[repr(u32)]
pub enum DepthFunction {
   Always = gl::ALWAYS,
   Less = gl::LESS,
   LessEqual = gl::LEQUAL,
}

#[repr(u32)]
#[derive(Copy, Clone, Debug, Eq, PartialEq)]
#[cfg_attr(feature = "capture", derive(Serialize))]
#[cfg_attr(feature = "replay", derive(Deserialize))]
pub enum TextureFilter {
   Nearest,
   Linear,
   Trilinear,
}

/// A structure defining a particular workflow of texture transfers.
#[derive(Clone, Debug)]
#[cfg_attr(feature = "capture", derive(Serialize))]
#[cfg_attr(feature = "replay", derive(Deserialize))]
pub struct TextureFormatPair<T> {
   /// Format the GPU natively stores texels in.
   pub internal: T,
   /// Format we expect the users to provide the texels in.
   pub external: T,
}

impl<T: Copy> From<T> for TextureFormatPair<T> {
   fn from(value: T) -> Self {
       TextureFormatPair {
           internal: value,
           external: value,
       }
   }
}

#[derive(Debug)]
pub enum VertexAttributeKind {
   F32,
   U8Norm,
   U16Norm,
   I32,
   U16,
}

#[derive(Debug)]
pub struct VertexAttribute {
   pub name: &'static str,
   pub count: u32,
   pub kind: VertexAttributeKind,
}

impl VertexAttribute {
   pub const fn quad_instance_vertex() -> Self {
       VertexAttribute {
           name: "aPosition",
           count: 2,
           kind: VertexAttributeKind::U8Norm,
       }
   }

   pub const fn gpu_buffer_address(name: &'static str) -> Self {
       VertexAttribute {
           name,
           count: 1,
           kind: VertexAttributeKind::I32,
       }
   }

   pub const fn f32x4(name: &'static str) -> Self {
       VertexAttribute {
           name,
           count: 4,
           kind: VertexAttributeKind::F32,
       }
   }

   pub const fn f32x3(name: &'static str) -> Self {
       VertexAttribute {
           name,
           count: 3,
           kind: VertexAttributeKind::F32,
       }
   }

   pub const fn f32x2(name: &'static str) -> Self {
       VertexAttribute {
           name,
           count: 2,
           kind: VertexAttributeKind::F32,
       }
   }

   pub const fn f32(name: &'static str) -> Self {
       VertexAttribute {
           name,
           count: 1,
           kind: VertexAttributeKind::F32,
       }
   }

   pub const fn i32x4(name: &'static str) -> Self {
       VertexAttribute {
           name,
           count: 4,
           kind: VertexAttributeKind::I32,
       }
   }

   pub const fn i32x2(name: &'static str) -> Self {
       VertexAttribute {
           name,
           count: 2,
           kind: VertexAttributeKind::I32,
       }
   }

   pub const fn i32(name: &'static str) -> Self {
       VertexAttribute {
           name,
           count: 1,
           kind: VertexAttributeKind::I32,
       }
   }

   pub const fn u16(name: &'static str) -> Self {
       VertexAttribute {
           name,
           count: 1,
           kind: VertexAttributeKind::U16,
       }
   }

   pub const fn u16x2(name: &'static str) -> Self {
       VertexAttribute {
           name,
           count: 2,
           kind: VertexAttributeKind::U16,
       }
   }
}

#[derive(Debug)]
pub struct VertexDescriptor {
   pub vertex_attributes: &'static [VertexAttribute],
   pub instance_attributes: &'static [VertexAttribute],
}

enum FBOTarget {
   Read,
   Draw,
}

/// Method of uploading texel data from CPU to GPU.
#[derive(Debug, Clone)]
pub enum UploadMethod {
   /// Just call `glTexSubImage` directly with the CPU data pointer
   Immediate,
   /// Accumulate the changes in PBO first before transferring to a texture.
   PixelBuffer(VertexUsageHint),
}

/// Plain old data that can be used to initialize a texture.
pub unsafe trait Texel: Copy + Default {
   fn image_format() -> ImageFormat;
}

unsafe impl Texel for u8 {
   fn image_format() -> ImageFormat { ImageFormat::R8 }
}

/// Returns the size in bytes of a depth target with the given dimensions.
fn depth_target_size_in_bytes(dimensions: &DeviceIntSize) -> usize {
   // DEPTH24 textures generally reserve 3 bytes for depth and 1 byte
   // for stencil, so we measure them as 32 bits.
   let pixels = dimensions.width * dimensions.height;
   (pixels as usize) * 4
}

pub fn get_gl_target(target: ImageBufferKind) -> gl::GLuint {
   match target {
       ImageBufferKind::Texture2D => gl::TEXTURE_2D,
       ImageBufferKind::TextureRect => gl::TEXTURE_RECTANGLE,
       ImageBufferKind::TextureExternal => gl::TEXTURE_EXTERNAL_OES,
       ImageBufferKind::TextureExternalBT709 => gl::TEXTURE_EXTERNAL_OES,
   }
}

pub fn from_gl_target(target: gl::GLuint) -> ImageBufferKind {
   match target {
       gl::TEXTURE_2D => ImageBufferKind::Texture2D,
       gl::TEXTURE_RECTANGLE => ImageBufferKind::TextureRect,
       gl::TEXTURE_EXTERNAL_OES => ImageBufferKind::TextureExternal,
       _ => panic!("Unexpected target {:?}", target),
   }
}

fn supports_extension(extensions: &[String], extension: &str) -> bool {
   extensions.iter().any(|s| s == extension)
}

fn get_shader_version(gl: &dyn gl::Gl) -> ShaderVersion {
   match gl.get_type() {
       gl::GlType::Gl => ShaderVersion::Gl,
       gl::GlType::Gles => ShaderVersion::Gles,
   }
}

// Get an unoptimized shader string by name, from the built in resources or
// an override path, if supplied.
pub fn get_unoptimized_shader_source(shader_name: &str, base_path: Option<&PathBuf>) -> Cow<'static, str> {
   if let Some(ref base) = base_path {
       let shader_path = base.join(&format!("{}.glsl", shader_name));
       Cow::Owned(shader_source_from_file(&shader_path))
   } else {
       Cow::Borrowed(
           UNOPTIMIZED_SHADERS
           .get(shader_name)
           .expect("Shader not found")
           .source
       )
   }
}

impl VertexAttributeKind {
   fn size_in_bytes(&self) -> u32 {
       match *self {
           VertexAttributeKind::F32 => 4,
           VertexAttributeKind::U8Norm => 1,
           VertexAttributeKind::U16Norm => 2,
           VertexAttributeKind::I32 => 4,
           VertexAttributeKind::U16 => 2,
       }
   }
}

impl VertexAttribute {
   fn size_in_bytes(&self) -> u32 {
       self.count * self.kind.size_in_bytes()
   }

   fn bind_to_vao(
       &self,
       attr_index: gl::GLuint,
       divisor: gl::GLuint,
       stride: gl::GLint,
       offset: gl::GLuint,
       gl: &dyn gl::Gl,
   ) {
       gl.enable_vertex_attrib_array(attr_index);
       gl.vertex_attrib_divisor(attr_index, divisor);

       match self.kind {
           VertexAttributeKind::F32 => {
               gl.vertex_attrib_pointer(
                   attr_index,
                   self.count as gl::GLint,
                   gl::FLOAT,
                   false,
                   stride,
                   offset,
               );
           }
           VertexAttributeKind::U8Norm => {
               gl.vertex_attrib_pointer(
                   attr_index,
                   self.count as gl::GLint,
                   gl::UNSIGNED_BYTE,
                   true,
                   stride,
                   offset,
               );
           }
           VertexAttributeKind::U16Norm => {
               gl.vertex_attrib_pointer(
                   attr_index,
                   self.count as gl::GLint,
                   gl::UNSIGNED_SHORT,
                   true,
                   stride,
                   offset,
               );
           }
           VertexAttributeKind::I32 => {
               gl.vertex_attrib_i_pointer(
                   attr_index,
                   self.count as gl::GLint,
                   gl::INT,
                   stride,
                   offset,
               );
           }
           VertexAttributeKind::U16 => {
               gl.vertex_attrib_i_pointer(
                   attr_index,
                   self.count as gl::GLint,
                   gl::UNSIGNED_SHORT,
                   stride,
                   offset,
               );
           }
       }
   }
}

impl VertexDescriptor {
   fn instance_stride(&self) -> u32 {
       self.instance_attributes
           .iter()
           .map(|attr| attr.size_in_bytes())
           .sum()
   }

   fn bind_attributes(
       attributes: &[VertexAttribute],
       start_index: usize,
       divisor: u32,
       gl: &dyn gl::Gl,
       vbo: VBOId,
   ) {
       vbo.bind(gl);

       let stride: u32 = attributes
           .iter()
           .map(|attr| attr.size_in_bytes())
           .sum();

       let mut offset = 0;
       for (i, attr) in attributes.iter().enumerate() {
           let attr_index = (start_index + i) as gl::GLuint;
           attr.bind_to_vao(attr_index, divisor, stride as _, offset, gl);
           offset += attr.size_in_bytes();
       }
   }

   fn bind(&self, gl: &dyn gl::Gl, main: VBOId, instance: VBOId, instance_divisor: u32) {
       Self::bind_attributes(self.vertex_attributes, 0, 0, gl, main);

       if !self.instance_attributes.is_empty() {
           Self::bind_attributes(
               self.instance_attributes,
               self.vertex_attributes.len(),
               instance_divisor,
               gl,
               instance,
           );
       }
   }
}

impl VBOId {
   fn bind(&self, gl: &dyn gl::Gl) {
       gl.bind_buffer(gl::ARRAY_BUFFER, self.0);
   }
}

impl IBOId {
   fn bind(&self, gl: &dyn gl::Gl) {
       gl.bind_buffer(gl::ELEMENT_ARRAY_BUFFER, self.0);
   }
}

impl FBOId {
   fn bind(&self, gl: &dyn gl::Gl, target: FBOTarget) {
       let target = match target {
           FBOTarget::Read => gl::READ_FRAMEBUFFER,
           FBOTarget::Draw => gl::DRAW_FRAMEBUFFER,
       };
       gl.bind_framebuffer(target, self.0);
   }
}

pub struct Stream<'a> {
   attributes: &'a [VertexAttribute],
   vbo: VBOId,
}

pub struct VBO<V> {
   id: gl::GLuint,
   target: gl::GLenum,
   allocated_count: usize,
   marker: PhantomData<V>,
}

impl<V> VBO<V> {
   pub fn allocated_count(&self) -> usize {
       self.allocated_count
   }

   pub fn stream_with<'a>(&self, attributes: &'a [VertexAttribute]) -> Stream<'a> {
       debug_assert_eq!(
           mem::size_of::<V>(),
           attributes.iter().map(|a| a.size_in_bytes() as usize).sum::<usize>()
       );
       Stream {
           attributes,
           vbo: VBOId(self.id),
       }
   }
}

impl<T> Drop for VBO<T> {
   fn drop(&mut self) {
       debug_assert!(thread::panicking() || self.id == 0);
   }
}

#[cfg_attr(feature = "replay", derive(Clone))]
#[derive(Debug)]
pub struct ExternalTexture {
   id: gl::GLuint,
   target: gl::GLuint,
   uv_rect: TexelRect,
   image_rendering: ImageRendering,
}

impl ExternalTexture {
   pub fn new(
       id: u32,
       target: ImageBufferKind,
       uv_rect: TexelRect,
       image_rendering: ImageRendering,
   ) -> Self {
       ExternalTexture {
           id,
           target: get_gl_target(target),
           uv_rect,
           image_rendering,
       }
   }

   #[cfg(feature = "replay")]
   pub fn internal_id(&self) -> gl::GLuint {
       self.id
   }

   pub fn get_uv_rect(&self) -> TexelRect {
       self.uv_rect
   }
}

bitflags! {
   #[derive(Default, Debug, Copy, PartialEq, Eq, Clone, PartialOrd, Ord, Hash)]
   pub struct TextureFlags: u32 {
       /// This texture corresponds to one of the shared texture caches.
       const IS_SHARED_TEXTURE_CACHE = 1 << 0;
   }
}

/// WebRender interface to an OpenGL texture.
///
/// Because freeing a texture requires various device handles that are not
/// reachable from this struct, manual destruction via `Device` is required.
/// Our `Drop` implementation asserts that this has happened.
#[derive(Debug)]
pub struct Texture {
   id: gl::GLuint,
   target: gl::GLuint,
   format: ImageFormat,
   size: DeviceIntSize,
   filter: TextureFilter,
   flags: TextureFlags,
   /// An internally mutable swizzling state that may change between batches.
   active_swizzle: Cell<Swizzle>,
   /// Framebuffer Object allowing this texture to be rendered to.
   ///
   /// Empty if this texture is not used as a render target or if a depth buffer is needed.
   fbo: Option<FBOId>,
   /// Same as the above, but with a depth buffer attached.
   ///
   /// FBOs are cheap to create but expensive to reconfigure (since doing so
   /// invalidates framebuffer completeness caching). Moreover, rendering with
   /// a depth buffer attached but the depth write+test disabled relies on the
   /// driver to optimize it out of the rendering pass, which most drivers
   /// probably do but, according to jgilbert, is best not to rely on.
   ///
   /// So we lazily generate a second list of FBOs with depth. This list is
   /// empty if this texture is not used as a render target _or_ if it is, but
   /// the depth buffer has never been requested.
   ///
   /// Note that we always fill fbo, and then lazily create fbo_with_depth
   /// when needed. We could make both lazy (i.e. render targets would have one
   /// or the other, but not both, unless they were actually used in both
   /// configurations). But that would complicate a lot of logic in this module,
   /// and FBOs are cheap enough to create.
   fbo_with_depth: Option<FBOId>,
   last_frame_used: GpuFrameId,
}

impl Texture {
   pub fn get_dimensions(&self) -> DeviceIntSize {
       self.size
   }

   pub fn get_format(&self) -> ImageFormat {
       self.format
   }

   pub fn get_filter(&self) -> TextureFilter {
       self.filter
   }

   pub fn get_target(&self) -> ImageBufferKind {
       from_gl_target(self.target)
   }

   pub fn supports_depth(&self) -> bool {
       self.fbo_with_depth.is_some()
   }

   pub fn last_frame_used(&self) -> GpuFrameId {
       self.last_frame_used
   }

   pub fn used_in_frame(&self, frame_id: GpuFrameId) -> bool {
       self.last_frame_used == frame_id
   }

   pub fn is_render_target(&self) -> bool {
       self.fbo.is_some()
   }

   /// Returns true if this texture was used within `threshold` frames of
   /// the current frame.
   pub fn used_recently(&self, current_frame_id: GpuFrameId, threshold: usize) -> bool {
       self.last_frame_used + threshold >= current_frame_id
   }

   /// Returns the flags for this texture.
   pub fn flags(&self) -> &TextureFlags {
       &self.flags
   }

   /// Returns a mutable borrow of the flags for this texture.
   pub fn flags_mut(&mut self) -> &mut TextureFlags {
       &mut self.flags
   }

   /// Returns the number of bytes (generally in GPU memory) that this texture
   /// consumes.
   pub fn size_in_bytes(&self) -> usize {
       let bpp = self.format.bytes_per_pixel() as usize;
       let w = self.size.width as usize;
       let h = self.size.height as usize;
       bpp * w * h
   }

   #[cfg(feature = "replay")]
   pub fn into_external(mut self) -> ExternalTexture {
       let ext = ExternalTexture {
           id: self.id,
           target: self.target,
           // TODO(gw): Support custom UV rect for external textures during captures
           uv_rect: TexelRect::new(
               0.0,
               0.0,
               self.size.width as f32,
               self.size.height as f32,
           ),
           image_rendering: ImageRendering::Auto,
       };
       self.id = 0; // don't complain, moved out
       ext
   }
}

impl Drop for Texture {
   fn drop(&mut self) {
       debug_assert!(thread::panicking() || self.id == 0);
   }
}

pub struct Program {
   id: gl::GLuint,
   u_transform: gl::GLint,
   u_texture_size: gl::GLint,
   source_info: ProgramSourceInfo,
   is_initialized: bool,
}

impl Program {
   pub fn is_initialized(&self) -> bool {
       self.is_initialized
   }
}

impl Drop for Program {
   fn drop(&mut self) {
       debug_assert!(
           thread::panicking() || self.id == 0,
           "renderer::deinit not called"
       );
   }
}

pub struct CustomVAO {
   id: gl::GLuint,
}

impl Drop for CustomVAO {
   fn drop(&mut self) {
       debug_assert!(
           thread::panicking() || self.id == 0,
           "renderer::deinit not called"
       );
   }
}

pub struct VAO {
   id: gl::GLuint,
   ibo_id: IBOId,
   main_vbo_id: VBOId,
   instance_vbo_id: VBOId,
   instance_stride: usize,
   instance_divisor: u32,
   owns_vertices_and_indices: bool,
}

impl Drop for VAO {
   fn drop(&mut self) {
       debug_assert!(
           thread::panicking() || self.id == 0,
           "renderer::deinit not called"
       );
   }
}

#[derive(Debug)]
pub struct PBO {
   id: gl::GLuint,
   reserved_size: usize,
}

impl PBO {
   pub fn get_reserved_size(&self) -> usize {
       self.reserved_size
   }
}

impl Drop for PBO {
   fn drop(&mut self) {
       debug_assert!(
           thread::panicking() || self.id == 0,
           "renderer::deinit not called or PBO not returned to pool"
       );
   }
}

pub struct BoundPBO<'a> {
   device: &'a mut Device,
   pub data: &'a [u8]
}

impl<'a> Drop for BoundPBO<'a> {
   fn drop(&mut self) {
       self.device.gl.unmap_buffer(gl::PIXEL_PACK_BUFFER);
       self.device.gl.bind_buffer(gl::PIXEL_PACK_BUFFER, 0);
   }
}

#[derive(PartialEq, Eq, Hash, Debug, Copy, Clone)]
pub struct FBOId(gl::GLuint);

#[derive(PartialEq, Eq, Hash, Debug, Copy, Clone)]
pub struct RBOId(gl::GLuint);

#[derive(PartialEq, Eq, Hash, Debug, Copy, Clone)]
pub struct VBOId(gl::GLuint);

#[derive(PartialEq, Eq, Hash, Debug, Copy, Clone)]
struct IBOId(gl::GLuint);

#[derive(Clone, Debug)]
enum ProgramSourceType {
   Unoptimized,
   Optimized(ShaderVersion),
}

#[derive(Clone, Debug)]
pub struct ProgramSourceInfo {
   base_filename: &'static str,
   features: Vec<&'static str>,
   full_name_cstr: Rc<std::ffi::CString>,
   source_type: ProgramSourceType,
   digest: ProgramSourceDigest,
}

impl ProgramSourceInfo {
   fn new(
       device: &Device,
       name: &'static str,
       features: &[&'static str],
   ) -> Self {

       // Compute the digest. Assuming the device has a `ProgramCache`, this
       // will always be needed, whereas the source is rarely needed.

       use std::collections::hash_map::DefaultHasher;
       use std::hash::Hasher;

       // Setup.
       let mut hasher = DefaultHasher::new();
       let gl_version = get_shader_version(&*device.gl());

       // Hash the renderer name.
       hasher.write(device.capabilities.renderer_name.as_bytes());

       let full_name = Self::make_full_name(name, features);

       let optimized_source = if device.use_optimized_shaders {
           OPTIMIZED_SHADERS.get(&(gl_version, &full_name)).or_else(|| {
               warn!("Missing optimized shader source for {}", &full_name);
               None
           })
       } else {
           None
       };

       let source_type = match optimized_source {
           Some(source_and_digest) => {
               // Optimized shader sources are used as-is, without any run-time processing.
               // The vertex and fragment shaders are different, so must both be hashed.
               // We use the hashes that were computed at build time, and verify it in debug builds.
               if cfg!(debug_assertions) {
                   let mut h = DefaultHasher::new();
                   h.write(source_and_digest.vert_source.as_bytes());
                   h.write(source_and_digest.frag_source.as_bytes());
                   let d: ProgramSourceDigest = h.into();
                   let digest = d.to_string();
                   debug_assert_eq!(digest, source_and_digest.digest);
                   hasher.write(digest.as_bytes());
               } else {
                   hasher.write(source_and_digest.digest.as_bytes());
               }

               ProgramSourceType::Optimized(gl_version)
           }
           None => {
               // For non-optimized sources we compute the hash by walking the static strings
               // in the same order as we would when concatenating the source, to avoid
               // heap-allocating in the common case.
               //
               // Note that we cheat a bit to make the hashing more efficient. First, the only
               // difference between the vertex and fragment shader is a single deterministic
               // define, so we don't need to hash both. Second, we precompute the digest of the
               // expanded source file at build time, and then just hash that digest here.
               let override_path = device.resource_override_path.as_ref();
               let source_and_digest = UNOPTIMIZED_SHADERS.get(&name).expect("Shader not found");

               // Hash the prefix string.
               build_shader_prefix_string(
                   gl_version,
                   &features,
                   ShaderKind::Vertex,
                   &name,
                   &mut |s| hasher.write(s.as_bytes()),
               );

               // Hash the shader file contents. We use a precomputed digest, and
               // verify it in debug builds.
               if override_path.is_some() || cfg!(debug_assertions) {
                   let mut h = DefaultHasher::new();
                   build_shader_main_string(
                       &name,
                       &|f| get_unoptimized_shader_source(f, override_path),
                       &mut |s| h.write(s.as_bytes())
                   );
                   let d: ProgramSourceDigest = h.into();
                   let digest = format!("{}", d);
                   debug_assert!(override_path.is_some() || digest == source_and_digest.digest);
                   hasher.write(digest.as_bytes());
               } else {
                   hasher.write(source_and_digest.digest.as_bytes());
               }

               ProgramSourceType::Unoptimized
           }
       };

       // Finish.
       ProgramSourceInfo {
           base_filename: name,
           features: features.to_vec(),
           full_name_cstr: Rc::new(std::ffi::CString::new(full_name).unwrap()),
           source_type,
           digest: hasher.into(),
       }
   }

   fn compute_source(&self, device: &Device, kind: ShaderKind) -> String {
       let full_name = self.full_name();
       match self.source_type {
           ProgramSourceType::Optimized(gl_version) => {
               let shader = OPTIMIZED_SHADERS
                   .get(&(gl_version, &full_name))
                   .unwrap_or_else(|| panic!("Missing optimized shader source for {}", full_name));

               match kind {
                   ShaderKind::Vertex => shader.vert_source.to_string(),
                   ShaderKind::Fragment => shader.frag_source.to_string(),
               }
           },
           ProgramSourceType::Unoptimized => {
               let mut src = String::new();
               device.build_shader_string(
                   &self.features,
                   kind,
                   self.base_filename,
                   |s| src.push_str(s),
               );
               src
           }
       }
   }

   fn make_full_name(base_filename: &'static str, features: &[&'static str]) -> String {
       if features.is_empty() {
           base_filename.to_string()
       } else {
           format!("{}_{}", base_filename, features.join("_"))
       }
   }

   fn full_name(&self) -> String {
       Self::make_full_name(self.base_filename, &self.features)
   }
}

#[cfg_attr(feature = "serialize_program", derive(Deserialize, Serialize))]
pub struct ProgramBinary {
   bytes: Vec<u8>,
   format: gl::GLenum,
   source_digest: ProgramSourceDigest,
}

impl ProgramBinary {
   fn new(bytes: Vec<u8>,
          format: gl::GLenum,
          source_digest: ProgramSourceDigest) -> Self {
       ProgramBinary {
           bytes,
           format,
           source_digest,
       }
   }

   /// Returns a reference to the source digest hash.
   pub fn source_digest(&self) -> &ProgramSourceDigest {
       &self.source_digest
   }
}

/// The interfaces that an application can implement to handle ProgramCache update
pub trait ProgramCacheObserver {
   fn save_shaders_to_disk(&self, entries: Vec<Arc<ProgramBinary>>);
   fn set_startup_shaders(&self, entries: Vec<Arc<ProgramBinary>>);
   fn try_load_shader_from_disk(&self, digest: &ProgramSourceDigest, program_cache: &Rc<ProgramCache>);
   fn notify_program_binary_failed(&self, program_binary: &Arc<ProgramBinary>);
}

struct ProgramCacheEntry {
   /// The binary.
   binary: Arc<ProgramBinary>,
   /// True if the binary has been linked, i.e. used for rendering.
   linked: bool,
}

pub struct ProgramCache {
   entries: RefCell<FastHashMap<ProgramSourceDigest, ProgramCacheEntry>>,

   /// Optional trait object that allows the client
   /// application to handle ProgramCache updating
   program_cache_handler: Option<Box<dyn ProgramCacheObserver>>,

   /// Programs that have not yet been cached to disk (by program_cache_handler)
   pending_entries: RefCell<Vec<Arc<ProgramBinary>>>,
}

impl ProgramCache {
   pub fn new(program_cache_observer: Option<Box<dyn ProgramCacheObserver>>) -> Rc<Self> {
       Rc::new(
           ProgramCache {
               entries: RefCell::new(FastHashMap::default()),
               program_cache_handler: program_cache_observer,
               pending_entries: RefCell::new(Vec::default()),
           }
       )
   }

   /// Save any new program binaries to the disk cache, and if startup has
   /// just completed then write the list of shaders to load on next startup.
   fn update_disk_cache(&self, startup_complete: bool) {
       if let Some(ref handler) = self.program_cache_handler {
           if !self.pending_entries.borrow().is_empty() {
               let pending_entries = self.pending_entries.replace(Vec::default());
               handler.save_shaders_to_disk(pending_entries);
           }

           if startup_complete {
               let startup_shaders = self.entries.borrow().values()
                   .filter(|e| e.linked).map(|e| e.binary.clone())
                   .collect::<Vec<_>>();
               handler.set_startup_shaders(startup_shaders);
           }
       }
   }

   /// Add a new ProgramBinary to the cache.
   /// This function is typically used after compiling and linking a new program.
   /// The binary will be saved to disk the next time update_disk_cache() is called.
   fn add_new_program_binary(&self, program_binary: Arc<ProgramBinary>) {
       self.pending_entries.borrow_mut().push(program_binary.clone());

       let digest = program_binary.source_digest.clone();
       let entry = ProgramCacheEntry {
           binary: program_binary,
           linked: true,
       };
       self.entries.borrow_mut().insert(digest, entry);
   }

   /// Load ProgramBinary to ProgramCache.
   /// The function is typically used to load ProgramBinary from disk.
   #[cfg(feature = "serialize_program")]
   pub fn load_program_binary(&self, program_binary: Arc<ProgramBinary>) {
       let digest = program_binary.source_digest.clone();
       let entry = ProgramCacheEntry {
           binary: program_binary,
           linked: false,
       };
       self.entries.borrow_mut().insert(digest, entry);
   }

   /// Returns the number of bytes allocated for shaders in the cache.
   pub fn report_memory(&self, op: VoidPtrToSizeFn) -> usize {
       self.entries.borrow().values()
           .map(|e| unsafe { op(e.binary.bytes.as_ptr() as *const c_void ) })
           .sum()
   }
}

#[derive(Debug, Copy, Clone)]
pub enum VertexUsageHint {
   Static,
   Dynamic,
   Stream,
}

impl VertexUsageHint {
   fn to_gl(&self) -> gl::GLuint {
       match *self {
           VertexUsageHint::Static => gl::STATIC_DRAW,
           VertexUsageHint::Dynamic => gl::DYNAMIC_DRAW,
           VertexUsageHint::Stream => gl::STREAM_DRAW,
       }
   }
}

#[derive(Copy, Clone, Debug)]
pub struct UniformLocation(#[allow(dead_code)] gl::GLint);

impl UniformLocation {
   pub const INVALID: Self = UniformLocation(-1);
}

#[derive(Debug)]
pub struct Capabilities {
   /// Whether multisampled render targets are supported.
   pub supports_multisampling: bool,
   /// Whether the function `glCopyImageSubData` is available.
   pub supports_copy_image_sub_data: bool,
   /// Whether the RGBAF32 textures can be bound to framebuffers.
   pub supports_color_buffer_float: bool,
   /// Whether the device supports persistently mapped buffers, via glBufferStorage.
   pub supports_buffer_storage: bool,
   /// Whether advanced blend equations are supported.
   pub supports_advanced_blend_equation: bool,
   /// Whether dual-source blending is supported.
   pub supports_dual_source_blending: bool,
   /// Whether KHR_debug is supported for getting debug messages from
   /// the driver.
   pub supports_khr_debug: bool,
   /// Whether we can configure texture units to do swizzling on sampling.
   pub supports_texture_swizzle: bool,
   /// Whether the driver supports uploading to textures from a non-zero
   /// offset within a PBO.
   pub supports_nonzero_pbo_offsets: bool,
   /// Whether the driver supports specifying the texture usage up front.
   pub supports_texture_usage: bool,
   /// Whether offscreen render targets can be partially updated.
   pub supports_render_target_partial_update: bool,
   /// Whether we can use SSBOs.
   pub supports_shader_storage_object: bool,
   /// Whether to enforce that texture uploads be batched regardless of what
   /// the pref says.
   pub requires_batched_texture_uploads: Option<bool>,
   /// Whether we are able to ue glClear to clear regions of an alpha render target.
   /// If false, we must use a shader to clear instead.
   pub supports_alpha_target_clears: bool,
   /// Whether we must perform a full unscissored glClear on alpha targets
   /// prior to rendering.
   pub requires_alpha_target_full_clear: bool,
   /// Whether clearing a render target (immediately after binding it) is faster using a scissor
   /// rect to clear just the required area, or clearing the entire target without a scissor rect.
   pub prefers_clear_scissor: bool,
   /// Whether the driver can correctly invalidate render targets. This can be
   /// a worthwhile optimization, but is buggy on some devices.
   pub supports_render_target_invalidate: bool,
   /// Whether the driver can reliably upload data to R8 format textures.
   pub supports_r8_texture_upload: bool,
   /// Whether the extension QCOM_tiled_rendering is supported.
   pub supports_qcom_tiled_rendering: bool,
   /// Whether clip-masking is supported natively by the GL implementation
   /// rather than emulated in shaders.
   pub uses_native_clip_mask: bool,
   /// Whether anti-aliasing is supported natively by the GL implementation
   /// rather than emulated in shaders.
   pub uses_native_antialiasing: bool,
   /// Whether the extension GL_OES_EGL_image_external_essl3 is supported. If true, external
   /// textures can be used as normal. If false, external textures can only be rendered with
   /// certain shaders, and must first be copied in to regular textures for others.
   pub supports_image_external_essl3: bool,
   /// Whether the VAO must be rebound after an attached VBO has been orphaned.
   pub requires_vao_rebind_after_orphaning: bool,
   /// The name of the renderer, as reported by GL
   pub renderer_name: String,
}

#[derive(Clone, Debug)]
pub enum ShaderError {
   Compilation(String, String), // name, error message
   Link(String, String),        // name, error message
}

/// A refcounted depth target, which may be shared by multiple textures across
/// the device.
struct SharedDepthTarget {
   /// The Render Buffer Object representing the depth target.
   rbo_id: RBOId,
   /// Reference count. When this drops to zero, the RBO is deleted.
   refcount: usize,
}

#[cfg(debug_assertions)]
impl Drop for SharedDepthTarget {
   fn drop(&mut self) {
       debug_assert!(thread::panicking() || self.refcount == 0);
   }
}

/// Describes for which texture formats to use the glTexStorage*
/// family of functions.
#[derive(PartialEq, Debug)]
enum TexStorageUsage {
   Never,
   NonBGRA8,
   Always,
}

/// Describes a required alignment for a stride,
/// which can either be represented in bytes or pixels.
#[derive(Copy, Clone, Debug)]
pub enum StrideAlignment {
   Bytes(NonZeroUsize),
   Pixels(NonZeroUsize),
}

impl StrideAlignment {
   pub fn num_bytes(&self, format: ImageFormat) -> NonZeroUsize {
       match *self {
           Self::Bytes(bytes) => bytes,
           Self::Pixels(pixels) => {
               assert!(format.bytes_per_pixel() > 0);
               NonZeroUsize::new(pixels.get() * format.bytes_per_pixel() as usize).unwrap()
           }
       }
   }
}

// We get 24 bits of Z value - use up 22 bits of it to give us
// 4 bits to account for GPU issues. This seems to manifest on
// some GPUs under certain perspectives due to z interpolation
// precision problems.
const RESERVE_DEPTH_BITS: i32 = 2;

pub struct Device {
   gl: Rc<dyn gl::Gl>,

   /// If non-None, |gl| points to a profiling wrapper, and this points to the
   /// underling Gl instance.
   base_gl: Option<Rc<dyn gl::Gl>>,

   // device state
   bound_textures: [gl::GLuint; 16],
   bound_program: gl::GLuint,
   bound_program_name: Rc<std::ffi::CString>,
   bound_vao: gl::GLuint,
   bound_read_fbo: (FBOId, DeviceIntPoint),
   bound_draw_fbo: FBOId,
   default_read_fbo: FBOId,
   default_draw_fbo: FBOId,

   /// Track depth state for assertions. Note that the default FBO has depth,
   /// so this defaults to true.
   depth_available: bool,

   upload_method: UploadMethod,
   use_batched_texture_uploads: bool,
   /// Whether to use draw calls instead of regular blitting commands.
   ///
   /// Note: this currently only applies to the batched texture uploads
   /// path.
   use_draw_calls_for_texture_copy: bool,
   /// Number of pixels below which we prefer batched uploads.
   batched_upload_threshold: i32,

   // HW or API capabilities
   capabilities: Capabilities,

   color_formats: TextureFormatPair<ImageFormat>,
   bgra_formats: TextureFormatPair<gl::GLuint>,
   bgra_pixel_type: gl::GLuint,
   swizzle_settings: SwizzleSettings,
   depth_format: gl::GLuint,

   /// Map from texture dimensions to shared depth buffers for render targets.
   ///
   /// Render targets often have the same width/height, so we can save memory
   /// by sharing these across targets.
   depth_targets: FastHashMap<DeviceIntSize, SharedDepthTarget>,

   // debug
   inside_frame: bool,
   crash_annotator: Option<Box<dyn CrashAnnotator>>,
   annotate_draw_call_crashes: bool,

   // resources
   resource_override_path: Option<PathBuf>,

   /// Whether to use shaders that have been optimized at build time.
   use_optimized_shaders: bool,

   max_texture_size: i32,
   cached_programs: Option<Rc<ProgramCache>>,

   // Frame counter. This is used to map between CPU
   // frames and GPU frames.
   frame_id: GpuFrameId,

   /// When to use glTexStorage*. We prefer this over glTexImage* because it
   /// guarantees that mipmaps won't be generated (which they otherwise are on
   /// some drivers, particularly ANGLE). However, it is not always supported
   /// at all, or for BGRA8 format. If it's not supported for the required
   /// format, we fall back to glTexImage*.
   texture_storage_usage: TexStorageUsage,

   /// Required stride alignment for pixel transfers. This may be required for
   /// correctness reasons due to driver bugs, or for performance reasons to
   /// ensure we remain on the fast-path for transfers.
   required_pbo_stride: StrideAlignment,

   /// Whether we must ensure the source strings passed to glShaderSource()
   /// are null-terminated, to work around driver bugs.
   requires_null_terminated_shader_source: bool,

   /// Whether we must unbind any texture from GL_TEXTURE_EXTERNAL_OES before
   /// binding to GL_TEXTURE_2D, to work around an android emulator bug.
   requires_texture_external_unbind: bool,

   ///
   is_software_webrender: bool,

   // GL extensions
   extensions: Vec<String>,

   /// Dumps the source of the shader with the given name
   dump_shader_source: Option<String>,

   surface_origin_is_top_left: bool,

   /// A debug boolean for tracking if the shader program has been set after
   /// a blend mode change.
   ///
   /// This is needed for compatibility with next-gen
   /// GPU APIs that switch states using "pipeline object" that bundles
   /// together the blending state with the shader.
   ///
   /// Having the constraint of always binding the shader last would allow
   /// us to have the "pipeline object" bound at that time. Without this
   /// constraint, we'd either have to eagerly bind the "pipeline object"
   /// on changing either the shader or the blend more, or lazily bind it
   /// at draw call time, neither of which is desirable.
   #[cfg(debug_assertions)]
   shader_is_ready: bool,

   // count created/deleted textures to report in the profiler.
   pub textures_created: u32,
   pub textures_deleted: u32,
}

/// Contains the parameters necessary to bind a draw target.
#[derive(Clone, Copy, Debug)]
pub enum DrawTarget {
   /// Use the device's default draw target, with the provided dimensions,
   /// which are used to set the viewport.
   Default {
       /// Target rectangle to draw.
       rect: FramebufferIntRect,
       /// Total size of the target.
       total_size: FramebufferIntSize,
       surface_origin_is_top_left: bool,
   },
   /// Use the provided texture.
   Texture {
       /// Size of the texture in pixels
       dimensions: DeviceIntSize,
       /// Whether to draw with the texture's associated depth target
       with_depth: bool,
       /// FBO that corresponds to the selected layer / depth mode
       fbo_id: FBOId,
       /// Native GL texture ID
       id: gl::GLuint,
       /// Native GL texture target
       target: gl::GLuint,
   },
   /// Use an FBO attached to an external texture.
   External {
       fbo: FBOId,
       size: FramebufferIntSize,
   },
   /// An OS compositor surface
   NativeSurface {
       offset: DeviceIntPoint,
       external_fbo_id: u32,
       dimensions: DeviceIntSize,
   },
}

impl DrawTarget {
   pub fn new_default(size: DeviceIntSize, surface_origin_is_top_left: bool) -> Self {
       let total_size = device_size_as_framebuffer_size(size);
       DrawTarget::Default {
           rect: total_size.into(),
           total_size,
           surface_origin_is_top_left,
       }
   }

   /// Returns true if this draw target corresponds to the default framebuffer.
   pub fn is_default(&self) -> bool {
       match *self {
           DrawTarget::Default {..} => true,
           _ => false,
       }
   }

   pub fn from_texture(
       texture: &Texture,
       with_depth: bool,
   ) -> Self {
       let fbo_id = if with_depth {
           texture.fbo_with_depth.unwrap()
       } else {
           texture.fbo.unwrap()
       };

       DrawTarget::Texture {
           dimensions: texture.get_dimensions(),
           fbo_id,
           with_depth,
           id: texture.id,
           target: texture.target,
       }
   }

   /// Returns the dimensions of this draw-target.
   pub fn dimensions(&self) -> DeviceIntSize {
       match *self {
           DrawTarget::Default { total_size, .. } => total_size.cast_unit(),
           DrawTarget::Texture { dimensions, .. } => dimensions,
           DrawTarget::External { size, .. } => size.cast_unit(),
           DrawTarget::NativeSurface { dimensions, .. } => dimensions,
       }
   }

   pub fn offset(&self) -> DeviceIntPoint {
       match *self {
           DrawTarget::Default { .. } |
           DrawTarget::Texture { .. } |
           DrawTarget::External { .. } => {
               DeviceIntPoint::zero()
           }
           DrawTarget::NativeSurface { offset, .. } => offset,
       }
   }

   pub fn to_framebuffer_rect(&self, device_rect: DeviceIntRect) -> FramebufferIntRect {
       let mut fb_rect = device_rect_as_framebuffer_rect(&device_rect);
       match *self {
           DrawTarget::Default { ref rect, surface_origin_is_top_left, .. } => {
               // perform a Y-flip here
               if !surface_origin_is_top_left {
                   let w = fb_rect.width();
                   let h = fb_rect.height();
                   fb_rect.min.x = fb_rect.min.x + rect.min.x;
                   fb_rect.min.y = rect.max.y - fb_rect.max.y;
                   fb_rect.max.x = fb_rect.min.x + w;
                   fb_rect.max.y = fb_rect.min.y + h;
               }
           }
           DrawTarget::Texture { .. } | DrawTarget::External { .. } | DrawTarget::NativeSurface { .. } => (),
       }
       fb_rect
   }

   pub fn surface_origin_is_top_left(&self) -> bool {
       match *self {
           DrawTarget::Default { surface_origin_is_top_left, .. } => surface_origin_is_top_left,
           DrawTarget::Texture { .. } | DrawTarget::External { .. } | DrawTarget::NativeSurface { .. } => true,
       }
   }

   /// Given a scissor rect, convert it to the right coordinate space
   /// depending on the draw target kind. If no scissor rect was supplied,
   /// returns a scissor rect that encloses the entire render target.
   pub fn build_scissor_rect(
       &self,
       scissor_rect: Option<DeviceIntRect>,
   ) -> FramebufferIntRect {
       let dimensions = self.dimensions();

       match scissor_rect {
           Some(scissor_rect) => match *self {
               DrawTarget::Default { ref rect, .. } => {
                   self.to_framebuffer_rect(scissor_rect)
                       .intersection(rect)
                       .unwrap_or_else(FramebufferIntRect::zero)
               }
               DrawTarget::NativeSurface { offset, .. } => {
                   device_rect_as_framebuffer_rect(&scissor_rect.translate(offset.to_vector()))
               }
               DrawTarget::Texture { .. } | DrawTarget::External { .. } => {
                   device_rect_as_framebuffer_rect(&scissor_rect)
               }
           }
           None => {
               FramebufferIntRect::from_size(
                   device_size_as_framebuffer_size(dimensions),
               )
           }
       }
   }
}

/// Contains the parameters necessary to bind a texture-backed read target.
#[derive(Clone, Copy, Debug)]
pub enum ReadTarget {
   /// Use the device's default draw target.
   Default,
   /// Use the provided texture,
   Texture {
       /// ID of the FBO to read from.
       fbo_id: FBOId,
   },
   /// Use an FBO attached to an external texture.
   External {
       fbo: FBOId,
   },
   /// An FBO bound to a native (OS compositor) surface
   NativeSurface {
       fbo_id: FBOId,
       offset: DeviceIntPoint,
   },
}

impl ReadTarget {
   pub fn from_texture(
       texture: &Texture,
   ) -> Self {
       ReadTarget::Texture {
           fbo_id: texture.fbo.unwrap(),
       }
   }

   fn offset(&self) -> DeviceIntPoint {
       match *self {
           ReadTarget::Default |
           ReadTarget::Texture { .. } |
           ReadTarget::External { .. } => {
               DeviceIntPoint::zero()
           }

           ReadTarget::NativeSurface { offset, .. } => {
               offset
           }
       }
   }
}

impl From<DrawTarget> for ReadTarget {
   fn from(t: DrawTarget) -> Self {
       match t {
           DrawTarget::Default { .. } => {
               ReadTarget::Default
           }
           DrawTarget::NativeSurface { external_fbo_id, offset, .. } => {
               ReadTarget::NativeSurface {
                   fbo_id: FBOId(external_fbo_id),
                   offset,
               }
           }
           DrawTarget::Texture { fbo_id, .. } => {
               ReadTarget::Texture { fbo_id }
           }
           DrawTarget::External { fbo, .. } => {
               ReadTarget::External { fbo }
           }
       }
   }
}

/// Parses the major, release, and patch versions from a GL_VERSION string on
/// Mali devices. For example, for the version string
/// "OpenGL ES 3.2 v1.r36p0-01eac0.28ab3a577f105e026887e2b4c93552fb" this
/// returns Some((1, 36, 0)). Returns None if the version cannot be parsed.
fn parse_mali_version(version_string: &str) -> Option<(u32, u32, u32)> {
   let (_prefix, version_string) = version_string.split_once("v")?;
   let (v_str, version_string) = version_string.split_once(".r")?;
   let v = v_str.parse().ok()?;

   let (r_str, version_string) = version_string.split_once("p")?;
   let r = r_str.parse().ok()?;

   // Not all devices have the trailing string following the "p" number.
   let (p_str, _) = version_string.split_once("-").unwrap_or((version_string, ""));
   let p = p_str.parse().ok()?;

   Some((v, r, p))
}

/// Returns whether this GPU belongs to the Mali Midgard family
fn is_mali_midgard(renderer_name: &str) -> bool {
   renderer_name.starts_with("Mali-T")
}

/// Returns whether this GPU belongs to the Mali Bifrost family
fn is_mali_bifrost(renderer_name: &str) -> bool {
   renderer_name == "Mali-G31"
       || renderer_name == "Mali-G51"
       || renderer_name == "Mali-G71"
       || renderer_name == "Mali-G52"
       || renderer_name == "Mali-G72"
       || renderer_name == "Mali-G76"
}

/// Returns whether this GPU belongs to the Mali Valhall family
fn is_mali_valhall(renderer_name: &str) -> bool {
   // As new Valhall GPUs may be released in the future we match all Mali-G models, apart from
   // Bifrost models (of which we don't expect any new ones to be released)
   renderer_name.starts_with("Mali-G") && !is_mali_bifrost(renderer_name)
}
#[inline(never)]
fn gl_error_string(code: u32) -> &'static str {
   match code {
       gl::INVALID_ENUM => "GL_INVALID_ENUM",
       gl::INVALID_VALUE => "GL_INVALID_VALUE",
       gl::INVALID_OPERATION => "GL_INVALID_OPERATION",
       gl::STACK_OVERFLOW => "GL_STACK_OVERFLOW",
       gl::STACK_UNDERFLOW => "GL_STACK_UNDERFLOW",
       gl::OUT_OF_MEMORY => "GL_OUT_OF_MEMORY",
       gl::INVALID_FRAMEBUFFER_OPERATION => "GL_INVALID_FRAMEBUFFER_OPERATION",
       0x507 => "GL_CONTEXT_LOST",
       _ => "(unknown error code)",
   }
}

impl Device {
   pub fn new(
       mut gl: Rc<dyn gl::Gl>,
       crash_annotator: Option<Box<dyn CrashAnnotator>>,
       resource_override_path: Option<PathBuf>,
       use_optimized_shaders: bool,
       upload_method: UploadMethod,
       batched_upload_threshold: i32,
       cached_programs: Option<Rc<ProgramCache>>,
       allow_texture_storage_support: bool,
       allow_texture_swizzling: bool,
       dump_shader_source: Option<String>,
       surface_origin_is_top_left: bool,
       panic_on_gl_error: bool,
   ) -> Device {
       let mut max_texture_size = [0];
       unsafe {
           gl.get_integer_v(gl::MAX_TEXTURE_SIZE, &mut max_texture_size);
       }

       // We cap the max texture size at 16384. Some hardware report higher
       // capabilities but get very unstable with very large textures.
       // Bug 1702494 tracks re-evaluating this cap.
       let max_texture_size = max_texture_size[0].min(16384);

       let renderer_name = gl.get_string(gl::RENDERER);
       info!("Renderer: {}", renderer_name);
       let version_string = gl.get_string(gl::VERSION);
       info!("Version: {}", version_string);
       info!("Max texture size: {}", max_texture_size);

       let mut extension_count = [0];
       unsafe {
           gl.get_integer_v(gl::NUM_EXTENSIONS, &mut extension_count);
       }
       let extension_count = extension_count[0] as gl::GLuint;
       let mut extensions = Vec::new();
       for i in 0 .. extension_count {
           extensions.push(gl.get_string_i(gl::EXTENSIONS, i));
       }

       // We block this on Mali Valhall GPUs as the extension's functions always return
       // GL_OUT_OF_MEMORY, causing us to panic in debug builds.
       let supports_khr_debug = supports_extension(&extensions, "GL_KHR_debug")
           && !is_mali_valhall(&renderer_name);

       // On debug builds, assert that each GL call is error-free. We don't do
       // this on release builds because the synchronous call can stall the
       // pipeline.
       if panic_on_gl_error || cfg!(debug_assertions) {
           gl = gl::ErrorReactingGl::wrap(gl, move |gl, name, code| {
               if supports_khr_debug {
                   Self::log_driver_messages(gl);
               }
               let err_name = gl_error_string(code);
               error!("Caught GL error 0x{:x} {} at {}", code, err_name, name);
               panic!("Caught GL error 0x{:x} {} at {}", code, err_name, name);
           });
       }

       if supports_extension(&extensions, "GL_ANGLE_provoking_vertex") {
           gl.provoking_vertex_angle(gl::FIRST_VERTEX_CONVENTION);
       }

       let supports_texture_usage = supports_extension(&extensions, "GL_ANGLE_texture_usage");

       // Our common-case image data in Firefox is BGRA, so we make an effort
       // to use BGRA as the internal texture storage format to avoid the need
       // to swizzle during upload. Currently we only do this on GLES (and thus
       // for Windows, via ANGLE).
       //
       // On Mac, Apple docs [1] claim that BGRA is a more efficient internal
       // format, but they don't support it with glTextureStorage. As a workaround,
       // we pretend that it's RGBA8 for the purposes of texture transfers,
       // but swizzle R with B for the texture sampling.
       //
       // We also need our internal format types to be sized, since glTexStorage*
       // will reject non-sized internal format types.
       //
       // Unfortunately, with GL_EXT_texture_format_BGRA8888, BGRA8 is not a
       // valid internal format (for glTexImage* or glTexStorage*) unless
       // GL_EXT_texture_storage is also available [2][3], which is usually
       // not the case on GLES 3 as the latter's functionality has been
       // included by default but the former has not been updated.
       // The extension is available on ANGLE, but on Android this usually
       // means we must fall back to using unsized BGRA and glTexImage*.
       //
       // Overall, we have the following factors in play when choosing the formats:
       //   - with glTexStorage, the internal format needs to match the external format,
       //     or the driver would have to do the conversion, which is slow
       //   - on desktop GL, there is no BGRA internal format. However, initializing
       //     the textures with glTexImage as RGBA appears to use BGRA internally,
       //     preferring BGRA external data [4].
       //   - when glTexStorage + BGRA internal format is not supported,
       //     and the external data is BGRA, we have the following options:
       //       1. use glTexImage with RGBA internal format, this costs us VRAM for mipmaps
       //       2. use glTexStorage with RGBA internal format, this costs us the conversion by the driver
       //       3. pretend we are uploading RGBA and set up the swizzling of the texture unit - this costs us batch breaks
       //
       // [1] https://developer.apple.com/library/archive/documentation/
       //     GraphicsImaging/Conceptual/OpenGL-MacProgGuide/opengl_texturedata/
       //     opengl_texturedata.html#//apple_ref/doc/uid/TP40001987-CH407-SW22
       // [2] https://www.khronos.org/registry/OpenGL/extensions/EXT/EXT_texture_format_BGRA8888.txt
       // [3] https://www.khronos.org/registry/OpenGL/extensions/EXT/EXT_texture_storage.txt
       // [4] http://http.download.nvidia.com/developer/Papers/2005/Fast_Texture_Transfers/Fast_Texture_Transfers.pdf

       // On the android emulator glTexImage fails to create textures larger than 3379.
       // So we must use glTexStorage instead. See bug 1591436.
       let is_emulator = renderer_name.starts_with("Android Emulator");
       let avoid_tex_image = is_emulator;
       let mut gl_version = [0; 2];
       unsafe {
           gl.get_integer_v(gl::MAJOR_VERSION, &mut gl_version[0..1]);
           gl.get_integer_v(gl::MINOR_VERSION, &mut gl_version[1..2]);
       }
       info!("GL context {:?} {}.{}", gl.get_type(), gl_version[0], gl_version[1]);

       // We block texture storage on mac because it doesn't support BGRA
       let supports_texture_storage = allow_texture_storage_support && !cfg!(target_os = "macos") &&
           match gl.get_type() {
               gl::GlType::Gl => supports_extension(&extensions, "GL_ARB_texture_storage"),
               gl::GlType::Gles => true,
           };

       // The GL_EXT_texture_format_BGRA8888 extension allows us to use BGRA as an internal format
       // with glTexImage on GLES. However, we can only use BGRA8 as an internal format for
       // glTexStorage when GL_EXT_texture_storage is also explicitly supported. This is because
       // glTexStorage was added in GLES 3, but GL_EXT_texture_format_BGRA8888 was written against
       // GLES 2 and GL_EXT_texture_storage.
       // To complicate things even further, some Intel devices claim to support both extensions
       // but in practice do not allow BGRA to be used with glTexStorage.
       let supports_gles_bgra = supports_extension(&extensions, "GL_EXT_texture_format_BGRA8888");
       let supports_texture_storage_with_gles_bgra = supports_gles_bgra
           && supports_extension(&extensions, "GL_EXT_texture_storage")
           && !renderer_name.starts_with("Intel(R) HD Graphics for BayTrail")
           && !renderer_name.starts_with("Intel(R) HD Graphics for Atom(TM) x5/x7");

       let supports_texture_swizzle = allow_texture_swizzling &&
           match gl.get_type() {
               // see https://www.g-truc.net/post-0734.html
               gl::GlType::Gl => gl_version >= [3, 3] ||
                   supports_extension(&extensions, "GL_ARB_texture_swizzle"),
               gl::GlType::Gles => true,
           };

       let (color_formats, bgra_formats, bgra_pixel_type, bgra8_sampling_swizzle, texture_storage_usage) = match gl.get_type() {
           // There is `glTexStorage`, use it and expect RGBA on the input.
           gl::GlType::Gl if supports_texture_storage && supports_texture_swizzle => (
               TextureFormatPair::from(ImageFormat::RGBA8),
               TextureFormatPair { internal: gl::RGBA8, external: gl::RGBA },
               gl::UNSIGNED_BYTE,
               Swizzle::Bgra, // pretend it's RGBA, rely on swizzling
               TexStorageUsage::Always
           ),
           // There is no `glTexStorage`, upload as `glTexImage` with BGRA input.
           gl::GlType::Gl => (
               TextureFormatPair { internal: ImageFormat::BGRA8, external: ImageFormat::BGRA8 },
               TextureFormatPair { internal: gl::RGBA, external: gl::BGRA },
               gl::UNSIGNED_INT_8_8_8_8_REV,
               Swizzle::Rgba, // converted on uploads by the driver, no swizzling needed
               TexStorageUsage::Never
           ),
           // glTexStorage is always supported in GLES 3, but because the GL_EXT_texture_storage
           // extension is supported we can use glTexStorage with BGRA8 as the internal format.
           // Prefer BGRA textures over RGBA.
           gl::GlType::Gles if supports_texture_storage_with_gles_bgra => (
               TextureFormatPair::from(ImageFormat::BGRA8),
               TextureFormatPair { internal: gl::BGRA8_EXT, external: gl::BGRA_EXT },
               gl::UNSIGNED_BYTE,
               Swizzle::Rgba, // no conversion needed
               TexStorageUsage::Always,
           ),
           // BGRA is not supported as an internal format with glTexStorage, therefore we will
           // use RGBA textures instead and pretend BGRA data is RGBA when uploading.
           // The swizzling will happen at the texture unit.
           gl::GlType::Gles if supports_texture_swizzle => (
               TextureFormatPair::from(ImageFormat::RGBA8),
               TextureFormatPair { internal: gl::RGBA8, external: gl::RGBA },
               gl::UNSIGNED_BYTE,
               Swizzle::Bgra, // pretend it's RGBA, rely on swizzling
               TexStorageUsage::Always,
           ),
           // BGRA is not supported as an internal format with glTexStorage, and we cannot use
           // swizzling either. Therefore prefer BGRA textures over RGBA, but use glTexImage
           // to initialize BGRA textures. glTexStorage can still be used for other formats.
           gl::GlType::Gles if supports_gles_bgra && !avoid_tex_image => (
               TextureFormatPair::from(ImageFormat::BGRA8),
               TextureFormatPair::from(gl::BGRA_EXT),
               gl::UNSIGNED_BYTE,
               Swizzle::Rgba, // no conversion needed
               TexStorageUsage::NonBGRA8,
           ),
           // Neither BGRA or swizzling are supported. GLES does not allow format conversion
           // during upload so we must use RGBA textures and pretend BGRA data is RGBA when
           // uploading. Images may be rendered incorrectly as a result.
           gl::GlType::Gles => {
               warn!("Neither BGRA or texture swizzling are supported. Images may be rendered incorrectly.");
               (
                   TextureFormatPair::from(ImageFormat::RGBA8),
                   TextureFormatPair { internal: gl::RGBA8, external: gl::RGBA },
                   gl::UNSIGNED_BYTE,
                   Swizzle::Rgba,
                   TexStorageUsage::Always,
               )
           }
       };

       let is_software_webrender = renderer_name.starts_with("Software WebRender");
       let upload_method = if is_software_webrender {
           // Uploads in SWGL generally reduce to simple memory copies.
           UploadMethod::Immediate
       } else {
           upload_method
       };
       // Prefer 24-bit depth format. While 16-bit depth also works, it may exhaust depth ids easily.
       let depth_format = gl::DEPTH_COMPONENT24;

       info!("GL texture cache {:?}, bgra {:?} swizzle {:?}, texture storage {:?}, depth {:?}",
           color_formats, bgra_formats, bgra8_sampling_swizzle, texture_storage_usage, depth_format);

       // On Mali-T devices glCopyImageSubData appears to stall the pipeline until any pending
       // renders to the source texture have completed. On Mali-G, it has been observed to
       // indefinitely hang in some circumstances. Using an alternative such as glBlitFramebuffer
       // is preferable on such devices, so pretend we don't support glCopyImageSubData.
       // See bugs 1669494 and 1677757.
       let supports_copy_image_sub_data = if renderer_name.starts_with("Mali") {
           false
       } else {
           supports_extension(&extensions, "GL_EXT_copy_image") ||
           supports_extension(&extensions, "GL_ARB_copy_image")
       };

       // We have seen crashes on x86 PowerVR Rogue G6430 devices during GPU cache
       // updates using the scatter shader. It seems likely that GL_EXT_color_buffer_float
       // is broken. See bug 1709408.
       let is_x86_powervr_rogue_g6430 = renderer_name.starts_with("PowerVR Rogue G6430")
           && cfg!(target_arch = "x86");
       let supports_color_buffer_float = match gl.get_type() {
           gl::GlType::Gl => true,
           gl::GlType::Gles if is_x86_powervr_rogue_g6430 => false,
           gl::GlType::Gles => supports_extension(&extensions, "GL_EXT_color_buffer_float"),
       };

       let is_adreno = renderer_name.starts_with("Adreno");

       // There appears to be a driver bug on older versions of the Adreno
       // driver which prevents usage of persistenly mapped buffers.
       // See bugs 1678585 and 1683936.
       // TODO: only disable feature for affected driver versions.
       let supports_buffer_storage = if is_adreno {
           false
       } else {
           supports_extension(&extensions, "GL_EXT_buffer_storage") ||
           supports_extension(&extensions, "GL_ARB_buffer_storage")
       };

       // KHR_blend_equation_advanced renders incorrectly on Adreno
       // devices. This has only been confirmed up to Adreno 5xx, and has been
       // fixed for Android 9, so this condition could be made more specific.
       let supports_advanced_blend_equation =
           supports_extension(&extensions, "GL_KHR_blend_equation_advanced") &&
           !is_adreno;

       let supports_dual_source_blending = match gl.get_type() {
           gl::GlType::Gl => supports_extension(&extensions,"GL_ARB_blend_func_extended") &&
               supports_extension(&extensions,"GL_ARB_explicit_attrib_location"),
           gl::GlType::Gles => supports_extension(&extensions,"GL_EXT_blend_func_extended"),
       };

       // Software webrender relies on the unoptimized shader source.
       let use_optimized_shaders = use_optimized_shaders && !is_software_webrender;

       // On the android emulator, and possibly some Mali devices, glShaderSource
       // can crash if the source strings are not null-terminated.
       // See bug 1591945 and bug 1799722.
       let requires_null_terminated_shader_source = is_emulator || renderer_name == "Mali-T628"
           || renderer_name == "Mali-T720" || renderer_name == "Mali-T760"
           || renderer_name == "Mali-G57";

       // The android emulator gets confused if you don't explicitly unbind any texture
       // from GL_TEXTURE_EXTERNAL_OES before binding another to GL_TEXTURE_2D. See bug 1636085.
       let requires_texture_external_unbind = is_emulator;

       let is_macos = cfg!(target_os = "macos");
            //  && renderer_name.starts_with("AMD");
            //  (XXX: we apply this restriction to all GPUs to handle switching)

       let is_windows_angle = cfg!(target_os = "windows")
           && renderer_name.starts_with("ANGLE");
       let is_adreno_3xx = renderer_name.starts_with("Adreno (TM) 3");

       // Some GPUs require the stride of the data during texture uploads to be
       // aligned to certain requirements, either for correctness or performance
       // reasons.
       let required_pbo_stride = if is_adreno_3xx {
           // On Adreno 3xx, alignments of < 128 bytes can result in corrupted
           // glyphs. See bug 1696039.
           StrideAlignment::Bytes(NonZeroUsize::new(128).unwrap())
       } else if is_adreno {
           // On later Adreno devices it must be a multiple of 64 *pixels* to
           // hit the fast path, meaning value in bytes varies with the texture
           // format. This is purely an optimization.
           StrideAlignment::Pixels(NonZeroUsize::new(64).unwrap())
       } else if is_macos {
           // On AMD Mac, it must always be a multiple of 256 bytes.
           // We apply this restriction to all GPUs to handle switching
           StrideAlignment::Bytes(NonZeroUsize::new(256).unwrap())
       } else if is_windows_angle {
           // On ANGLE-on-D3D, PBO texture uploads get incorrectly truncated
           // if the stride is greater than the width * bpp.
           StrideAlignment::Bytes(NonZeroUsize::new(1).unwrap())
       } else {
           // Other platforms may have similar requirements and should be added
           // here. The default value should be 4 bytes.
           StrideAlignment::Bytes(NonZeroUsize::new(4).unwrap())
       };

       // On AMD Macs there is a driver bug which causes some texture uploads
       // from a non-zero offset within a PBO to fail. See bug 1603783.
       let supports_nonzero_pbo_offsets = !is_macos;

       // We have encountered several issues when only partially updating render targets on a
       // variety of Mali GPUs. As a precaution avoid doing so on all Midgard and Bifrost GPUs.
       // Valhall (eg Mali-Gx7 onwards) appears to be unaffected. See bug 1691955, bug 1558374,
       // and bug 1663355.
       // We have Additionally encountered issues on PowerVR D-Series. See bug 2005312.
       let supports_render_target_partial_update = !is_mali_midgard(&renderer_name)
           && !is_mali_bifrost(&renderer_name)
           && !renderer_name.starts_with("PowerVR D-Series");

       let supports_shader_storage_object = match gl.get_type() {
           // see https://www.g-truc.net/post-0734.html
           gl::GlType::Gl => supports_extension(&extensions, "GL_ARB_shader_storage_buffer_object"),
           gl::GlType::Gles => gl_version >= [3, 1],
       };

       // SWGL uses swgl_clipMask() instead of implementing clip-masking in shaders.
       // This allows certain shaders to potentially bypass the more expensive alpha-
       // pass variants if they know the alpha-pass was only required to deal with
       // clip-masking.
       let uses_native_clip_mask = is_software_webrender;

       // SWGL uses swgl_antiAlias() instead of implementing anti-aliasing in shaders.
       // As above, this allows bypassing certain alpha-pass variants.
       let uses_native_antialiasing = is_software_webrender;

       // If running on android with a mesa driver (eg intel chromebooks), parse the mesa version.
       let mut android_mesa_version = None;
       if cfg!(target_os = "android") && renderer_name.starts_with("Mesa") {
           if let Some((_, mesa_version)) = version_string.split_once("Mesa ") {
               if let Some((major_str, _)) = mesa_version.split_once(".") {
                   if let Ok(major) = major_str.parse::<i32>() {
                       android_mesa_version = Some(major);
                   }
               }
           }
       }

       // If the device supports OES_EGL_image_external_essl3 we can use it to render
       // external images. If not, we must use the ESSL 1.0 OES_EGL_image_external
       // extension instead.
       // Mesa versions prior to 20.0 do not implement textureSize(samplerExternalOES),
       // so we must use the fallback path.
       let supports_image_external_essl3 = match android_mesa_version {
           Some(major) if major < 20 => false,
           _ => supports_extension(&extensions, "GL_OES_EGL_image_external_essl3"),
       };

       let mut requires_batched_texture_uploads = None;
       if is_software_webrender {
           // No benefit to batching texture uploads with swgl.
           requires_batched_texture_uploads = Some(false);
       } else if renderer_name.starts_with("Mali-G") {
           // On Mali-Gxx the driver really struggles with many small texture uploads,
           // and handles fewer, larger uploads better.
           requires_batched_texture_uploads = Some(true);
       }

       // On Mali-Txxx devices we have observed crashes during draw calls when rendering
       // to an alpha target immediately after using glClear to clear regions of it.
       // Using a shader to clear the regions avoids the crash. See bug 1638593.
       // On Adreno 510 devices we have seen garbage being used as masks when clearing
       // alpha targets with glClear. Using quads to clear avoids this. See bug 1941154.
       let is_adreno_510 = renderer_name.starts_with("Adreno (TM) 510");
       let supports_alpha_target_clears = !is_mali_midgard(&renderer_name) && !is_adreno_510;

       // On Adreno 4xx devices with older drivers we have seen render tasks to alpha targets have
       // no effect unless the target is fully cleared prior to rendering. See bug 1714227.
       let is_adreno_4xx = renderer_name.starts_with("Adreno (TM) 4");
       let requires_alpha_target_full_clear = is_adreno_4xx;

       // Testing on Intel and nVidia GPUs, as well as software webrender, showed large performance
       // wins applying a scissor rect when clearing render targets. Assume this is the best
       // default. On mobile GPUs, however, it can be much more efficient to clear the entire
       // render target. For now, enable the scissor everywhere except Android hardware
       // webrender. We can tweak this further if needs be.
       let prefers_clear_scissor = !cfg!(target_os = "android") || is_software_webrender;

       let mut supports_render_target_invalidate = true;

       // On PowerVR Rogue devices we have seen that invalidating render targets after we are done
       // with them can incorrectly cause pending renders to be written to different targets
       // instead. See bug 1719345.
       let is_powervr_rogue = renderer_name.starts_with("PowerVR Rogue");
       if is_powervr_rogue {
           supports_render_target_invalidate = false;
       }

       // On Mali Valhall devices with a driver version v1.r36p0 we have seen that invalidating
       // render targets can result in image corruption, perhaps due to subsequent reuses of the
       // render target not correctly reinitializing them to a valid state. See bug 1787520.
       if is_mali_valhall(&renderer_name) {
           match parse_mali_version(&version_string) {
               Some(version) if version >= (1, 36, 0) => supports_render_target_invalidate = false,
               _ => {}
           }
       }

       // On Linux we we have seen uploads to R8 format textures result in
       // corruption on some AMD cards.
       // See https://bugzilla.mozilla.org/show_bug.cgi?id=1687554#c13
       let supports_r8_texture_upload = if cfg!(target_os = "linux")
           && renderer_name.starts_with("AMD Radeon RX")
       {
           false
       } else {
           true
       };

       let supports_qcom_tiled_rendering = if is_adreno && version_string.contains("V@0490") {
           // We have encountered rendering errors on a variety of Adreno GPUs specifically on
           // driver version V@0490, so block this extension on that driver version. See bug 1828248.
           false
       } else if renderer_name == "Adreno (TM) 308" {
           // And specifically on Areno 308 GPUs we have encountered rendering errors on driver
           // versions V@331, V@415, and V@0502. We presume this therefore affects all driver
           // versions. See bug 1843749 and bug 1847319.
           false
       } else {
           supports_extension(&extensions, "GL_QCOM_tiled_rendering")
       };

       // On some Adreno 3xx devices the vertex array object must be unbound and rebound after
       // an attached buffer has been orphaned.
       let requires_vao_rebind_after_orphaning = is_adreno_3xx;

       Device {
           gl,
           base_gl: None,
           crash_annotator,
           annotate_draw_call_crashes: false,
           resource_override_path,
           use_optimized_shaders,
           upload_method,
           use_batched_texture_uploads: requires_batched_texture_uploads.unwrap_or(false),
           use_draw_calls_for_texture_copy: false,
           batched_upload_threshold,

           inside_frame: false,

           capabilities: Capabilities {
               supports_multisampling: false, //TODO
               supports_copy_image_sub_data,
               supports_color_buffer_float,
               supports_buffer_storage,
               supports_advanced_blend_equation,
               supports_dual_source_blending,
               supports_khr_debug,
               supports_texture_swizzle,
               supports_nonzero_pbo_offsets,
               supports_texture_usage,
               supports_render_target_partial_update,
               supports_shader_storage_object,
               requires_batched_texture_uploads,
               supports_alpha_target_clears,
               requires_alpha_target_full_clear,
               prefers_clear_scissor,
               supports_render_target_invalidate,
               supports_r8_texture_upload,
               supports_qcom_tiled_rendering,
               uses_native_clip_mask,
               uses_native_antialiasing,
               supports_image_external_essl3,
               requires_vao_rebind_after_orphaning,
               renderer_name,
           },

           color_formats,
           bgra_formats,
           bgra_pixel_type,
           swizzle_settings: SwizzleSettings {
               bgra8_sampling_swizzle,
           },
           depth_format,

           depth_targets: FastHashMap::default(),

           bound_textures: [0; 16],
           bound_program: 0,
           bound_program_name: Rc::new(std::ffi::CString::new("").unwrap()),
           bound_vao: 0,
           bound_read_fbo: (FBOId(0), DeviceIntPoint::zero()),
           bound_draw_fbo: FBOId(0),
           default_read_fbo: FBOId(0),
           default_draw_fbo: FBOId(0),

           depth_available: true,

           max_texture_size,
           cached_programs,
           frame_id: GpuFrameId(0),
           extensions,
           texture_storage_usage,
           requires_null_terminated_shader_source,
           requires_texture_external_unbind,
           is_software_webrender,
           required_pbo_stride,
           dump_shader_source,
           surface_origin_is_top_left,

           #[cfg(debug_assertions)]
           shader_is_ready: false,

           textures_created: 0,
           textures_deleted: 0,
       }
   }

   pub fn gl(&self) -> &dyn gl::Gl {
       &*self.gl
   }

   pub fn rc_gl(&self) -> &Rc<dyn gl::Gl> {
       &self.gl
   }

   pub fn set_parameter(&mut self, param: &Parameter) {
       match param {
           Parameter::Bool(BoolParameter::PboUploads, enabled) => {
               if !self.is_software_webrender {
                   self.upload_method = if *enabled {
                       UploadMethod::PixelBuffer(crate::ONE_TIME_USAGE_HINT)
                   } else {
                       UploadMethod::Immediate
                   };
               }
           }
           Parameter::Bool(BoolParameter::BatchedUploads, enabled) => {
               if self.capabilities.requires_batched_texture_uploads.is_none() {
                   self.use_batched_texture_uploads = *enabled;
               }
           }
           Parameter::Bool(BoolParameter::DrawCallsForTextureCopy, enabled) => {
               self.use_draw_calls_for_texture_copy = *enabled;
           }
           Parameter::Int(IntParameter::BatchedUploadThreshold, threshold) => {
               self.batched_upload_threshold = *threshold;
           }
           _ => {}
       }
   }

   /// Ensures that the maximum texture size is less than or equal to the
   /// provided value. If the provided value is less than the value supported
   /// by the driver, the latter is used.
   pub fn clamp_max_texture_size(&mut self, size: i32) {
       self.max_texture_size = self.max_texture_size.min(size);
   }

   /// Returns the limit on texture dimensions (width or height).
   pub fn max_texture_size(&self) -> i32 {
       self.max_texture_size
   }

   pub fn surface_origin_is_top_left(&self) -> bool {
       self.surface_origin_is_top_left
   }

   pub fn get_capabilities(&self) -> &Capabilities {
       &self.capabilities
   }

   pub fn preferred_color_formats(&self) -> TextureFormatPair<ImageFormat> {
       self.color_formats.clone()
   }

   pub fn swizzle_settings(&self) -> Option<SwizzleSettings> {
       if self.capabilities.supports_texture_swizzle {
           Some(self.swizzle_settings)
       } else {
           None
       }
   }

   pub fn depth_bits(&self) -> i32 {
       match self.depth_format {
           gl::DEPTH_COMPONENT16 => 16,
           gl::DEPTH_COMPONENT24 => 24,
           _ => panic!("Unknown depth format {:?}", self.depth_format),
       }
   }

   // See gpu_types.rs where we declare the number of possible documents and
   // number of items per document. This should match up with that.
   pub fn max_depth_ids(&self) -> i32 {
       return 1 << (self.depth_bits() - RESERVE_DEPTH_BITS);
   }

   pub fn ortho_near_plane(&self) -> f32 {
       return -self.max_depth_ids() as f32;
   }

   pub fn ortho_far_plane(&self) -> f32 {
       return (self.max_depth_ids() - 1) as f32;
   }

   pub fn required_pbo_stride(&self) -> StrideAlignment {
       self.required_pbo_stride
   }

   pub fn upload_method(&self) -> &UploadMethod {
       &self.upload_method
   }

   pub fn use_batched_texture_uploads(&self) -> bool {
       self.use_batched_texture_uploads
   }

   pub fn use_draw_calls_for_texture_copy(&self) -> bool {
       self.use_draw_calls_for_texture_copy
   }

   pub fn batched_upload_threshold(&self) -> i32 {
       self.batched_upload_threshold
   }

   pub fn reset_state(&mut self) {
       for i in 0 .. self.bound_textures.len() {
           self.bound_textures[i] = 0;
           self.gl.active_texture(gl::TEXTURE0 + i as gl::GLuint);
           self.gl.bind_texture(gl::TEXTURE_2D, 0);
       }

       self.bound_vao = 0;
       self.gl.bind_vertex_array(0);

       self.bound_read_fbo = (self.default_read_fbo, DeviceIntPoint::zero());
       self.gl.bind_framebuffer(gl::READ_FRAMEBUFFER, self.default_read_fbo.0);

       self.bound_draw_fbo = self.default_draw_fbo;
       self.gl.bind_framebuffer(gl::DRAW_FRAMEBUFFER, self.bound_draw_fbo.0);
   }

   #[cfg(debug_assertions)]
   fn print_shader_errors(source: &str, log: &str) {
       // hacky way to extract the offending lines
       if !log.starts_with("0:") && !log.starts_with("0(") {
           return;
       }
       let end_pos = match log[2..].chars().position(|c| !c.is_digit(10)) {
           Some(pos) => 2 + pos,
           None => return,
       };
       let base_line_number = match log[2 .. end_pos].parse::<usize>() {
           Ok(number) if number >= 2 => number - 2,
           _ => return,
       };
       for (line, prefix) in source.lines().skip(base_line_number).zip(&["|",">","|"]) {
           error!("{}\t{}", prefix, line);
       }
   }

   pub fn compile_shader(
       &self,
       name: &str,
       shader_type: gl::GLenum,
       source: &String,
   ) -> Result<gl::GLuint, ShaderError> {
       debug!("compile {}", name);
       let id = self.gl.create_shader(shader_type);

       let mut new_source = Cow::from(source.as_str());
       // Ensure the source strings we pass to glShaderSource are
       // null-terminated on buggy platforms.
       if self.requires_null_terminated_shader_source {
           new_source.to_mut().push('\0');
       }

       self.gl.shader_source(id, &[new_source.as_bytes()]);
       self.gl.compile_shader(id);
       let log = self.gl.get_shader_info_log(id);
       let mut status = [0];
       unsafe {
           self.gl.get_shader_iv(id, gl::COMPILE_STATUS, &mut status);
       }
       if status[0] == 0 {
           let type_str = match shader_type {
               gl::VERTEX_SHADER => "vertex",
               gl::FRAGMENT_SHADER => "fragment",
               _ => panic!("Unexpected shader type {:x}", shader_type),
           };
           error!("Failed to compile {} shader: {}\n{}", type_str, name, log);
           #[cfg(debug_assertions)]
           Self::print_shader_errors(source, &log);
           Err(ShaderError::Compilation(name.to_string(), log))
       } else {
           if !log.is_empty() {
               warn!("Warnings detected on shader: {}\n{}", name, log);
           }
           Ok(id)
       }
   }

   pub fn begin_frame(&mut self) -> GpuFrameId {
       debug_assert!(!self.inside_frame);
       self.inside_frame = true;
       #[cfg(debug_assertions)]
       {
           self.shader_is_ready = false;
       }

       self.textures_created = 0;
       self.textures_deleted = 0;

       // If our profiler state has changed, apply or remove the profiling
       // wrapper from our GL context.
       let being_profiled = profiler::thread_is_being_profiled();
       let using_wrapper = self.base_gl.is_some();

       // We can usually unwind driver stacks on OSes other than Android, so we don't need to
       // manually instrument gl calls there. Timestamps can be pretty expensive on Windows (2us
       // each and perhaps an opportunity to be descheduled?) which makes the profiles gathered
       // with this turned on less useful so only profile on ARM Android.
       if cfg!(any(target_arch = "arm", target_arch = "aarch64"))
           && cfg!(target_os = "android")
           && being_profiled
           && !using_wrapper
       {
           fn note(name: &str, duration: Duration) {
               profiler::add_text_marker("OpenGL Calls", name, duration);
           }
           let threshold = Duration::from_millis(1);
           let wrapped = gl::ProfilingGl::wrap(self.gl.clone(), threshold, note);
           let base = mem::replace(&mut self.gl, wrapped);
           self.base_gl = Some(base);
       } else if !being_profiled && using_wrapper {
           self.gl = self.base_gl.take().unwrap();
       }

       // Retrieve the currently set FBO.
       let mut default_read_fbo = [0];
       unsafe {
           self.gl.get_integer_v(gl::READ_FRAMEBUFFER_BINDING, &mut default_read_fbo);
       }
       self.default_read_fbo = FBOId(default_read_fbo[0] as gl::GLuint);
       let mut default_draw_fbo = [0];
       unsafe {
           self.gl.get_integer_v(gl::DRAW_FRAMEBUFFER_BINDING, &mut default_draw_fbo);
       }
       self.default_draw_fbo = FBOId(default_draw_fbo[0] as gl::GLuint);

       // Shader state
       self.bound_program = 0;
       self.gl.use_program(0);

       // Reset common state
       self.reset_state();

       // Pixel op state
       self.gl.pixel_store_i(gl::UNPACK_ALIGNMENT, 1);
       self.gl.bind_buffer(gl::PIXEL_UNPACK_BUFFER, 0);

       // Default is sampler 0, always
       self.gl.active_texture(gl::TEXTURE0);

       self.frame_id
   }

   fn bind_texture_impl(
       &mut self,
       slot: TextureSlot,
       id: gl::GLuint,
       target: gl::GLenum,
       set_swizzle: Option<Swizzle>,
       image_rendering: Option<ImageRendering>,
   ) {
       debug_assert!(self.inside_frame);

       if self.bound_textures[slot.0] != id || set_swizzle.is_some() || image_rendering.is_some() {
           self.gl.active_texture(gl::TEXTURE0 + slot.0 as gl::GLuint);
           // The android emulator gets confused if you don't explicitly unbind any texture
           // from GL_TEXTURE_EXTERNAL_OES before binding to GL_TEXTURE_2D. See bug 1636085.
           if target == gl::TEXTURE_2D && self.requires_texture_external_unbind {
               self.gl.bind_texture(gl::TEXTURE_EXTERNAL_OES, 0);
           }
           self.gl.bind_texture(target, id);
           if let Some(swizzle) = set_swizzle {
               if self.capabilities.supports_texture_swizzle {
                   let components = match swizzle {
                       Swizzle::Rgba => [gl::RED, gl::GREEN, gl::BLUE, gl::ALPHA],
                       Swizzle::Bgra => [gl::BLUE, gl::GREEN, gl::RED, gl::ALPHA],
                   };
                   self.gl.tex_parameter_i(target, gl::TEXTURE_SWIZZLE_R, components[0] as i32);
                   self.gl.tex_parameter_i(target, gl::TEXTURE_SWIZZLE_G, components[1] as i32);
                   self.gl.tex_parameter_i(target, gl::TEXTURE_SWIZZLE_B, components[2] as i32);
                   self.gl.tex_parameter_i(target, gl::TEXTURE_SWIZZLE_A, components[3] as i32);
               } else {
                   debug_assert_eq!(swizzle, Swizzle::default());
               }
           }
           if let Some(image_rendering) = image_rendering {
               let filter = match image_rendering {
                   ImageRendering::Auto | ImageRendering::CrispEdges => gl::LINEAR,
                   ImageRendering::Pixelated => gl::NEAREST,
               };
               self.gl.tex_parameter_i(target, gl::TEXTURE_MIN_FILTER, filter as i32);
               self.gl.tex_parameter_i(target, gl::TEXTURE_MAG_FILTER, filter as i32);
           }
           self.gl.active_texture(gl::TEXTURE0);
           self.bound_textures[slot.0] = id;
       }
   }

   pub fn bind_texture<S>(&mut self, slot: S, texture: &Texture, swizzle: Swizzle)
   where
       S: Into<TextureSlot>,
   {
       let old_swizzle = texture.active_swizzle.replace(swizzle);
       let set_swizzle = if old_swizzle != swizzle {
           Some(swizzle)
       } else {
           None
       };
       self.bind_texture_impl(slot.into(), texture.id, texture.target, set_swizzle, None);
   }

   pub fn bind_external_texture<S>(&mut self, slot: S, external_texture: &ExternalTexture)
   where
       S: Into<TextureSlot>,
   {
       self.bind_texture_impl(
           slot.into(),
           external_texture.id,
           external_texture.target,
           None,
           Some(external_texture.image_rendering),
       );
   }

   pub fn bind_read_target_impl(
       &mut self,
       fbo_id: FBOId,
       offset: DeviceIntPoint,
   ) {
       debug_assert!(self.inside_frame);

       if self.bound_read_fbo != (fbo_id, offset) {
           fbo_id.bind(self.gl(), FBOTarget::Read);
       }

       self.bound_read_fbo = (fbo_id, offset);
   }

   pub fn bind_read_target(&mut self, target: ReadTarget) {
       let fbo_id = match target {
           ReadTarget::Default => self.default_read_fbo,
           ReadTarget::Texture { fbo_id } => fbo_id,
           ReadTarget::External { fbo } => fbo,
           ReadTarget::NativeSurface { fbo_id, .. } => fbo_id,
       };

       self.bind_read_target_impl(fbo_id, target.offset())
   }

   fn bind_draw_target_impl(&mut self, fbo_id: FBOId) {
       debug_assert!(self.inside_frame);

       if self.bound_draw_fbo != fbo_id {
           self.bound_draw_fbo = fbo_id;
           fbo_id.bind(self.gl(), FBOTarget::Draw);
       }
   }

   pub fn reset_read_target(&mut self) {
       let fbo = self.default_read_fbo;
       self.bind_read_target_impl(fbo, DeviceIntPoint::zero());
   }


   pub fn reset_draw_target(&mut self) {
       let fbo = self.default_draw_fbo;
       self.bind_draw_target_impl(fbo);
       self.depth_available = true;
   }

   pub fn bind_draw_target(
       &mut self,
       target: DrawTarget,
   ) {
       let (fbo_id, rect, depth_available) = match target {
           DrawTarget::Default { rect, .. } => {
               (self.default_draw_fbo, rect, false)
           }
           DrawTarget::Texture { dimensions, fbo_id, with_depth, .. } => {
               let rect = FramebufferIntRect::from_size(
                   device_size_as_framebuffer_size(dimensions),
               );
               (fbo_id, rect, with_depth)
           },
           DrawTarget::External { fbo, size } => {
               (fbo, size.into(), false)
           }
           DrawTarget::NativeSurface { external_fbo_id, offset, dimensions, .. } => {
               (
                   FBOId(external_fbo_id),
                   device_rect_as_framebuffer_rect(&DeviceIntRect::from_origin_and_size(offset, dimensions)),
                   true
               )
           }
       };

       self.depth_available = depth_available;
       self.bind_draw_target_impl(fbo_id);
       self.gl.viewport(
           rect.min.x,
           rect.min.y,
           rect.width(),
           rect.height(),
       );
   }

   /// Creates an unbound FBO object. Additional attachment API calls are
   /// required to make it complete.
   pub fn create_fbo(&mut self) -> FBOId {
       FBOId(self.gl.gen_framebuffers(1)[0])
   }

   /// Creates an FBO with the given texture bound as the color attachment.
   pub fn create_fbo_for_external_texture(&mut self, texture_id: u32) -> FBOId {
       let fbo = self.create_fbo();
       fbo.bind(self.gl(), FBOTarget::Draw);
       self.gl.framebuffer_texture_2d(
           gl::DRAW_FRAMEBUFFER,
           gl::COLOR_ATTACHMENT0,
           gl::TEXTURE_2D,
           texture_id,
           0,
       );
       debug_assert_eq!(
           self.gl.check_frame_buffer_status(gl::DRAW_FRAMEBUFFER),
           gl::FRAMEBUFFER_COMPLETE,
           "Incomplete framebuffer",
       );
       self.bound_draw_fbo.bind(self.gl(), FBOTarget::Draw);
       fbo
   }

   pub fn delete_fbo(&mut self, fbo: FBOId) {
       self.gl.delete_framebuffers(&[fbo.0]);
   }

   pub fn bind_external_draw_target(&mut self, fbo_id: FBOId) {
       debug_assert!(self.inside_frame);

       if self.bound_draw_fbo != fbo_id {
           self.bound_draw_fbo = fbo_id;
           fbo_id.bind(self.gl(), FBOTarget::Draw);
       }
   }

   /// Link a program, attaching the supplied vertex format.
   ///
   /// If `create_program()` finds a binary shader on disk, it will kick
   /// off linking immediately, which some drivers (notably ANGLE) run
   /// in parallel on background threads. As such, this function should
   /// ideally be run sometime later, to give the driver time to do that
   /// before blocking due to an API call accessing the shader.
   ///
   /// This generally means that the first run of the application will have
   /// to do a bunch of blocking work to compile the shader from source, but
   /// subsequent runs should load quickly.
   pub fn link_program(
       &mut self,
       program: &mut Program,
       descriptor: &VertexDescriptor,
   ) -> Result<(), ShaderError> {
       profile_scope!("compile shader");

       let _guard = CrashAnnotatorGuard::new(
           &self.crash_annotator,
           CrashAnnotation::CompileShader,
           &program.source_info.full_name_cstr
       );

       assert!(!program.is_initialized());
       let mut build_program = true;
       let info = &program.source_info;

       // See if we hit the binary shader cache
       if let Some(ref cached_programs) = self.cached_programs {
           // If the shader is not in the cache, attempt to load it from disk
           if cached_programs.entries.borrow().get(&program.source_info.digest).is_none() {
               if let Some(ref handler) = cached_programs.program_cache_handler {
                   handler.try_load_shader_from_disk(&program.source_info.digest, cached_programs);
                   if let Some(entry) = cached_programs.entries.borrow().get(&program.source_info.digest) {
                       self.gl.program_binary(program.id, entry.binary.format, &entry.binary.bytes);
                   }
               }
           }

           if let Some(entry) = cached_programs.entries.borrow_mut().get_mut(&info.digest) {
               let mut link_status = [0];
               unsafe {
                   self.gl.get_program_iv(program.id, gl::LINK_STATUS, &mut link_status);
               }
               if link_status[0] == 0 {
                   let error_log = self.gl.get_program_info_log(program.id);
                   error!(
                     "Failed to load a program object with a program binary: {} renderer {}\n{}",
                     &info.base_filename,
                     self.capabilities.renderer_name,
                     error_log
                   );
                   if let Some(ref program_cache_handler) = cached_programs.program_cache_handler {
                       program_cache_handler.notify_program_binary_failed(&entry.binary);
                   }
               } else {
                   entry.linked = true;
                   build_program = false;
               }
           }
       }

       // If not, we need to do a normal compile + link pass.
       if build_program {
           // Compile the vertex shader
           let vs_source = info.compute_source(self, ShaderKind::Vertex);
           let vs_id = match self.compile_shader(&info.full_name(), gl::VERTEX_SHADER, &vs_source) {
                   Ok(vs_id) => vs_id,
                   Err(err) => return Err(err),
               };

           // Compile the fragment shader
           let fs_source = info.compute_source(self, ShaderKind::Fragment);
           let fs_id =
               match self.compile_shader(&info.full_name(), gl::FRAGMENT_SHADER, &fs_source) {
                   Ok(fs_id) => fs_id,
                   Err(err) => {
                       self.gl.delete_shader(vs_id);
                       return Err(err);
                   }
               };

           // Check if shader source should be dumped
           if Some(info.base_filename) == self.dump_shader_source.as_ref().map(String::as_ref) {
               let path = std::path::Path::new(info.base_filename);
               std::fs::write(path.with_extension("vert"), vs_source).unwrap();
               std::fs::write(path.with_extension("frag"), fs_source).unwrap();
           }

           // Attach shaders
           self.gl.attach_shader(program.id, vs_id);
           self.gl.attach_shader(program.id, fs_id);

           // Bind vertex attributes
           for (i, attr) in descriptor
               .vertex_attributes
               .iter()
               .chain(descriptor.instance_attributes.iter())
               .enumerate()
           {
               self.gl
                   .bind_attrib_location(program.id, i as gl::GLuint, attr.name);
           }

           if self.cached_programs.is_some() {
               self.gl.program_parameter_i(program.id, gl::PROGRAM_BINARY_RETRIEVABLE_HINT, gl::TRUE as gl::GLint);
           }

           // Link!
           self.gl.link_program(program.id);

           // GL recommends detaching and deleting shaders once the link
           // is complete (whether successful or not). This allows the driver
           // to free any memory associated with the parsing and compilation.
           self.gl.detach_shader(program.id, vs_id);
           self.gl.detach_shader(program.id, fs_id);
           self.gl.delete_shader(vs_id);
           self.gl.delete_shader(fs_id);

           let mut link_status = [0];
           unsafe {
               self.gl.get_program_iv(program.id, gl::LINK_STATUS, &mut link_status);
           }
           if link_status[0] == 0 {
               let error_log = self.gl.get_program_info_log(program.id);
               error!(
                   "Failed to link shader program: {}\n{}",
                   &info.base_filename,
                   error_log
               );
               self.gl.delete_program(program.id);
               return Err(ShaderError::Link(info.base_filename.to_owned(), error_log));
           }

           if let Some(ref cached_programs) = self.cached_programs {
               if !cached_programs.entries.borrow().contains_key(&info.digest) {
                   let (buffer, format) = self.gl.get_program_binary(program.id);
                   if buffer.len() > 0 {
                       let binary = Arc::new(ProgramBinary::new(buffer, format, info.digest.clone()));
                       cached_programs.add_new_program_binary(binary);
                   }
               }
           }
       }

       // If we get here, the link succeeded, so get the uniforms.
       program.is_initialized = true;
       program.u_transform = self.gl.get_uniform_location(program.id, "uTransform");
       program.u_texture_size = self.gl.get_uniform_location(program.id, "uTextureSize");

       Ok(())
   }

   pub fn bind_program(&mut self, program: &Program) -> bool {
       debug_assert!(self.inside_frame);
       debug_assert!(program.is_initialized());
       if !program.is_initialized() {
           return false;
       }
       #[cfg(debug_assertions)]
       {
           self.shader_is_ready = true;
       }

       if self.bound_program != program.id {
           self.gl.use_program(program.id);
           self.bound_program = program.id;
           self.bound_program_name = program.source_info.full_name_cstr.clone();
       }
       true
   }

   pub fn create_texture(
       &mut self,
       target: ImageBufferKind,
       format: ImageFormat,
       mut width: i32,
       mut height: i32,
       filter: TextureFilter,
       render_target: Option<RenderTargetInfo>,
   ) -> Texture {
       debug_assert!(self.inside_frame);

       if width > self.max_texture_size || height > self.max_texture_size {
           error!("Attempting to allocate a texture of size {}x{} above the limit, trimming", width, height);
           width = width.min(self.max_texture_size);
           height = height.min(self.max_texture_size);
       }

       // Set up the texture book-keeping.
       let mut texture = Texture {
           id: self.gl.gen_textures(1)[0],
           target: get_gl_target(target),
           size: DeviceIntSize::new(width, height),
           format,
           filter,
           active_swizzle: Cell::default(),
           fbo: None,
           fbo_with_depth: None,
           last_frame_used: self.frame_id,
           flags: TextureFlags::default(),
       };
       self.bind_texture(DEFAULT_TEXTURE, &texture, Swizzle::default());
       self.set_texture_parameters(texture.target, filter);

       if self.capabilities.supports_texture_usage && render_target.is_some() {
           self.gl.tex_parameter_i(texture.target, gl::TEXTURE_USAGE_ANGLE, gl::FRAMEBUFFER_ATTACHMENT_ANGLE as gl::GLint);
       }

       // Allocate storage.
       let desc = self.gl_describe_format(texture.format);

       // Firefox doesn't use mipmaps, but Servo uses them for standalone image
       // textures images larger than 512 pixels. This is the only case where
       // we set the filter to trilinear.
       let mipmap_levels =  if texture.filter == TextureFilter::Trilinear {
           let max_dimension = cmp::max(width, height);
           ((max_dimension) as f64).log2() as gl::GLint + 1
       } else {
           1
       };

       // We never want to upload texture data at the same time as allocating the texture.
       self.gl.bind_buffer(gl::PIXEL_UNPACK_BUFFER, 0);

       // Use glTexStorage where available, since it avoids allocating
       // unnecessary mipmap storage and generally improves performance with
       // stronger invariants.
       let use_texture_storage = match self.texture_storage_usage {
           TexStorageUsage::Always => true,
           TexStorageUsage::NonBGRA8 => texture.format != ImageFormat::BGRA8,
           TexStorageUsage::Never => false,
       };
       if use_texture_storage {
           self.gl.tex_storage_2d(
               texture.target,
               mipmap_levels,
               desc.internal,
               texture.size.width as gl::GLint,
               texture.size.height as gl::GLint,
           );
       } else {
           self.gl.tex_image_2d(
               texture.target,
               0,
               desc.internal as gl::GLint,
               texture.size.width as gl::GLint,
               texture.size.height as gl::GLint,
               0,
               desc.external,
               desc.pixel_type,
               None,
           );
       }

       // Set up FBOs, if required.
       if let Some(rt_info) = render_target {
           self.init_fbos(&mut texture, false);
           if rt_info.has_depth {
               self.init_fbos(&mut texture, true);
           }
       }

       self.textures_created += 1;

       texture
   }

   fn set_texture_parameters(&mut self, target: gl::GLuint, filter: TextureFilter) {
       let mag_filter = match filter {
           TextureFilter::Nearest => gl::NEAREST,
           TextureFilter::Linear | TextureFilter::Trilinear => gl::LINEAR,
       };

       let min_filter = match filter {
           TextureFilter::Nearest => gl::NEAREST,
           TextureFilter::Linear => gl::LINEAR,
           TextureFilter::Trilinear => gl::LINEAR_MIPMAP_LINEAR,
       };

       self.gl
           .tex_parameter_i(target, gl::TEXTURE_MAG_FILTER, mag_filter as gl::GLint);
       self.gl
           .tex_parameter_i(target, gl::TEXTURE_MIN_FILTER, min_filter as gl::GLint);

       self.gl
           .tex_parameter_i(target, gl::TEXTURE_WRAP_S, gl::CLAMP_TO_EDGE as gl::GLint);
       self.gl
           .tex_parameter_i(target, gl::TEXTURE_WRAP_T, gl::CLAMP_TO_EDGE as gl::GLint);
   }

   /// Copies the entire contents of one texture to another. The dest texture must be at least
   /// as large as the source texture in each dimension. No scaling is performed, so if the dest
   /// texture is larger than the source texture then some of its pixels will not be written to.
   pub fn copy_entire_texture(
       &mut self,
       dst: &mut Texture,
       src: &Texture,
   ) {
       debug_assert!(self.inside_frame);
       debug_assert!(dst.size.width >= src.size.width);
       debug_assert!(dst.size.height >= src.size.height);

       self.copy_texture_sub_region(
           src,
           0,
           0,
           dst,
           0,
           0,
           src.size.width as _,
           src.size.height as _,
       );
   }

   /// Copies the specified subregion from src_texture to dest_texture.
   pub fn copy_texture_sub_region(
       &mut self,
       src_texture: &Texture,
       src_x: usize,
       src_y: usize,
       dest_texture: &Texture,
       dest_x: usize,
       dest_y: usize,
       width: usize,
       height: usize,
   ) {
       if self.capabilities.supports_copy_image_sub_data {
           assert_ne!(
               src_texture.id, dest_texture.id,
               "glCopyImageSubData's behaviour is undefined if src and dst images are identical and the rectangles overlap."
           );
           unsafe {
               self.gl.copy_image_sub_data(
                   src_texture.id,
                   src_texture.target,
                   0,
                   src_x as _,
                   src_y as _,
                   0,
                   dest_texture.id,
                   dest_texture.target,
                   0,
                   dest_x as _,
                   dest_y as _,
                   0,
                   width as _,
                   height as _,
                   1,
               );
           }
       } else {
           let src_offset = FramebufferIntPoint::new(src_x as i32, src_y as i32);
           let dest_offset = FramebufferIntPoint::new(dest_x as i32, dest_y as i32);
           let size = FramebufferIntSize::new(width as i32, height as i32);

           self.blit_render_target(
               ReadTarget::from_texture(src_texture),
               FramebufferIntRect::from_origin_and_size(src_offset, size),
               DrawTarget::from_texture(dest_texture, false),
               FramebufferIntRect::from_origin_and_size(dest_offset, size),
               // In most cases the filter shouldn't matter, as there is no scaling involved
               // in the blit. We were previously using Linear, but this caused issues when
               // blitting RGBAF32 textures on Mali, so use Nearest to be safe.
               TextureFilter::Nearest,
           );
       }
   }

   /// Notifies the device that the contents of a render target are no longer
   /// needed.
   pub fn invalidate_render_target(&mut self, texture: &Texture) {
       if self.capabilities.supports_render_target_invalidate {
           let (fbo, attachments) = if texture.supports_depth() {
               (&texture.fbo_with_depth,
                &[gl::COLOR_ATTACHMENT0, gl::DEPTH_ATTACHMENT] as &[gl::GLenum])
           } else {
               (&texture.fbo, &[gl::COLOR_ATTACHMENT0] as &[gl::GLenum])
           };

           if let Some(fbo_id) = fbo {
               let original_bound_fbo = self.bound_draw_fbo;
               // Note: The invalidate extension may not be supported, in which
               // case this is a no-op. That's ok though, because it's just a
               // hint.
               self.bind_external_draw_target(*fbo_id);
               self.gl.invalidate_framebuffer(gl::FRAMEBUFFER, attachments);
               self.bind_external_draw_target(original_bound_fbo);
           }
       }
   }

   /// Notifies the device that the contents of the current framebuffer's depth
   /// attachment is no longer needed. Unlike invalidate_render_target, this can
   /// be called even when the contents of the colour attachment is still required.
   /// This should be called before unbinding the framebuffer at the end of a pass,
   /// to allow tiled GPUs to avoid writing the contents back to memory.
   pub fn invalidate_depth_target(&mut self) {
       assert!(self.depth_available);
       let attachments = if self.bound_draw_fbo == self.default_draw_fbo {
           &[gl::DEPTH] as &[gl::GLenum]
       } else {
           &[gl::DEPTH_ATTACHMENT] as &[gl::GLenum]
       };
       self.gl.invalidate_framebuffer(gl::DRAW_FRAMEBUFFER, attachments);
   }

   /// Notifies the device that a render target is about to be reused.
   ///
   /// This method adds or removes a depth target as necessary.
   pub fn reuse_render_target<T: Texel>(
       &mut self,
       texture: &mut Texture,
       rt_info: RenderTargetInfo,
   ) {
       texture.last_frame_used = self.frame_id;

       // Add depth support if needed.
       if rt_info.has_depth && !texture.supports_depth() {
           self.init_fbos(texture, true);
       }
   }

   fn init_fbos(&mut self, texture: &mut Texture, with_depth: bool) {
       let (fbo, depth_rb) = if with_depth {
           let depth_target = self.acquire_depth_target(texture.get_dimensions());
           (&mut texture.fbo_with_depth, Some(depth_target))
       } else {
           (&mut texture.fbo, None)
       };

       // Generate the FBOs.
       assert!(fbo.is_none());
       let fbo_id = FBOId(*self.gl.gen_framebuffers(1).first().unwrap());
       *fbo = Some(fbo_id);

       // Bind the FBOs.
       let original_bound_fbo = self.bound_draw_fbo;

       self.bind_external_draw_target(fbo_id);

       self.gl.framebuffer_texture_2d(
           gl::DRAW_FRAMEBUFFER,
           gl::COLOR_ATTACHMENT0,
           texture.target,
           texture.id,
           0,
       );

       if let Some(depth_rb) = depth_rb {
           self.gl.framebuffer_renderbuffer(
               gl::DRAW_FRAMEBUFFER,
               gl::DEPTH_ATTACHMENT,
               gl::RENDERBUFFER,
               depth_rb.0,
           );
       }

       debug_assert_eq!(
           self.gl.check_frame_buffer_status(gl::DRAW_FRAMEBUFFER),
           gl::FRAMEBUFFER_COMPLETE,
           "Incomplete framebuffer",
       );

       self.bind_external_draw_target(original_bound_fbo);
   }

   fn acquire_depth_target(&mut self, dimensions: DeviceIntSize) -> RBOId {
       let gl = &self.gl;
       let depth_format = self.depth_format;
       let target = self.depth_targets.entry(dimensions).or_insert_with(|| {
           let renderbuffer_ids = gl.gen_renderbuffers(1);
           let depth_rb = renderbuffer_ids[0];
           gl.bind_renderbuffer(gl::RENDERBUFFER, depth_rb);
           gl.renderbuffer_storage(
               gl::RENDERBUFFER,
               depth_format,
               dimensions.width as _,
               dimensions.height as _,
           );
           SharedDepthTarget {
               rbo_id: RBOId(depth_rb),
               refcount: 0,
           }
       });
       target.refcount += 1;
       target.rbo_id
   }

   fn release_depth_target(&mut self, dimensions: DeviceIntSize) {
       let mut entry = match self.depth_targets.entry(dimensions) {
           Entry::Occupied(x) => x,
           Entry::Vacant(..) => panic!("Releasing unknown depth target"),
       };
       debug_assert!(entry.get().refcount != 0);
       entry.get_mut().refcount -= 1;
       if entry.get().refcount == 0 {
           let (_, target) = entry.remove_entry();
           self.gl.delete_renderbuffers(&[target.rbo_id.0]);
       }
   }

   /// Perform a blit between self.bound_read_fbo and self.bound_draw_fbo.
   fn blit_render_target_impl(
       &mut self,
       src_rect: FramebufferIntRect,
       dest_rect: FramebufferIntRect,
       filter: TextureFilter,
   ) {
       debug_assert!(self.inside_frame);

       let filter = match filter {
           TextureFilter::Nearest => gl::NEAREST,
           TextureFilter::Linear | TextureFilter::Trilinear => gl::LINEAR,
       };

       let src_x0 = src_rect.min.x + self.bound_read_fbo.1.x;
       let src_y0 = src_rect.min.y + self.bound_read_fbo.1.y;

       self.gl.blit_framebuffer(
           src_x0,
           src_y0,
           src_x0 + src_rect.width(),
           src_y0 + src_rect.height(),
           dest_rect.min.x,
           dest_rect.min.y,
           dest_rect.max.x,
           dest_rect.max.y,
           gl::COLOR_BUFFER_BIT,
           filter,
       );
   }

   /// Perform a blit between src_target and dest_target.
   /// This will overwrite self.bound_read_fbo and self.bound_draw_fbo.
   pub fn blit_render_target(
       &mut self,
       src_target: ReadTarget,
       src_rect: FramebufferIntRect,
       dest_target: DrawTarget,
       dest_rect: FramebufferIntRect,
       filter: TextureFilter,
   ) {
       debug_assert!(self.inside_frame);

       self.bind_read_target(src_target);

       self.bind_draw_target(dest_target);

       self.blit_render_target_impl(src_rect, dest_rect, filter);
   }

   /// Performs a blit while flipping vertically. Useful for blitting textures
   /// (which use origin-bottom-left) to the main framebuffer (which uses
   /// origin-top-left).
   pub fn blit_render_target_invert_y(
       &mut self,
       src_target: ReadTarget,
       src_rect: FramebufferIntRect,
       dest_target: DrawTarget,
       dest_rect: FramebufferIntRect,
   ) {
       debug_assert!(self.inside_frame);

       let mut inverted_dest_rect = dest_rect;
       inverted_dest_rect.min.y = dest_rect.max.y;
       inverted_dest_rect.max.y = dest_rect.min.y;

       self.blit_render_target(
           src_target,
           src_rect,
           dest_target,
           inverted_dest_rect,
           TextureFilter::Linear,
       );
   }

   pub fn delete_texture(&mut self, mut texture: Texture) {
       debug_assert!(self.inside_frame);
       let had_depth = texture.supports_depth();
       if let Some(fbo) = texture.fbo {
           self.gl.delete_framebuffers(&[fbo.0]);
           texture.fbo = None;
       }
       if let Some(fbo) = texture.fbo_with_depth {
           self.gl.delete_framebuffers(&[fbo.0]);
           texture.fbo_with_depth = None;
       }

       if had_depth {
           self.release_depth_target(texture.get_dimensions());
       }

       self.gl.delete_textures(&[texture.id]);

       for bound_texture in &mut self.bound_textures {
           if *bound_texture == texture.id {
               *bound_texture = 0;
           }
       }

       self.textures_deleted += 1;

       // Disarm the assert in Texture::drop().
       texture.id = 0;
   }

   #[cfg(feature = "replay")]
   pub fn delete_external_texture(&mut self, mut external: ExternalTexture) {
       self.gl.delete_textures(&[external.id]);
       external.id = 0;
   }

   pub fn delete_program(&mut self, mut program: Program) {
       self.gl.delete_program(program.id);
       program.id = 0;
   }

   /// Create a shader program and link it immediately.
   pub fn create_program_linked(
       &mut self,
       base_filename: &'static str,
       features: &[&'static str],
       descriptor: &VertexDescriptor,
   ) -> Result<Program, ShaderError> {
       let mut program = self.create_program(base_filename, features)?;
       self.link_program(&mut program, descriptor)?;
       Ok(program)
   }

   /// Create a shader program. This does minimal amount of work to start
   /// loading a binary shader. If a binary shader is found, we invoke
   /// glProgramBinary, which, at least on ANGLE, will load and link the
   /// binary on a background thread. This can speed things up later when
   /// we invoke `link_program()`.
   pub fn create_program(
       &mut self,
       base_filename: &'static str,
       features: &[&'static str],
   ) -> Result<Program, ShaderError> {
       debug_assert!(self.inside_frame);

       let source_info = ProgramSourceInfo::new(self, base_filename, features);

       // Create program
       let pid = self.gl.create_program();

       // Attempt to load a cached binary if possible.
       if let Some(ref cached_programs) = self.cached_programs {
           if let Some(entry) = cached_programs.entries.borrow().get(&source_info.digest) {
               self.gl.program_binary(pid, entry.binary.format, &entry.binary.bytes);
           }
       }

       // Use 0 for the uniforms as they are initialized by link_program.
       let program = Program {
           id: pid,
           u_transform: 0,
           u_texture_size: 0,
           source_info,
           is_initialized: false,
       };

       Ok(program)
   }

   fn build_shader_string<F: FnMut(&str)>(
       &self,
       features: &[&'static str],
       kind: ShaderKind,
       base_filename: &str,
       output: F,
   ) {
       do_build_shader_string(
           get_shader_version(&*self.gl),
           features,
           kind,
           base_filename,
           &|f| get_unoptimized_shader_source(f, self.resource_override_path.as_ref()),
           output,
       )
   }

   pub fn bind_shader_samplers<S>(&mut self, program: &Program, bindings: &[(&'static str, S)])
   where
       S: Into<TextureSlot> + Copy,
   {
       // bind_program() must be called before calling bind_shader_samplers
       assert_eq!(self.bound_program, program.id);

       for binding in bindings {
           let u_location = self.gl.get_uniform_location(program.id, binding.0);
           if u_location != -1 {
               self.bind_program(program);
               self.gl
                   .uniform_1i(u_location, binding.1.into().0 as gl::GLint);
           }
       }
   }

   pub fn get_uniform_location(&self, program: &Program, name: &str) -> UniformLocation {
       UniformLocation(self.gl.get_uniform_location(program.id, name))
   }

   pub fn set_uniforms(
       &self,
       program: &Program,
       transform: &Transform3D<f32>,
   ) {
       debug_assert!(self.inside_frame);
       #[cfg(debug_assertions)]
       debug_assert!(self.shader_is_ready);

       self.gl
           .uniform_matrix_4fv(program.u_transform, false, &transform.to_array());
   }

   /// Sets the uTextureSize uniform. Most shaders do not require this to be called
   /// as they use the textureSize GLSL function instead.
   pub fn set_shader_texture_size(
       &self,
       program: &Program,
       texture_size: DeviceSize,
   ) {
       debug_assert!(self.inside_frame);
       #[cfg(debug_assertions)]
       debug_assert!(self.shader_is_ready);

       if program.u_texture_size != -1 {
           self.gl.uniform_2f(program.u_texture_size, texture_size.width, texture_size.height);
       }
   }

   pub fn create_pbo(&mut self) -> PBO {
       let id = self.gl.gen_buffers(1)[0];
       PBO {
           id,
           reserved_size: 0,
       }
   }

   pub fn create_pbo_with_size(&mut self, size: usize) -> PBO {
       let mut pbo = self.create_pbo();

       self.gl.bind_buffer(gl::PIXEL_PACK_BUFFER, pbo.id);
       self.gl.pixel_store_i(gl::PACK_ALIGNMENT, 1);
       self.gl.buffer_data_untyped(
           gl::PIXEL_PACK_BUFFER,
           size as _,
           ptr::null(),
           gl::STREAM_READ,
       );
       self.gl.bind_buffer(gl::PIXEL_UNPACK_BUFFER, 0);

       pbo.reserved_size = size;
       pbo
   }

   pub fn read_pixels_into_pbo(
       &mut self,
       read_target: ReadTarget,
       rect: DeviceIntRect,
       format: ImageFormat,
       pbo: &PBO,
   ) {
       let byte_size = rect.area() as usize * format.bytes_per_pixel() as usize;

       assert!(byte_size <= pbo.reserved_size);

       self.bind_read_target(read_target);

       self.gl.bind_buffer(gl::PIXEL_PACK_BUFFER, pbo.id);
       self.gl.pixel_store_i(gl::PACK_ALIGNMENT, 1);

       let gl_format = self.gl_describe_format(format);

       unsafe {
           self.gl.read_pixels_into_pbo(
               rect.min.x as _,
               rect.min.y as _,
               rect.width() as _,
               rect.height() as _,
               gl_format.read,
               gl_format.pixel_type,
           );
       }

       self.gl.bind_buffer(gl::PIXEL_PACK_BUFFER, 0);
   }

   pub fn map_pbo_for_readback<'a>(&'a mut self, pbo: &'a PBO) -> Option<BoundPBO<'a>> {
       self.gl.bind_buffer(gl::PIXEL_PACK_BUFFER, pbo.id);

       let buf_ptr = match self.gl.get_type() {
           gl::GlType::Gl => {
               self.gl.map_buffer(gl::PIXEL_PACK_BUFFER, gl::READ_ONLY)
           }

           gl::GlType::Gles => {
               self.gl.map_buffer_range(
                   gl::PIXEL_PACK_BUFFER,
                   0,
                   pbo.reserved_size as _,
                   gl::MAP_READ_BIT)
           }
       };

       if buf_ptr.is_null() {
           return None;
       }

       let buffer = unsafe { slice::from_raw_parts(buf_ptr as *const u8, pbo.reserved_size) };

       Some(BoundPBO {
           device: self,
           data: buffer,
       })
   }

   pub fn delete_pbo(&mut self, mut pbo: PBO) {
       self.gl.delete_buffers(&[pbo.id]);
       pbo.id = 0;
       pbo.reserved_size = 0
   }

   /// Returns the size and stride in bytes required to upload an area of pixels
   /// of the specified size, to a texture of the specified format.
   pub fn required_upload_size_and_stride(&self, size: DeviceIntSize, format: ImageFormat) -> (usize, usize) {
       assert!(size.width >= 0);
       assert!(size.height >= 0);

       let bytes_pp = format.bytes_per_pixel() as usize;
       let width_bytes = size.width as usize * bytes_pp;

       let dst_stride = round_up_to_multiple(width_bytes, self.required_pbo_stride.num_bytes(format));

       // The size of the chunk should only need to be (height - 1) * dst_stride + width_bytes,
       // however, the android emulator will error unless it is height * dst_stride.
       // See bug 1587047 for details.
       // Using the full final row also ensures that the offset of the next chunk is
       // optimally aligned.
       let dst_size = dst_stride * size.height as usize;

       (dst_size, dst_stride)
   }

   /// Returns a `TextureUploader` which can be used to upload texture data to `texture`.
   /// Once uploads have been performed the uploader must be flushed with `TextureUploader::flush()`.
   pub fn upload_texture<'a>(
       &mut self,
       pbo_pool: &'a mut UploadPBOPool,
   ) -> TextureUploader<'a> {
       debug_assert!(self.inside_frame);

       pbo_pool.begin_frame(self);

       TextureUploader {
           buffers: Vec::new(),
           pbo_pool,
       }
   }

   /// Performs an immediate (non-PBO) texture upload.
   pub fn upload_texture_immediate<T: Texel>(
       &mut self,
       texture: &Texture,
       pixels: &[T]
   ) {
       self.bind_texture(DEFAULT_TEXTURE, texture, Swizzle::default());
       let desc = self.gl_describe_format(texture.format);
       self.gl.tex_sub_image_2d(
           texture.target,
           0,
           0,
           0,
           texture.size.width as gl::GLint,
           texture.size.height as gl::GLint,
           desc.external,
           desc.pixel_type,
           texels_to_u8_slice(pixels),
       );
   }

   pub fn read_pixels(&mut self, img_desc: &ImageDescriptor) -> Vec<u8> {
       let desc = self.gl_describe_format(img_desc.format);
       self.gl.read_pixels(
           0, 0,
           img_desc.size.width as i32,
           img_desc.size.height as i32,
           desc.read,
           desc.pixel_type,
       )
   }

   /// Read rectangle of pixels into the specified output slice.
   pub fn read_pixels_into(
       &mut self,
       rect: FramebufferIntRect,
       format: ImageFormat,
       output: &mut [u8],
   ) {
       let bytes_per_pixel = format.bytes_per_pixel();
       let desc = self.gl_describe_format(format);
       let size_in_bytes = (bytes_per_pixel * rect.area()) as usize;
       assert_eq!(output.len(), size_in_bytes);

       self.gl.flush();
       self.gl.read_pixels_into_buffer(
           rect.min.x as _,
           rect.min.y as _,
           rect.width() as _,
           rect.height() as _,
           desc.read,
           desc.pixel_type,
           output,
       );
   }

   /// Get texels of a texture into the specified output slice.
   pub fn get_tex_image_into(
       &mut self,
       texture: &Texture,
       format: ImageFormat,
       output: &mut [u8],
   ) {
       self.bind_texture(DEFAULT_TEXTURE, texture, Swizzle::default());
       let desc = self.gl_describe_format(format);
       self.gl.get_tex_image_into_buffer(
           texture.target,
           0,
           desc.external,
           desc.pixel_type,
           output,
       );
   }

   /// Attaches the provided texture to the current Read FBO binding.
   fn attach_read_texture_raw(&mut self, texture_id: gl::GLuint, target: gl::GLuint) {
       self.gl.framebuffer_texture_2d(
           gl::READ_FRAMEBUFFER,
           gl::COLOR_ATTACHMENT0,
           target,
           texture_id,
           0,
       )
   }

   pub fn attach_read_texture_external(
       &mut self, texture_id: gl::GLuint, target: ImageBufferKind
   ) {
       self.attach_read_texture_raw(texture_id, get_gl_target(target))
   }

   pub fn attach_read_texture(&mut self, texture: &Texture) {
       self.attach_read_texture_raw(texture.id, texture.target)
   }

   fn bind_vao_impl(&mut self, id: gl::GLuint) {
       debug_assert!(self.inside_frame);

       if self.bound_vao != id {
           self.bound_vao = id;
           self.gl.bind_vertex_array(id);
       }
   }

   pub fn bind_vao(&mut self, vao: &VAO) {
       self.bind_vao_impl(vao.id)
   }

   pub fn bind_custom_vao(&mut self, vao: &CustomVAO) {
       self.bind_vao_impl(vao.id)
   }

   fn create_vao_with_vbos(
       &mut self,
       descriptor: &VertexDescriptor,
       main_vbo_id: VBOId,
       instance_vbo_id: VBOId,
       instance_divisor: u32,
       ibo_id: IBOId,
       owns_vertices_and_indices: bool,
   ) -> VAO {
       let instance_stride = descriptor.instance_stride() as usize;
       let vao_id = self.gl.gen_vertex_arrays(1)[0];

       self.bind_vao_impl(vao_id);

       descriptor.bind(self.gl(), main_vbo_id, instance_vbo_id, instance_divisor);
       ibo_id.bind(self.gl()); // force it to be a part of VAO

       VAO {
           id: vao_id,
           ibo_id,
           main_vbo_id,
           instance_vbo_id,
           instance_stride,
           instance_divisor,
           owns_vertices_and_indices,
       }
   }

   pub fn create_custom_vao(
       &mut self,
       streams: &[Stream],
   ) -> CustomVAO {
       debug_assert!(self.inside_frame);

       let vao_id = self.gl.gen_vertex_arrays(1)[0];
       self.bind_vao_impl(vao_id);

       let mut attrib_index = 0;
       for stream in streams {
           VertexDescriptor::bind_attributes(
               stream.attributes,
               attrib_index,
               0,
               self.gl(),
               stream.vbo,
           );
           attrib_index += stream.attributes.len();
       }

       CustomVAO {
           id: vao_id,
       }
   }

   pub fn delete_custom_vao(&mut self, mut vao: CustomVAO) {
       self.gl.delete_vertex_arrays(&[vao.id]);
       vao.id = 0;
   }

   pub fn create_vbo<T>(&mut self) -> VBO<T> {
       let ids = self.gl.gen_buffers(1);
       VBO {
           id: ids[0],
           target: gl::ARRAY_BUFFER,
           allocated_count: 0,
           marker: PhantomData,
       }
   }

   pub fn delete_vbo<T>(&mut self, mut vbo: VBO<T>) {
       self.gl.delete_buffers(&[vbo.id]);
       vbo.id = 0;
   }

   pub fn create_vao(&mut self, descriptor: &VertexDescriptor, instance_divisor: u32) -> VAO {
       debug_assert!(self.inside_frame);

       let buffer_ids = self.gl.gen_buffers(3);
       let ibo_id = IBOId(buffer_ids[0]);
       let main_vbo_id = VBOId(buffer_ids[1]);
       let intance_vbo_id = VBOId(buffer_ids[2]);

       self.create_vao_with_vbos(descriptor, main_vbo_id, intance_vbo_id, instance_divisor, ibo_id, true)
   }

   pub fn delete_vao(&mut self, mut vao: VAO) {
       self.gl.delete_vertex_arrays(&[vao.id]);
       vao.id = 0;

       if vao.owns_vertices_and_indices {
           self.gl.delete_buffers(&[vao.ibo_id.0]);
           self.gl.delete_buffers(&[vao.main_vbo_id.0]);
       }

       self.gl.delete_buffers(&[vao.instance_vbo_id.0])
   }

   pub fn allocate_vbo<V>(
       &mut self,
       vbo: &mut VBO<V>,
       count: usize,
       usage_hint: VertexUsageHint,
   ) {
       debug_assert!(self.inside_frame);
       vbo.allocated_count = count;

       self.gl.bind_buffer(vbo.target, vbo.id);
       self.gl.buffer_data_untyped(
           vbo.target,
           (count * mem::size_of::<V>()) as _,
           ptr::null(),
           usage_hint.to_gl(),
       );
   }

   pub fn fill_vbo<V>(
       &mut self,
       vbo: &VBO<V>,
       data: &[V],
       offset: usize,
   ) {
       debug_assert!(self.inside_frame);
       assert!(offset + data.len() <= vbo.allocated_count);
       let stride = mem::size_of::<V>();

       self.gl.bind_buffer(vbo.target, vbo.id);
       self.gl.buffer_sub_data_untyped(
           vbo.target,
           (offset * stride) as _,
           (data.len() * stride) as _,
           data.as_ptr() as _,
       );
   }

   fn update_vbo_data<V>(
       &mut self,
       vbo: VBOId,
       vertices: &[V],
       usage_hint: VertexUsageHint,
   ) {
       debug_assert!(self.inside_frame);

       vbo.bind(self.gl());
       gl::buffer_data(self.gl(), gl::ARRAY_BUFFER, vertices, usage_hint.to_gl());
   }

   pub fn create_vao_with_new_instances(
       &mut self,
       descriptor: &VertexDescriptor,
       base_vao: &VAO,
   ) -> VAO {
       debug_assert!(self.inside_frame);

       let buffer_ids = self.gl.gen_buffers(1);
       let intance_vbo_id = VBOId(buffer_ids[0]);

       self.create_vao_with_vbos(
           descriptor,
           base_vao.main_vbo_id,
           intance_vbo_id,
           base_vao.instance_divisor,
           base_vao.ibo_id,
           false,
       )
   }

   pub fn update_vao_main_vertices<V>(
       &mut self,
       vao: &VAO,
       vertices: &[V],
       usage_hint: VertexUsageHint,
   ) {
       debug_assert_eq!(self.bound_vao, vao.id);
       self.update_vbo_data(vao.main_vbo_id, vertices, usage_hint)
   }

   pub fn update_vao_instances<V: Clone>(
       &mut self,
       vao: &VAO,
       instances: &[V],
       usage_hint: VertexUsageHint,
       // if `Some(count)`, each instance is repeated `count` times
       repeat: Option<NonZeroUsize>,
   ) {
       debug_assert_eq!(self.bound_vao, vao.id);
       debug_assert_eq!(vao.instance_stride as usize, mem::size_of::<V>());

       match repeat {
           Some(count) => {
               let target = gl::ARRAY_BUFFER;
               self.gl.bind_buffer(target, vao.instance_vbo_id.0);
               let size = instances.len() * count.get() * mem::size_of::<V>();
               self.gl.buffer_data_untyped(
                   target,
                   size as _,
                   ptr::null(),
                   usage_hint.to_gl(),
               );

               let ptr = match self.gl.get_type() {
                   gl::GlType::Gl => {
                       self.gl.map_buffer(target, gl::WRITE_ONLY)
                   }
                   gl::GlType::Gles => {
                       self.gl.map_buffer_range(target, 0, size as _, gl::MAP_WRITE_BIT)
                   }
               };
               assert!(!ptr.is_null());

               let buffer_slice = unsafe {
                   slice::from_raw_parts_mut(ptr as *mut V, instances.len() * count.get())
               };
               for (quad, instance) in buffer_slice.chunks_mut(4).zip(instances) {
                   quad[0] = instance.clone();
                   quad[1] = instance.clone();
                   quad[2] = instance.clone();
                   quad[3] = instance.clone();
               }
               self.gl.unmap_buffer(target);
           }
           None => {
               self.update_vbo_data(vao.instance_vbo_id, instances, usage_hint);
           }
       }

       // On some devices the VAO must be manually unbound and rebound after an attached buffer has
       // been orphaned. Failure to do so appeared to result in the orphaned buffer's contents
       // being used for the subsequent draw call, rather than the new buffer's contents.
       if self.capabilities.requires_vao_rebind_after_orphaning {
           self.bind_vao_impl(0);
           self.bind_vao_impl(vao.id);
       }
   }

   pub fn update_vao_indices<I>(&mut self, vao: &VAO, indices: &[I], usage_hint: VertexUsageHint) {
       debug_assert!(self.inside_frame);
       debug_assert_eq!(self.bound_vao, vao.id);

       vao.ibo_id.bind(self.gl());
       gl::buffer_data(
           self.gl(),
           gl::ELEMENT_ARRAY_BUFFER,
           indices,
           usage_hint.to_gl(),
       );
   }

   pub fn draw_triangles_u16(&mut self, first_vertex: i32, index_count: i32) {
       debug_assert!(self.inside_frame);
       #[cfg(debug_assertions)]
       debug_assert!(self.shader_is_ready);

       let _guard = if self.annotate_draw_call_crashes {
           Some(CrashAnnotatorGuard::new(
               &self.crash_annotator,
               CrashAnnotation::DrawShader,
               &self.bound_program_name,
           ))
       } else {
           None
       };

       self.gl.draw_elements(
           gl::TRIANGLES,
           index_count,
           gl::UNSIGNED_SHORT,
           first_vertex as u32 * 2,
       );
   }

   pub fn draw_triangles_u32(&mut self, first_vertex: i32, index_count: i32) {
       debug_assert!(self.inside_frame);
       #[cfg(debug_assertions)]
       debug_assert!(self.shader_is_ready);

       let _guard = if self.annotate_draw_call_crashes {
           Some(CrashAnnotatorGuard::new(
               &self.crash_annotator,
               CrashAnnotation::DrawShader,
               &self.bound_program_name,
           ))
       } else {
           None
       };

       self.gl.draw_elements(
           gl::TRIANGLES,
           index_count,
           gl::UNSIGNED_INT,
           first_vertex as u32 * 4,
       );
   }

   pub fn draw_nonindexed_points(&mut self, first_vertex: i32, vertex_count: i32) {
       debug_assert!(self.inside_frame);
       #[cfg(debug_assertions)]
       debug_assert!(self.shader_is_ready);

       let _guard = if self.annotate_draw_call_crashes {
           Some(CrashAnnotatorGuard::new(
               &self.crash_annotator,
               CrashAnnotation::DrawShader,
               &self.bound_program_name,
           ))
       } else {
           None
       };

       self.gl.draw_arrays(gl::POINTS, first_vertex, vertex_count);
   }

   pub fn draw_nonindexed_lines(&mut self, first_vertex: i32, vertex_count: i32) {
       debug_assert!(self.inside_frame);
       #[cfg(debug_assertions)]
       debug_assert!(self.shader_is_ready);

       let _guard = if self.annotate_draw_call_crashes {
           Some(CrashAnnotatorGuard::new(
               &self.crash_annotator,
               CrashAnnotation::DrawShader,
               &self.bound_program_name,
           ))
       } else {
           None
       };

       self.gl.draw_arrays(gl::LINES, first_vertex, vertex_count);
   }

   pub fn draw_indexed_triangles(&mut self, index_count: i32) {
       debug_assert!(self.inside_frame);
       #[cfg(debug_assertions)]
       debug_assert!(self.shader_is_ready);

       let _guard = if self.annotate_draw_call_crashes {
           Some(CrashAnnotatorGuard::new(
               &self.crash_annotator,
               CrashAnnotation::DrawShader,
               &self.bound_program_name,
           ))
       } else {
           None
       };

       self.gl.draw_elements(
           gl::TRIANGLES,
           index_count,
           gl::UNSIGNED_SHORT,
           0,
       );
   }

   pub fn draw_indexed_triangles_instanced_u16(&mut self, index_count: i32, instance_count: i32) {
       debug_assert!(self.inside_frame);
       #[cfg(debug_assertions)]
       debug_assert!(self.shader_is_ready);

       let _guard = if self.annotate_draw_call_crashes {
           Some(CrashAnnotatorGuard::new(
               &self.crash_annotator,
               CrashAnnotation::DrawShader,
               &self.bound_program_name,
           ))
       } else {
           None
       };

       self.gl.draw_elements_instanced(
           gl::TRIANGLES,
           index_count,
           gl::UNSIGNED_SHORT,
           0,
           instance_count,
       );
   }

   pub fn end_frame(&mut self) {
       self.reset_draw_target();
       self.reset_read_target();

       debug_assert!(self.inside_frame);
       self.inside_frame = false;

       self.gl.bind_texture(gl::TEXTURE_2D, 0);
       self.gl.use_program(0);

       for i in 0 .. self.bound_textures.len() {
           self.gl.active_texture(gl::TEXTURE0 + i as gl::GLuint);
           self.gl.bind_texture(gl::TEXTURE_2D, 0);
       }

       self.gl.active_texture(gl::TEXTURE0);

       self.frame_id.0 += 1;

       // Save any shaders compiled this frame to disk.
       // If this is the tenth frame then treat startup as complete, meaning the
       // current set of in-use shaders are the ones to load on the next startup.
       if let Some(ref cache) = self.cached_programs {
           cache.update_disk_cache(self.frame_id.0 == 10);
       }
   }

   pub fn clear_target(
       &self,
       color: Option<[f32; 4]>,
       depth: Option<f32>,
       rect: Option<FramebufferIntRect>,
   ) {
       let mut clear_bits = 0;

       if let Some(color) = color {
           self.gl.clear_color(color[0], color[1], color[2], color[3]);
           clear_bits |= gl::COLOR_BUFFER_BIT;
       }

       if let Some(depth) = depth {
           if cfg!(debug_assertions) {
               let mut mask = [0];
               unsafe {
                   self.gl.get_boolean_v(gl::DEPTH_WRITEMASK, &mut mask);
               }
               assert_ne!(mask[0], 0);
           }
           self.gl.clear_depth(depth as f64);
           clear_bits |= gl::DEPTH_BUFFER_BIT;
       }

       if clear_bits != 0 {
           match rect {
               Some(rect) => {
                   self.gl.enable(gl::SCISSOR_TEST);
                   self.gl.scissor(
                       rect.min.x,
                       rect.min.y,
                       rect.width(),
                       rect.height(),
                   );
                   self.gl.clear(clear_bits);
                   self.gl.disable(gl::SCISSOR_TEST);
               }
               None => {
                   self.gl.clear(clear_bits);
               }
           }
       }
   }

   pub fn enable_depth(&self, depth_func: DepthFunction) {
       assert!(self.depth_available, "Enabling depth test without depth target");
       self.gl.enable(gl::DEPTH_TEST);
       self.gl.depth_func(depth_func as gl::GLuint);
   }

   pub fn disable_depth(&self) {
       self.gl.disable(gl::DEPTH_TEST);
   }

   pub fn enable_depth_write(&self) {
       assert!(self.depth_available, "Enabling depth write without depth target");
       self.gl.depth_mask(true);
   }

   pub fn disable_depth_write(&self) {
       self.gl.depth_mask(false);
   }

   pub fn disable_stencil(&self) {
       self.gl.disable(gl::STENCIL_TEST);
   }

   pub fn set_scissor_rect(&self, rect: FramebufferIntRect) {
       self.gl.scissor(
           rect.min.x,
           rect.min.y,
           rect.width(),
           rect.height(),
       );
   }

   pub fn enable_scissor(&self) {
       self.gl.enable(gl::SCISSOR_TEST);
   }

   pub fn disable_scissor(&self) {
       self.gl.disable(gl::SCISSOR_TEST);
   }

   pub fn enable_color_write(&self) {
       self.gl.color_mask(true, true, true, true);
   }

   pub fn disable_color_write(&self) {
       self.gl.color_mask(false, false, false, false);
   }

   pub fn set_blend(&mut self, enable: bool) {
       if enable {
           self.gl.enable(gl::BLEND);
       } else {
           self.gl.disable(gl::BLEND);
       }
       #[cfg(debug_assertions)]
       {
           self.shader_is_ready = false;
       }
   }

   fn set_blend_factors(
       &mut self,
       color: (gl::GLenum, gl::GLenum),
       alpha: (gl::GLenum, gl::GLenum),
   ) {
       self.gl.blend_equation(gl::FUNC_ADD);
       if color == alpha {
           self.gl.blend_func(color.0, color.1);
       } else {
           self.gl.blend_func_separate(color.0, color.1, alpha.0, alpha.1);
       }
       #[cfg(debug_assertions)]
       {
           self.shader_is_ready = false;
       }
   }

   pub fn set_blend_mode_alpha(&mut self) {
       self.set_blend_factors(
           (gl::SRC_ALPHA, gl::ONE_MINUS_SRC_ALPHA),
           (gl::ONE, gl::ONE_MINUS_SRC_ALPHA),
       );
   }

   pub fn set_blend_mode_premultiplied_alpha(&mut self) {
       self.set_blend_factors(
           (gl::ONE, gl::ONE_MINUS_SRC_ALPHA),
           (gl::ONE, gl::ONE_MINUS_SRC_ALPHA),
       );
   }

   pub fn set_blend_mode_premultiplied_dest_out(&mut self) {
       self.set_blend_factors(
           (gl::ZERO, gl::ONE_MINUS_SRC_ALPHA),
           (gl::ZERO, gl::ONE_MINUS_SRC_ALPHA),
       );
   }

   pub fn set_blend_mode_multiply(&mut self) {
       self.set_blend_factors(
           (gl::ZERO, gl::SRC_COLOR),
           (gl::ZERO, gl::SRC_ALPHA),
       );
   }
   pub fn set_blend_mode_subpixel_pass0(&mut self) {
       self.set_blend_factors(
           (gl::ZERO, gl::ONE_MINUS_SRC_COLOR),
           (gl::ZERO, gl::ONE_MINUS_SRC_ALPHA),
       );
   }
   pub fn set_blend_mode_subpixel_pass1(&mut self) {
       self.set_blend_factors(
           (gl::ONE, gl::ONE),
           (gl::ONE, gl::ONE),
       );
   }
   pub fn set_blend_mode_subpixel_dual_source(&mut self) {
       self.set_blend_factors(
           (gl::ONE, gl::ONE_MINUS_SRC1_COLOR),
           (gl::ONE, gl::ONE_MINUS_SRC1_ALPHA),
       );
   }
   pub fn set_blend_mode_multiply_dual_source(&mut self) {
       self.set_blend_factors(
           (gl::ONE_MINUS_DST_ALPHA, gl::ONE_MINUS_SRC1_COLOR),
           (gl::ONE, gl::ONE_MINUS_SRC_ALPHA),
       );
   }
   pub fn set_blend_mode_screen(&mut self) {
       self.set_blend_factors(
           (gl::ONE, gl::ONE_MINUS_SRC_COLOR),
           (gl::ONE, gl::ONE_MINUS_SRC_ALPHA),
       );
   }
   pub fn set_blend_mode_plus_lighter(&mut self) {
       self.set_blend_factors(
           (gl::ONE, gl::ONE),
           (gl::ONE, gl::ONE),
       );
   }
   pub fn set_blend_mode_exclusion(&mut self) {
       self.set_blend_factors(
           (gl::ONE_MINUS_DST_COLOR, gl::ONE_MINUS_SRC_COLOR),
           (gl::ONE, gl::ONE_MINUS_SRC_ALPHA),
       );
   }
   pub fn set_blend_mode_show_overdraw(&mut self) {
       self.set_blend_factors(
           (gl::ONE, gl::ONE_MINUS_SRC_ALPHA),
           (gl::ONE, gl::ONE_MINUS_SRC_ALPHA),
       );
   }

   pub fn set_blend_mode_max(&mut self) {
       self.gl
           .blend_func_separate(gl::ONE, gl::ONE, gl::ONE, gl::ONE);
       self.gl.blend_equation_separate(gl::MAX, gl::FUNC_ADD);
       #[cfg(debug_assertions)]
       {
           self.shader_is_ready = false;
       }
   }
   pub fn set_blend_mode_min(&mut self) {
       self.gl
           .blend_func_separate(gl::ONE, gl::ONE, gl::ONE, gl::ONE);
       self.gl.blend_equation_separate(gl::MIN, gl::FUNC_ADD);
       #[cfg(debug_assertions)]
       {
           self.shader_is_ready = false;
       }
   }
   pub fn set_blend_mode_advanced(&mut self, mode: MixBlendMode) {
       self.gl.blend_equation(match mode {
           MixBlendMode::Normal => {
               // blend factor only make sense for the normal mode
               self.gl.blend_func_separate(gl::ZERO, gl::SRC_COLOR, gl::ZERO, gl::SRC_ALPHA);
               gl::FUNC_ADD
           },
           MixBlendMode::PlusLighter => {
               return self.set_blend_mode_plus_lighter();
           },
           MixBlendMode::Multiply => gl::MULTIPLY_KHR,
           MixBlendMode::Screen => gl::SCREEN_KHR,
           MixBlendMode::Overlay => gl::OVERLAY_KHR,
           MixBlendMode::Darken => gl::DARKEN_KHR,
           MixBlendMode::Lighten => gl::LIGHTEN_KHR,
           MixBlendMode::ColorDodge => gl::COLORDODGE_KHR,
           MixBlendMode::ColorBurn => gl::COLORBURN_KHR,
           MixBlendMode::HardLight => gl::HARDLIGHT_KHR,
           MixBlendMode::SoftLight => gl::SOFTLIGHT_KHR,
           MixBlendMode::Difference => gl::DIFFERENCE_KHR,
           MixBlendMode::Exclusion => gl::EXCLUSION_KHR,
           MixBlendMode::Hue => gl::HSL_HUE_KHR,
           MixBlendMode::Saturation => gl::HSL_SATURATION_KHR,
           MixBlendMode::Color => gl::HSL_COLOR_KHR,
           MixBlendMode::Luminosity => gl::HSL_LUMINOSITY_KHR,
       });
       #[cfg(debug_assertions)]
       {
           self.shader_is_ready = false;
       }
   }

   pub fn supports_extension(&self, extension: &str) -> bool {
       supports_extension(&self.extensions, extension)
   }

   pub fn echo_driver_messages(&self) {
       if self.capabilities.supports_khr_debug {
           Device::log_driver_messages(self.gl());
       }
   }

   fn log_driver_messages(gl: &dyn gl::Gl) {
       for msg in gl.get_debug_messages() {
           let level = match msg.severity {
               gl::DEBUG_SEVERITY_HIGH => Level::Error,
               gl::DEBUG_SEVERITY_MEDIUM => Level::Warn,
               gl::DEBUG_SEVERITY_LOW => Level::Info,
               gl::DEBUG_SEVERITY_NOTIFICATION => Level::Debug,
               _ => Level::Trace,
           };
           let ty = match msg.ty {
               gl::DEBUG_TYPE_ERROR => "error",
               gl::DEBUG_TYPE_DEPRECATED_BEHAVIOR => "deprecated",
               gl::DEBUG_TYPE_UNDEFINED_BEHAVIOR => "undefined",
               gl::DEBUG_TYPE_PORTABILITY => "portability",
               gl::DEBUG_TYPE_PERFORMANCE => "perf",
               gl::DEBUG_TYPE_MARKER => "marker",
               gl::DEBUG_TYPE_PUSH_GROUP => "group push",
               gl::DEBUG_TYPE_POP_GROUP => "group pop",
               gl::DEBUG_TYPE_OTHER => "other",
               _ => "?",
           };
           log!(level, "({}) {}", ty, msg.message);
       }
   }

   pub fn gl_describe_format(&self, format: ImageFormat) -> FormatDesc {
       match format {
           ImageFormat::R8 => FormatDesc {
               internal: gl::R8,
               external: gl::RED,
               read: gl::RED,
               pixel_type: gl::UNSIGNED_BYTE,
           },
           ImageFormat::R16 => FormatDesc {
               internal: gl::R16,
               external: gl::RED,
               read: gl::RED,
               pixel_type: gl::UNSIGNED_SHORT,
           },
           ImageFormat::BGRA8 => {
               FormatDesc {
                   internal: self.bgra_formats.internal,
                   external: self.bgra_formats.external,
                   read: gl::BGRA,
                   pixel_type: self.bgra_pixel_type,
               }
           },
           ImageFormat::RGBA8 => {
               FormatDesc {
                   internal: gl::RGBA8,
                   external: gl::RGBA,
                   read: gl::RGBA,
                   pixel_type: gl::UNSIGNED_BYTE,
               }
           },
           ImageFormat::RGBAF32 => FormatDesc {
               internal: gl::RGBA32F,
               external: gl::RGBA,
               read: gl::RGBA,
               pixel_type: gl::FLOAT,
           },
           ImageFormat::RGBAI32 => FormatDesc {
               internal: gl::RGBA32I,
               external: gl::RGBA_INTEGER,
               read: gl::RGBA_INTEGER,
               pixel_type: gl::INT,
           },
           ImageFormat::RG8 => FormatDesc {
               internal: gl::RG8,
               external: gl::RG,
               read: gl::RG,
               pixel_type: gl::UNSIGNED_BYTE,
           },
           ImageFormat::RG16 => FormatDesc {
               internal: gl::RG16,
               external: gl::RG,
               read: gl::RG,
               pixel_type: gl::UNSIGNED_SHORT,
           },
       }
   }

   /// Generates a memory report for the resources managed by the device layer.
   pub fn report_memory(&self, size_op_funs: &MallocSizeOfOps, swgl: *mut c_void) -> MemoryReport {
       let mut report = MemoryReport::default();
       report.depth_target_textures += self.depth_targets_memory();

       #[cfg(feature = "sw_compositor")]
       if !swgl.is_null() {
           report.swgl += swgl::Context::from(swgl).report_memory(size_op_funs.size_of_op);
       }
       // unconditionally use swgl stuff
       let _ = size_op_funs;
       let _ = swgl;
       report
   }

   pub fn depth_targets_memory(&self) -> usize {
       let mut total = 0;
       for dim in self.depth_targets.keys() {
           total += depth_target_size_in_bytes(dim);
       }

       total
   }
}

pub struct FormatDesc {
   /// Format the texel data is internally stored in within a texture.
   pub internal: gl::GLenum,
   /// Format that we expect the data to be provided when filling the texture.
   pub external: gl::GLuint,
   /// Format to read the texels as, so that they can be uploaded as `external`
   /// later on.
   pub read: gl::GLuint,
   /// Associated pixel type.
   pub pixel_type: gl::GLuint,
}

#[derive(Debug)]
struct UploadChunk<'a> {
   rect: DeviceIntRect,
   stride: Option<i32>,
   offset: usize,
   format_override: Option<ImageFormat>,
   texture: &'a Texture,
}

#[derive(Debug)]
struct PixelBuffer<'a> {
   size_used: usize,
   // small vector avoids heap allocation for a single chunk
   chunks: SmallVec<[UploadChunk<'a>; 1]>,
   inner: UploadPBO,
   mapping: &'a mut [mem::MaybeUninit<u8>],
}

impl<'a> PixelBuffer<'a> {
   fn new(
       pbo: UploadPBO,
   ) -> Self {
       let mapping = unsafe {
           slice::from_raw_parts_mut(pbo.mapping.get_ptr().as_ptr(), pbo.pbo.reserved_size)
       };
       Self {
           size_used: 0,
           chunks: SmallVec::new(),
           inner: pbo,
           mapping,
       }
   }

   fn flush_chunks(&mut self, device: &mut Device) {
       for chunk in self.chunks.drain(..) {
           TextureUploader::update_impl(device, chunk);
       }
   }
}

impl<'a> Drop for PixelBuffer<'a> {
   fn drop(&mut self) {
       assert_eq!(self.chunks.len(), 0, "PixelBuffer must be flushed before dropping.");
   }
}

#[derive(Debug)]
enum PBOMapping {
   Unmapped,
   Transient(ptr::NonNull<mem::MaybeUninit<u8>>),
   Persistent(ptr::NonNull<mem::MaybeUninit<u8>>),
}

impl PBOMapping {
   fn get_ptr(&self) -> ptr::NonNull<mem::MaybeUninit<u8>> {
       match self {
           PBOMapping::Unmapped => unreachable!("Cannot get pointer to unmapped PBO."),
           PBOMapping::Transient(ptr) => *ptr,
           PBOMapping::Persistent(ptr) => *ptr,
       }
   }
}

/// A PBO for uploading texture data, managed by UploadPBOPool.
#[derive(Debug)]
struct UploadPBO {
   pbo: PBO,
   mapping: PBOMapping,
   can_recycle: bool,
}

impl UploadPBO {
   fn empty() -> Self {
       Self {
           pbo: PBO {
               id: 0,
               reserved_size: 0,
           },
           mapping: PBOMapping::Unmapped,
           can_recycle: false,
       }
   }
}

/// Allocates and recycles PBOs used for uploading texture data.
/// Tries to allocate and recycle PBOs of a fixed size, but will make exceptions when
/// a larger buffer is required or to work around driver bugs.
pub struct UploadPBOPool {
   /// Usage hint to provide to the driver for optimizations.
   usage_hint: VertexUsageHint,
   /// The preferred size, in bytes, of the buffers to allocate.
   default_size: usize,
   /// List of allocated PBOs ready to be re-used.
   available_buffers: Vec<UploadPBO>,
   /// PBOs which have been returned during the current frame,
   /// and do not yet have an associated sync object.
   returned_buffers: Vec<UploadPBO>,
   /// PBOs which are waiting until their sync object is signalled,
   /// indicating they can are ready to be re-used.
   waiting_buffers: Vec<(gl::GLsync, Vec<UploadPBO>)>,
   /// PBOs which have been orphaned.
   /// We can recycle their IDs but must reallocate their storage.
   orphaned_buffers: Vec<PBO>,
}

impl UploadPBOPool {
   pub fn new(device: &mut Device, default_size: usize) -> Self {
       let usage_hint = match device.upload_method {
           UploadMethod::Immediate => VertexUsageHint::Stream,
           UploadMethod::PixelBuffer(usage_hint) => usage_hint,
       };
       Self {
           usage_hint,
           default_size,
           available_buffers: Vec::new(),
           returned_buffers: Vec::new(),
           waiting_buffers: Vec::new(),
           orphaned_buffers: Vec::new(),
       }
   }

   /// To be called at the beginning of a series of uploads.
   /// Moves any buffers which are now ready to be used from the waiting list to the ready list.
   pub fn begin_frame(&mut self, device: &mut Device) {
       // Iterate through the waiting buffers and check if each fence has been signalled.
       // If a fence is signalled, move its corresponding buffers to the available list.
       // On error, delete the buffers. Stop when we find the first non-signalled fence,
       // and clean up the signalled fences.
       let mut first_not_signalled = self.waiting_buffers.len();
       for (i, (sync, buffers)) in self.waiting_buffers.iter_mut().enumerate() {
           match device.gl.client_wait_sync(*sync, 0, 0) {
               gl::TIMEOUT_EXPIRED => {
                   first_not_signalled = i;
                   break;
               },
               gl::ALREADY_SIGNALED | gl::CONDITION_SATISFIED => {
                   self.available_buffers.extend(buffers.drain(..));
               }
               gl::WAIT_FAILED | _ => {
                   warn!("glClientWaitSync error in UploadPBOPool::begin_frame()");
                   for buffer in buffers.drain(..) {
                       device.delete_pbo(buffer.pbo);
                   }
               }
           }
       }

       // Delete signalled fences, and remove their now-empty Vecs from waiting_buffers.
       for (sync, _) in self.waiting_buffers.drain(0..first_not_signalled) {
           device.gl.delete_sync(sync);
       }
   }

   // To be called at the end of a series of uploads.
   // Creates a sync object, and adds the buffers returned during this frame to waiting_buffers.
   pub fn end_frame(&mut self, device: &mut Device) {
       if !self.returned_buffers.is_empty() {
           let sync = device.gl.fence_sync(gl::SYNC_GPU_COMMANDS_COMPLETE, 0);
           if !sync.is_null() {
               self.waiting_buffers.push((sync, mem::replace(&mut self.returned_buffers, Vec::new())))
           } else {
               warn!("glFenceSync error in UploadPBOPool::end_frame()");

               for buffer in self.returned_buffers.drain(..) {
                   device.delete_pbo(buffer.pbo);
               }
           }
       }
   }

   /// Obtain a PBO, either by reusing an existing PBO or allocating a new one.
   /// min_size specifies the minimum required size of the PBO. The returned PBO
   /// may be larger than required.
   fn get_pbo(&mut self, device: &mut Device, min_size: usize) -> Result<UploadPBO, String> {

       // If min_size is smaller than our default size, then use the default size.
       // The exception to this is when due to driver bugs we cannot upload from
       // offsets other than zero within a PBO. In this case, there is no point in
       // allocating buffers larger than required, as they cannot be shared.
       let (can_recycle, size) = if min_size <= self.default_size && device.capabilities.supports_nonzero_pbo_offsets {
           (true, self.default_size)
       } else {
           (false, min_size)
       };

       // Try to recycle an already allocated PBO.
       if can_recycle {
           if let Some(mut buffer) = self.available_buffers.pop() {
               assert_eq!(buffer.pbo.reserved_size, size);
               assert!(buffer.can_recycle);

               device.gl.bind_buffer(gl::PIXEL_UNPACK_BUFFER, buffer.pbo.id);

               match buffer.mapping {
                   PBOMapping::Unmapped => {
                       // If buffer was unmapped then transiently map it.
                       let ptr = device.gl.map_buffer_range(
                           gl::PIXEL_UNPACK_BUFFER,
                           0,
                           buffer.pbo.reserved_size as _,
                           gl::MAP_WRITE_BIT | gl::MAP_UNSYNCHRONIZED_BIT,
                       ) as *mut _;

                       let ptr = ptr::NonNull::new(ptr).ok_or_else(
                           || format!("Failed to transiently map PBO of size {} bytes", buffer.pbo.reserved_size)
                       )?;

                       buffer.mapping = PBOMapping::Transient(ptr);
                   }
                   PBOMapping::Transient(_) => {
                       unreachable!("Transiently mapped UploadPBO must be unmapped before returning to pool.");
                   }
                   PBOMapping::Persistent(_) => {
                   }
               }

               return Ok(buffer);
           }
       }

       // Try to recycle a PBO ID (but not its allocation) from a previously allocated PBO.
       // If there are none available, create a new PBO.
       let mut pbo = match self.orphaned_buffers.pop() {
           Some(pbo) => pbo,
           None => device.create_pbo(),
       };

       assert_eq!(pbo.reserved_size, 0);
       pbo.reserved_size = size;

       device.gl.bind_buffer(gl::PIXEL_UNPACK_BUFFER, pbo.id);
       let mapping = if device.capabilities.supports_buffer_storage && can_recycle {
           device.gl.buffer_storage(
               gl::PIXEL_UNPACK_BUFFER,
               pbo.reserved_size as _,
               ptr::null(),
               gl::MAP_WRITE_BIT | gl::MAP_PERSISTENT_BIT,
           );
           let ptr = device.gl.map_buffer_range(
               gl::PIXEL_UNPACK_BUFFER,
               0,
               pbo.reserved_size as _,
               // GL_MAP_COHERENT_BIT doesn't seem to work on Adreno, so use glFlushMappedBufferRange.
               // kvark notes that coherent memory can be faster on some platforms, such as nvidia,
               // so in the future we could choose which to use at run time.
               gl::MAP_WRITE_BIT | gl::MAP_PERSISTENT_BIT | gl::MAP_FLUSH_EXPLICIT_BIT,
           ) as *mut _;

           let ptr = ptr::NonNull::new(ptr).ok_or_else(
               || format!("Failed to transiently map PBO of size {} bytes", pbo.reserved_size)
           )?;

           PBOMapping::Persistent(ptr)
       } else {
           device.gl.buffer_data_untyped(
               gl::PIXEL_UNPACK_BUFFER,
               pbo.reserved_size as _,
               ptr::null(),
               self.usage_hint.to_gl(),
           );
           let ptr = device.gl.map_buffer_range(
               gl::PIXEL_UNPACK_BUFFER,
               0,
               pbo.reserved_size as _,
               // Unlike the above code path, where we are re-mapping a buffer that has previously been unmapped,
               // this buffer has just been created there is no need for GL_MAP_UNSYNCHRONIZED_BIT.
               gl::MAP_WRITE_BIT,
           ) as *mut _;

           let ptr = ptr::NonNull::new(ptr).ok_or_else(
               || format!("Failed to transiently map PBO of size {} bytes", pbo.reserved_size)
           )?;

           PBOMapping::Transient(ptr)
       };

       Ok(UploadPBO { pbo, mapping, can_recycle })
   }

   /// Returns a PBO to the pool. If the PBO is recyclable it is placed in the waiting list.
   /// Otherwise we orphan the allocation immediately, and will subsequently reuse just the ID.
   fn return_pbo(&mut self, device: &mut Device, mut buffer: UploadPBO) {
       assert!(
           !matches!(buffer.mapping, PBOMapping::Transient(_)),
           "Transiently mapped UploadPBO must be unmapped before returning to pool.",
       );

       if buffer.can_recycle {
           self.returned_buffers.push(buffer);
       } else {
           device.gl.bind_buffer(gl::PIXEL_UNPACK_BUFFER, buffer.pbo.id);
           device.gl.buffer_data_untyped(
               gl::PIXEL_UNPACK_BUFFER,
               0,
               ptr::null(),
               gl::STREAM_DRAW,
           );
           buffer.pbo.reserved_size = 0;
           self.orphaned_buffers.push(buffer.pbo);
       }

       device.gl.bind_buffer(gl::PIXEL_UNPACK_BUFFER, 0);
   }

   /// Frees all allocated buffers in response to a memory pressure event.
   pub fn on_memory_pressure(&mut self, device: &mut Device) {
       for buffer in self.available_buffers.drain(..) {
           device.delete_pbo(buffer.pbo);
       }
       for buffer in self.returned_buffers.drain(..) {
           device.delete_pbo(buffer.pbo)
       }
       for (sync, buffers) in self.waiting_buffers.drain(..) {
           device.gl.delete_sync(sync);
           for buffer in buffers {
               device.delete_pbo(buffer.pbo)
           }
       }
       // There is no need to delete orphaned PBOs on memory pressure.
   }

   /// Generates a memory report.
   pub fn report_memory(&self) -> MemoryReport {
       let mut report = MemoryReport::default();
       for buffer in &self.available_buffers {
           report.texture_upload_pbos += buffer.pbo.reserved_size;
       }
       for buffer in &self.returned_buffers {
           report.texture_upload_pbos += buffer.pbo.reserved_size;
       }
       for (_, buffers) in &self.waiting_buffers {
           for buffer in buffers {
               report.texture_upload_pbos += buffer.pbo.reserved_size;
           }
       }
       report
   }

   pub fn deinit(&mut self, device: &mut Device) {
       for buffer in self.available_buffers.drain(..) {
           device.delete_pbo(buffer.pbo);
       }
       for buffer in self.returned_buffers.drain(..) {
           device.delete_pbo(buffer.pbo)
       }
       for (sync, buffers) in self.waiting_buffers.drain(..) {
           device.gl.delete_sync(sync);
           for buffer in buffers {
               device.delete_pbo(buffer.pbo)
           }
       }
       for pbo in self.orphaned_buffers.drain(..) {
           device.delete_pbo(pbo);
       }
   }
}

/// Used to perform a series of texture uploads.
/// Create using Device::upload_texture(). Perform a series of uploads using either
/// upload(), or stage() and upload_staged(), then call flush().
pub struct TextureUploader<'a> {
   /// A list of buffers containing uploads that need to be flushed.
   buffers: Vec<PixelBuffer<'a>>,
   /// Pool used to obtain PBOs to fill with texture data.
   pub pbo_pool: &'a mut UploadPBOPool,
}

impl<'a> Drop for TextureUploader<'a> {
   fn drop(&mut self) {
       assert!(
           thread::panicking() || self.buffers.is_empty(),
           "TextureUploader must be flushed before it is dropped."
       );
   }
}

/// A buffer used to manually stage data to be uploaded to a texture.
/// Created by calling TextureUploader::stage(), the data can then be written to via get_mapping().
#[derive(Debug)]
pub struct UploadStagingBuffer<'a> {
   /// The PixelBuffer containing this upload.
   buffer: PixelBuffer<'a>,
   /// The offset of this upload within the PixelBuffer.
   offset: usize,
   /// The size of this upload.
   size: usize,
   /// The stride of the data within the buffer.
   stride: usize,
}

impl<'a> UploadStagingBuffer<'a> {
   /// Returns the required stride of the data to be written to the buffer.
   pub fn get_stride(&self) -> usize {
       self.stride
   }

   /// Returns a mapping of the data in the buffer, to be written to.
   pub fn get_mapping(&mut self) -> &mut [mem::MaybeUninit<u8>] {
       &mut self.buffer.mapping[self.offset..self.offset + self.size]
   }
}

impl<'a> TextureUploader<'a> {
   /// Returns an UploadStagingBuffer which can be used to manually stage data to be uploaded.
   /// Once the data has been staged, it can be uploaded with upload_staged().
   pub fn stage(
       &mut self,
       device: &mut Device,
       format: ImageFormat,
       size: DeviceIntSize,
   ) -> Result<UploadStagingBuffer<'a>, String> {
       assert!(matches!(device.upload_method, UploadMethod::PixelBuffer(_)), "Texture uploads should only be staged when using pixel buffers.");

       // for optimal PBO texture uploads the offset and stride of the data in
       // the buffer may have to be a multiple of a certain value.
       let (dst_size, dst_stride) = device.required_upload_size_and_stride(
           size,
           format,
       );

       // Find a pixel buffer with enough space remaining, creating a new one if required.
       let buffer_index = self.buffers.iter().position(|buffer| {
           buffer.size_used + dst_size <= buffer.inner.pbo.reserved_size
       });
       let buffer = match buffer_index {
           Some(i) => self.buffers.swap_remove(i),
           None => PixelBuffer::new(self.pbo_pool.get_pbo(device, dst_size)?),
       };

       if !device.capabilities.supports_nonzero_pbo_offsets {
           assert_eq!(buffer.size_used, 0, "PBO uploads from non-zero offset are not supported.");
       }
       assert!(buffer.size_used + dst_size <= buffer.inner.pbo.reserved_size, "PixelBuffer is too small");

       let offset = buffer.size_used;

       Ok(UploadStagingBuffer {
           buffer,
           offset,
           size: dst_size,
           stride: dst_stride,
       })
   }

   /// Uploads manually staged texture data to the specified texture.
   pub fn upload_staged(
       &mut self,
       device: &mut Device,
       texture: &'a Texture,
       rect: DeviceIntRect,
       format_override: Option<ImageFormat>,
       mut staging_buffer: UploadStagingBuffer<'a>,
   ) -> usize {
       let size = staging_buffer.size;

       staging_buffer.buffer.chunks.push(UploadChunk {
           rect,
           stride: Some(staging_buffer.stride as i32),
           offset: staging_buffer.offset,
           format_override,
           texture,
       });
       staging_buffer.buffer.size_used += staging_buffer.size;

       // Flush the buffer if it is full, otherwise return it to the uploader for further use.
       if staging_buffer.buffer.size_used < staging_buffer.buffer.inner.pbo.reserved_size {
           self.buffers.push(staging_buffer.buffer);
       } else {
           Self::flush_buffer(device, self.pbo_pool, staging_buffer.buffer);
       }

       size
   }

   /// Uploads texture data to the specified texture.
   pub fn upload<T>(
       &mut self,
       device: &mut Device,
       texture: &'a Texture,
       mut rect: DeviceIntRect,
       stride: Option<i32>,
       format_override: Option<ImageFormat>,
       data: *const T,
       len: usize,
   ) -> usize {
       // Textures dimensions may have been clamped by the hardware. Crop the
       // upload region to match.
       let cropped = rect.intersection(
           &DeviceIntRect::from_size(texture.get_dimensions())
       );
       if cfg!(debug_assertions) && cropped.map_or(true, |r| r != rect) {
           warn!("Cropping texture upload {:?} to {:?}", rect, cropped);
       }
       rect = match cropped {
           None => return 0,
           Some(r) => r,
       };

       let bytes_pp = texture.format.bytes_per_pixel() as usize;
       let width_bytes = rect.width() as usize * bytes_pp;

       let src_stride = stride.map_or(width_bytes, |stride| {
           assert!(stride >= 0);
           stride as usize
       });
       let src_size = (rect.height() as usize - 1) * src_stride + width_bytes;
       assert!(src_size <= len * mem::size_of::<T>());

       match device.upload_method {
           UploadMethod::Immediate => {
               if cfg!(debug_assertions) {
                   let mut bound_buffer = [0];
                   unsafe {
                       device.gl.get_integer_v(gl::PIXEL_UNPACK_BUFFER_BINDING, &mut bound_buffer);
                   }
                   assert_eq!(bound_buffer[0], 0, "GL_PIXEL_UNPACK_BUFFER must not be bound for immediate uploads.");
               }

               Self::update_impl(device, UploadChunk {
                   rect,
                   stride: Some(src_stride as i32),
                   offset: data as _,
                   format_override,
                   texture,
               });

               width_bytes * rect.height() as usize
           }
           UploadMethod::PixelBuffer(_) => {
               let mut staging_buffer = match self.stage(device, texture.format, rect.size()) {
                   Ok(staging_buffer) => staging_buffer,
                   Err(_) => return 0,
               };
               let dst_stride = staging_buffer.get_stride();

               unsafe {
                   let src: &[mem::MaybeUninit<u8>] = slice::from_raw_parts(data as *const _, src_size);

                   if src_stride == dst_stride {
                       // the stride is already optimal, so simply copy
                       // the data as-is in to the buffer
                       staging_buffer.get_mapping()[..src_size].copy_from_slice(src);
                   } else {
                       // copy the data line-by-line in to the buffer so
                       // that it has an optimal stride
                       for y in 0..rect.height() as usize {
                           let src_start = y * src_stride;
                           let src_end = src_start + width_bytes;
                           let dst_start = y * staging_buffer.get_stride();
                           let dst_end = dst_start + width_bytes;

                           staging_buffer.get_mapping()[dst_start..dst_end].copy_from_slice(&src[src_start..src_end])
                       }
                   }
               }

               self.upload_staged(device, texture, rect, format_override, staging_buffer)
           }
       }
   }

   fn flush_buffer(device: &mut Device, pbo_pool: &mut UploadPBOPool, mut buffer: PixelBuffer) {
       device.gl.bind_buffer(gl::PIXEL_UNPACK_BUFFER, buffer.inner.pbo.id);
       match buffer.inner.mapping {
           PBOMapping::Unmapped => unreachable!("UploadPBO should be mapped at this stage."),
           PBOMapping::Transient(_) => {
               device.gl.unmap_buffer(gl::PIXEL_UNPACK_BUFFER);
               buffer.inner.mapping = PBOMapping::Unmapped;
           }
           PBOMapping::Persistent(_) => {
               device.gl.flush_mapped_buffer_range(gl::PIXEL_UNPACK_BUFFER, 0, buffer.size_used as _);
           }
       }
       buffer.flush_chunks(device);
       let pbo = mem::replace(&mut buffer.inner, UploadPBO::empty());
       pbo_pool.return_pbo(device, pbo);
   }

   /// Flushes all pending texture uploads. Must be called after all
   /// required upload() or upload_staged() calls have been made.
   pub fn flush(mut self, device: &mut Device) {
       for buffer in self.buffers.drain(..) {
           Self::flush_buffer(device, self.pbo_pool, buffer);
       }

       device.gl.bind_buffer(gl::PIXEL_UNPACK_BUFFER, 0);
   }

   fn update_impl(device: &mut Device, chunk: UploadChunk) {
       device.bind_texture(DEFAULT_TEXTURE, chunk.texture, Swizzle::default());

       let format = chunk.format_override.unwrap_or(chunk.texture.format);
       let (gl_format, bpp, data_type) = match format {
           ImageFormat::R8 => (gl::RED, 1, gl::UNSIGNED_BYTE),
           ImageFormat::R16 => (gl::RED, 2, gl::UNSIGNED_SHORT),
           ImageFormat::BGRA8 => (device.bgra_formats.external, 4, device.bgra_pixel_type),
           ImageFormat::RGBA8 => (gl::RGBA, 4, gl::UNSIGNED_BYTE),
           ImageFormat::RG8 => (gl::RG, 2, gl::UNSIGNED_BYTE),
           ImageFormat::RG16 => (gl::RG, 4, gl::UNSIGNED_SHORT),
           ImageFormat::RGBAF32 => (gl::RGBA, 16, gl::FLOAT),
           ImageFormat::RGBAI32 => (gl::RGBA_INTEGER, 16, gl::INT),
       };

       let row_length = match chunk.stride {
           Some(value) => value / bpp,
           None => chunk.texture.size.width,
       };

       if chunk.stride.is_some() {
           device.gl.pixel_store_i(
               gl::UNPACK_ROW_LENGTH,
               row_length as _,
           );
       }

       let pos = chunk.rect.min;
       let size = chunk.rect.size();

       match chunk.texture.target {
           gl::TEXTURE_2D | gl::TEXTURE_RECTANGLE | gl::TEXTURE_EXTERNAL_OES => {
               device.gl.tex_sub_image_2d_pbo(
                   chunk.texture.target,
                   0,
                   pos.x as _,
                   pos.y as _,
                   size.width as _,
                   size.height as _,
                   gl_format,
                   data_type,
                   chunk.offset,
               );
           }
           _ => panic!("BUG: Unexpected texture target!"),
       }

       // If using tri-linear filtering, build the mip-map chain for this texture.
       if chunk.texture.filter == TextureFilter::Trilinear {
           device.gl.generate_mipmap(chunk.texture.target);
       }

       // Reset row length to 0, otherwise the stride would apply to all texture uploads.
       if chunk.stride.is_some() {
           device.gl.pixel_store_i(gl::UNPACK_ROW_LENGTH, 0 as _);
       }
   }
}

fn texels_to_u8_slice<T: Texel>(texels: &[T]) -> &[u8] {
   unsafe {
       slice::from_raw_parts(texels.as_ptr() as *const u8, texels.len() * mem::size_of::<T>())
   }
}