tor-browser

cord_internal.h (33441B)

---

// Copyright 2021 The Abseil Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//      https://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

#ifndef ABSL_STRINGS_INTERNAL_CORD_INTERNAL_H_
#define ABSL_STRINGS_INTERNAL_CORD_INTERNAL_H_

#include <atomic>
#include <cassert>
#include <cstddef>
#include <cstdint>
#include <cstring>
#include <string>

#include "absl/base/attributes.h"
#include "absl/base/config.h"
#include "absl/base/internal/endian.h"
#include "absl/base/macros.h"
#include "absl/base/nullability.h"
#include "absl/base/optimization.h"
#include "absl/container/internal/compressed_tuple.h"
#include "absl/container/internal/container_memory.h"
#include "absl/strings/string_view.h"

// We can only add poisoning if we can detect consteval executions.
#if defined(ABSL_HAVE_CONSTANT_EVALUATED) && \
   (defined(ABSL_HAVE_ADDRESS_SANITIZER) || \
    defined(ABSL_HAVE_MEMORY_SANITIZER))
#define ABSL_INTERNAL_CORD_HAVE_SANITIZER 1
#endif

#define ABSL_CORD_INTERNAL_NO_SANITIZE \
 ABSL_ATTRIBUTE_NO_SANITIZE_ADDRESS ABSL_ATTRIBUTE_NO_SANITIZE_MEMORY

namespace absl {
ABSL_NAMESPACE_BEGIN
namespace cord_internal {

// The overhead of a vtable is too much for Cord, so we roll our own subclasses
// using only a single byte to differentiate classes from each other - the "tag"
// byte.  Define the subclasses first so we can provide downcasting helper
// functions in the base class.
struct CordRep;
struct CordRepConcat;
struct CordRepExternal;
struct CordRepFlat;
struct CordRepSubstring;
struct CordRepCrc;
class CordRepBtree;

class CordzInfo;

// Default feature enable states for cord ring buffers
enum CordFeatureDefaults { kCordShallowSubcordsDefault = false };

extern std::atomic<bool> shallow_subcords_enabled;

inline void enable_shallow_subcords(bool enable) {
 shallow_subcords_enabled.store(enable, std::memory_order_relaxed);
}

enum Constants {
 // The inlined size to use with absl::InlinedVector.
 //
 // Note: The InlinedVectors in this file (and in cord.h) do not need to use
 // the same value for their inlined size. The fact that they do is historical.
 // It may be desirable for each to use a different inlined size optimized for
 // that InlinedVector's usage.
 //
 // TODO(jgm): Benchmark to see if there's a more optimal value than 47 for
 // the inlined vector size (47 exists for backward compatibility).
 kInlinedVectorSize = 47,

 // Prefer copying blocks of at most this size, otherwise reference count.
 kMaxBytesToCopy = 511
};

// Emits a fatal error "Unexpected node type: xyz" and aborts the program.
[[noreturn]] void LogFatalNodeType(CordRep* rep);

// Fast implementation of memmove for up to 15 bytes. This implementation is
// safe for overlapping regions. If nullify_tail is true, the destination is
// padded with '\0' up to 15 bytes.
template <bool nullify_tail = false>
inline void SmallMemmove(char* dst, const char* src, size_t n) {
 if (n >= 8) {
   assert(n <= 15);
   uint64_t buf1;
   uint64_t buf2;
   memcpy(&buf1, src, 8);
   memcpy(&buf2, src + n - 8, 8);
   if (nullify_tail) {
     memset(dst + 7, 0, 8);
   }
   // GCC 12 has a false-positive -Wstringop-overflow warning here.
#if ABSL_INTERNAL_HAVE_MIN_GNUC_VERSION(12, 0)
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wstringop-overflow"
#endif
   memcpy(dst, &buf1, 8);
   memcpy(dst + n - 8, &buf2, 8);
#if ABSL_INTERNAL_HAVE_MIN_GNUC_VERSION(12, 0)
#pragma GCC diagnostic pop
#endif
 } else if (n >= 4) {
   uint32_t buf1;
   uint32_t buf2;
   memcpy(&buf1, src, 4);
   memcpy(&buf2, src + n - 4, 4);
   if (nullify_tail) {
     memset(dst + 4, 0, 4);
     memset(dst + 7, 0, 8);
   }
   memcpy(dst, &buf1, 4);
   memcpy(dst + n - 4, &buf2, 4);
 } else {
   if (n != 0) {
     dst[0] = src[0];
     dst[n / 2] = src[n / 2];
     dst[n - 1] = src[n - 1];
   }
   if (nullify_tail) {
     memset(dst + 7, 0, 8);
     memset(dst + n, 0, 8);
   }
 }
}

// Compact class for tracking the reference count and state flags for CordRep
// instances.  Data is stored in an atomic int32_t for compactness and speed.
class RefcountAndFlags {
public:
 constexpr RefcountAndFlags() : count_{kRefIncrement} {}
 struct Immortal {};
 explicit constexpr RefcountAndFlags(Immortal) : count_(kImmortalFlag) {}

 // Increments the reference count. Imposes no memory ordering.
 inline void Increment() {
   count_.fetch_add(kRefIncrement, std::memory_order_relaxed);
 }

 // Asserts that the current refcount is greater than 0. If the refcount is
 // greater than 1, decrements the reference count.
 //
 // Returns false if there are no references outstanding; true otherwise.
 // Inserts barriers to ensure that state written before this method returns
 // false will be visible to a thread that just observed this method returning
 // false.  Always returns false when the immortal bit is set.
 inline bool Decrement() {
   int32_t refcount = count_.load(std::memory_order_acquire);
   assert(refcount > 0 || refcount & kImmortalFlag);
   return refcount != kRefIncrement &&
          count_.fetch_sub(kRefIncrement, std::memory_order_acq_rel) !=
              kRefIncrement;
 }

 // Same as Decrement but expect that refcount is greater than 1.
 inline bool DecrementExpectHighRefcount() {
   int32_t refcount =
       count_.fetch_sub(kRefIncrement, std::memory_order_acq_rel);
   assert(refcount > 0 || refcount & kImmortalFlag);
   return refcount != kRefIncrement;
 }

 // Returns the current reference count using acquire semantics.
 inline size_t Get() const {
   return static_cast<size_t>(count_.load(std::memory_order_acquire) >>
                              kNumFlags);
 }

 // Returns whether the atomic integer is 1.
 // If the reference count is used in the conventional way, a
 // reference count of 1 implies that the current thread owns the
 // reference and no other thread shares it.
 // This call performs the test for a reference count of one, and
 // performs the memory barrier needed for the owning thread
 // to act on the object, knowing that it has exclusive access to the
 // object. Always returns false when the immortal bit is set.
 inline bool IsOne() {
   return count_.load(std::memory_order_acquire) == kRefIncrement;
 }

 bool IsImmortal() const {
   return (count_.load(std::memory_order_relaxed) & kImmortalFlag) != 0;
 }

private:
 // We reserve the bottom bit for flag.
 // kImmortalBit indicates that this entity should never be collected; it is
 // used for the StringConstant constructor to avoid collecting immutable
 // constant cords.
 enum Flags {
   kNumFlags = 1,

   kImmortalFlag = 0x1,
   kRefIncrement = (1 << kNumFlags),
 };

 std::atomic<int32_t> count_;
};

// Various representations that we allow
enum CordRepKind {
 UNUSED_0 = 0,
 SUBSTRING = 1,
 CRC = 2,
 BTREE = 3,
 UNUSED_4 = 4,
 EXTERNAL = 5,

 // We have different tags for different sized flat arrays,
 // starting with FLAT, and limited to MAX_FLAT_TAG. The below values map to an
 // allocated range of 32 bytes to 256 KB. The current granularity is:
 // - 8 byte granularity for flat sizes in [32 - 512]
 // - 64 byte granularity for flat sizes in (512 - 8KiB]
 // - 4KiB byte granularity for flat sizes in (8KiB, 256 KiB]
 // If a new tag is needed in the future, then 'FLAT' and 'MAX_FLAT_TAG' should
 // be adjusted as well as the Tag <---> Size mapping logic so that FLAT still
 // represents the minimum flat allocation size. (32 bytes as of now).
 FLAT = 6,
 MAX_FLAT_TAG = 248
};

// There are various locations where we want to check if some rep is a 'plain'
// data edge, i.e. an external or flat rep. By having FLAT == EXTERNAL + 1, we
// can perform this check in a single branch as 'tag >= EXTERNAL'
// Note that we can leave this optimization to the compiler. The compiler will
// DTRT when it sees a condition like `tag == EXTERNAL || tag >= FLAT`.
static_assert(FLAT == EXTERNAL + 1, "EXTERNAL and FLAT not consecutive");

struct CordRep {
 // Result from an `extract edge` operation. Contains the (possibly changed)
 // tree node as well as the extracted edge, or {tree, nullptr} if no edge
 // could be extracted.
 // On success, the returned `tree` value is null if `extracted` was the only
 // data edge inside the tree, a data edge if there were only two data edges in
 // the tree, or the (possibly new / smaller) remaining tree with the extracted
 // data edge removed.
 struct ExtractResult {
   CordRep* tree;
   CordRep* extracted;
 };

 CordRep() = default;
 constexpr CordRep(RefcountAndFlags::Immortal immortal, size_t l)
     : length(l), refcount(immortal), tag(EXTERNAL), storage{} {}

 // The following three fields have to be less than 32 bytes since
 // that is the smallest supported flat node size. Some code optimizations rely
 // on the specific layout of these fields. Notably: the non-trivial field
 // `refcount` being preceded by `length`, and being tailed by POD data
 // members only.
 // LINT.IfChange
 size_t length;
 RefcountAndFlags refcount;
 // If tag < FLAT, it represents CordRepKind and indicates the type of node.
 // Otherwise, the node type is CordRepFlat and the tag is the encoded size.
 uint8_t tag;

 // `storage` provides two main purposes:
 // - the starting point for FlatCordRep.Data() [flexible-array-member]
 // - 3 bytes of additional storage for use by derived classes.
 // The latter is used by CordrepConcat and CordRepBtree. CordRepConcat stores
 // a 'depth' value in storage[0], and the (future) CordRepBtree class stores
 // `height`, `begin` and `end` in the 3 entries. Otherwise we would need to
 // allocate room for these in the derived class, as not all compilers reuse
 // padding space from the base class (clang and gcc do, MSVC does not, etc)
 uint8_t storage[3];
 // LINT.ThenChange(cord_rep_btree.h:copy_raw)

 // Returns true if this instance's tag matches the requested type.
 constexpr bool IsSubstring() const { return tag == SUBSTRING; }
 constexpr bool IsCrc() const { return tag == CRC; }
 constexpr bool IsExternal() const { return tag == EXTERNAL; }
 constexpr bool IsFlat() const { return tag >= FLAT; }
 constexpr bool IsBtree() const { return tag == BTREE; }

 inline CordRepSubstring* substring();
 inline const CordRepSubstring* substring() const;
 inline CordRepCrc* crc();
 inline const CordRepCrc* crc() const;
 inline CordRepExternal* external();
 inline const CordRepExternal* external() const;
 inline CordRepFlat* flat();
 inline const CordRepFlat* flat() const;
 inline CordRepBtree* btree();
 inline const CordRepBtree* btree() const;

 // --------------------------------------------------------------------
 // Memory management

 // Destroys the provided `rep`.
 static void Destroy(CordRep* rep);

 // Increments the reference count of `rep`.
 // Requires `rep` to be a non-null pointer value.
 static inline CordRep* Ref(CordRep* rep);

 // Decrements the reference count of `rep`. Destroys rep if count reaches
 // zero. Requires `rep` to be a non-null pointer value.
 static inline void Unref(CordRep* rep);
};

struct CordRepSubstring : public CordRep {
 size_t start;  // Starting offset of substring in child
 CordRep* child;

 // Creates a substring on `child`, adopting a reference on `child`.
 // Requires `child` to be either a flat or external node, and `pos` and `n` to
 // form a non-empty partial sub range of `'child`, i.e.:
 // `n > 0 && n < length && n + pos <= length`
 static inline CordRepSubstring* Create(CordRep* child, size_t pos, size_t n);

 // Creates a substring of `rep`. Does not adopt a reference on `rep`.
 // Requires `IsDataEdge(rep) && n > 0 && pos + n <= rep->length`.
 // If `n == rep->length` then this method returns `CordRep::Ref(rep)`
 // If `rep` is a substring of a flat or external node, then this method will
 // return a new substring of that flat or external node with `pos` adjusted
 // with the original `start` position.
 static inline CordRep* Substring(CordRep* rep, size_t pos, size_t n);
};

// Type for function pointer that will invoke the releaser function and also
// delete the `CordRepExternalImpl` corresponding to the passed in
// `CordRepExternal`.
using ExternalReleaserInvoker = void (*)(CordRepExternal*);

// External CordReps are allocated together with a type erased releaser. The
// releaser is stored in the memory directly following the CordRepExternal.
struct CordRepExternal : public CordRep {
 CordRepExternal() = default;
 explicit constexpr CordRepExternal(absl::string_view str)
     : CordRep(RefcountAndFlags::Immortal{}, str.size()),
       base(str.data()),
       releaser_invoker(nullptr) {}

 const char* base;
 // Pointer to function that knows how to call and destroy the releaser.
 ExternalReleaserInvoker releaser_invoker;

 // Deletes (releases) the external rep.
 // Requires rep != nullptr and rep->IsExternal()
 static void Delete(CordRep* rep);
};

// Use go/ranked-overloads for dispatching.
struct Rank0 {};
struct Rank1 : Rank0 {};

template <typename Releaser,
         typename = ::std::invoke_result_t<Releaser, absl::string_view>>
void InvokeReleaser(Rank1, Releaser&& releaser, absl::string_view data) {
 ::std::invoke(std::forward<Releaser>(releaser), data);
}

template <typename Releaser, typename = ::std::invoke_result_t<Releaser>>
void InvokeReleaser(Rank0, Releaser&& releaser, absl::string_view) {
 ::std::invoke(std::forward<Releaser>(releaser));
}

// We use CompressedTuple so that we can benefit from EBCO.
template <typename Releaser>
struct CordRepExternalImpl
   : public CordRepExternal,
     public ::absl::container_internal::CompressedTuple<Releaser> {
 // The extra int arg is so that we can avoid interfering with copy/move
 // constructors while still benefitting from perfect forwarding.
 template <typename T>
 CordRepExternalImpl(T&& releaser, int)
     : CordRepExternalImpl::CompressedTuple(std::forward<T>(releaser)) {
   this->releaser_invoker = &Release;
 }

 ~CordRepExternalImpl() {
   InvokeReleaser(Rank1{}, std::move(this->template get<0>()),
                  absl::string_view(base, length));
 }

 static void Release(CordRepExternal* rep) {
   delete static_cast<CordRepExternalImpl*>(rep);
 }
};

inline CordRepSubstring* CordRepSubstring::Create(CordRep* child, size_t pos,
                                                 size_t n) {
 assert(child != nullptr);
 assert(n > 0);
 assert(n < child->length);
 assert(pos < child->length);
 assert(n <= child->length - pos);

 // Move to strategical places inside the Cord logic and make this an assert.
 if (ABSL_PREDICT_FALSE(!(child->IsExternal() || child->IsFlat()))) {
   LogFatalNodeType(child);
 }

 CordRepSubstring* rep = new CordRepSubstring();
 rep->length = n;
 rep->tag = SUBSTRING;
 rep->start = pos;
 rep->child = child;
 return rep;
}

inline CordRep* CordRepSubstring::Substring(CordRep* rep, size_t pos,
                                           size_t n) {
 assert(rep != nullptr);
 assert(n != 0);
 assert(pos < rep->length);
 assert(n <= rep->length - pos);
 if (n == rep->length) return CordRep::Ref(rep);
 if (rep->IsSubstring()) {
   pos += rep->substring()->start;
   rep = rep->substring()->child;
 }
 CordRepSubstring* substr = new CordRepSubstring();
 substr->length = n;
 substr->tag = SUBSTRING;
 substr->start = pos;
 substr->child = CordRep::Ref(rep);
 return substr;
}

inline void CordRepExternal::Delete(CordRep* rep) {
 assert(rep != nullptr && rep->IsExternal());
 auto* rep_external = static_cast<CordRepExternal*>(rep);
 assert(rep_external->releaser_invoker != nullptr);
 rep_external->releaser_invoker(rep_external);
}

template <typename Str>
struct ConstInitExternalStorage {
 ABSL_CONST_INIT static CordRepExternal value;
};

template <typename Str>
ABSL_CONST_INIT CordRepExternal
   ConstInitExternalStorage<Str>::value(Str::value);

enum {
 kMaxInline = 15,
};

constexpr char GetOrNull(absl::string_view data, size_t pos) {
 return pos < data.size() ? data[pos] : '\0';
}

// We store cordz_info as 64 bit pointer value in little endian format. This
// guarantees that the least significant byte of cordz_info matches the first
// byte of the inline data representation in `data`, which holds the inlined
// size or the 'is_tree' bit.
using cordz_info_t = int64_t;

// Assert that the `cordz_info` pointer value perfectly overlaps the last half
// of `data` and can hold a pointer value.
static_assert(sizeof(cordz_info_t) * 2 == kMaxInline + 1, "");
static_assert(sizeof(cordz_info_t) >= sizeof(intptr_t), "");

// LittleEndianByte() creates a little endian representation of 'value', i.e.:
// a little endian value where the first byte in the host's representation
// holds 'value`, with all other bytes being 0.
static constexpr cordz_info_t LittleEndianByte(unsigned char value) {
#if defined(ABSL_IS_BIG_ENDIAN)
 return static_cast<cordz_info_t>(value) << ((sizeof(cordz_info_t) - 1) * 8);
#else
 return value;
#endif
}

class InlineData {
public:
 // DefaultInitType forces the use of the default initialization constructor.
 enum DefaultInitType { kDefaultInit };

 // kNullCordzInfo holds the little endian representation of intptr_t(1)
 // This is the 'null' / initial value of 'cordz_info'. The null value
 // is specifically big endian 1 as with 64-bit pointers, the last
 // byte of cordz_info overlaps with the last byte holding the tag.
 static constexpr cordz_info_t kNullCordzInfo = LittleEndianByte(1);

 // kTagOffset contains the offset of the control byte / tag. This constant is
 // intended mostly for debugging purposes: do not remove this constant as it
 // is actively inspected and used by gdb pretty printing code.
 static constexpr size_t kTagOffset = 0;

 // Implement `~InlineData()` conditionally: we only need this destructor to
 // unpoison poisoned instances under *SAN, and it will only compile correctly
 // if the current compiler supports `absl::is_constant_evaluated()`.
#ifdef ABSL_INTERNAL_CORD_HAVE_SANITIZER
 ~InlineData() noexcept { unpoison(); }
#endif

 constexpr InlineData() noexcept { poison_this(); }

 explicit InlineData(DefaultInitType) noexcept : rep_(kDefaultInit) {
   poison_this();
 }

 explicit InlineData(CordRep* rep) noexcept : rep_(rep) {
   ABSL_ASSERT(rep != nullptr);
 }

 // Explicit constexpr constructor to create a constexpr InlineData
 // value. Creates an inlined SSO value if `rep` is null, otherwise
 // creates a tree instance value.
 constexpr InlineData(absl::string_view sv, CordRep* rep) noexcept
     : rep_(rep ? Rep(rep) : Rep(sv)) {
   poison();
 }

 constexpr InlineData(const InlineData& rhs) noexcept;
 InlineData& operator=(const InlineData& rhs) noexcept;
 friend void swap(InlineData& lhs, InlineData& rhs) noexcept;

 friend bool operator==(const InlineData& lhs, const InlineData& rhs) {
#ifdef ABSL_INTERNAL_CORD_HAVE_SANITIZER
   const Rep l = lhs.rep_.SanitizerSafeCopy();
   const Rep r = rhs.rep_.SanitizerSafeCopy();
   return memcmp(&l, &r, sizeof(l)) == 0;
#else
   return memcmp(&lhs, &rhs, sizeof(lhs)) == 0;
#endif
 }
 friend bool operator!=(const InlineData& lhs, const InlineData& rhs) {
   return !operator==(lhs, rhs);
 }

 // Poisons the unused inlined SSO data if the current instance
 // is inlined, else un-poisons the entire instance.
 constexpr void poison();

 // Un-poisons this instance.
 constexpr void unpoison();

 // Poisons the current instance. This is used on default initialization.
 constexpr void poison_this();

 // Returns true if the current instance is empty.
 // The 'empty value' is an inlined data value of zero length.
 bool is_empty() const { return rep_.tag() == 0; }

 // Returns true if the current instance holds a tree value.
 bool is_tree() const { return (rep_.tag() & 1) != 0; }

 // Returns true if the current instance holds a cordz_info value.
 // Requires the current instance to hold a tree value.
 bool is_profiled() const {
   assert(is_tree());
   return rep_.cordz_info() != kNullCordzInfo;
 }

 // Returns true if either of the provided instances hold a cordz_info value.
 // This method is more efficient than the equivalent `data1.is_profiled() ||
 // data2.is_profiled()`. Requires both arguments to hold a tree.
 static bool is_either_profiled(const InlineData& data1,
                                const InlineData& data2) {
   assert(data1.is_tree() && data2.is_tree());
   return (data1.rep_.cordz_info() | data2.rep_.cordz_info()) !=
          kNullCordzInfo;
 }

 // Returns the cordz_info sampling instance for this instance, or nullptr
 // if the current instance is not sampled and does not have CordzInfo data.
 // Requires the current instance to hold a tree value.
 CordzInfo* cordz_info() const {
   assert(is_tree());
   intptr_t info = static_cast<intptr_t>(absl::little_endian::ToHost64(
       static_cast<uint64_t>(rep_.cordz_info())));
   assert(info & 1);
   return reinterpret_cast<CordzInfo*>(info - 1);
 }

 // Sets the current cordz_info sampling instance for this instance, or nullptr
 // if the current instance is not sampled and does not have CordzInfo data.
 // Requires the current instance to hold a tree value.
 void set_cordz_info(CordzInfo* cordz_info) {
   assert(is_tree());
   uintptr_t info = reinterpret_cast<uintptr_t>(cordz_info) | 1;
   rep_.set_cordz_info(
       static_cast<cordz_info_t>(absl::little_endian::FromHost64(info)));
 }

 // Resets the current cordz_info to null / empty.
 void clear_cordz_info() {
   assert(is_tree());
   rep_.set_cordz_info(kNullCordzInfo);
 }

 // Returns a read only pointer to the character data inside this instance.
 // Requires the current instance to hold inline data.
 const char* as_chars() const {
   assert(!is_tree());
   return rep_.as_chars();
 }

 // Returns a mutable pointer to the character data inside this instance.
 // Should be used for 'write only' operations setting an inlined value.
 // Applications can set the value of inlined data either before or after
 // setting the inlined size, i.e., both of the below are valid:
 //
 //   // Set inlined data and inline size
 //   memcpy(data_.as_chars(), data, size);
 //   data_.set_inline_size(size);
 //
 //   // Set inlined size and inline data
 //   data_.set_inline_size(size);
 //   memcpy(data_.as_chars(), data, size);
 //
 // It's an error to read from the returned pointer without a preceding write
 // if the current instance does not hold inline data, i.e.: is_tree() == true.
 char* as_chars() { return rep_.as_chars(); }

 // Returns the tree value of this value.
 // Requires the current instance to hold a tree value.
 CordRep* as_tree() const {
   assert(is_tree());
   return rep_.tree();
 }

 void set_inline_data(const char* data, size_t n) {
   ABSL_ASSERT(n <= kMaxInline);
   unpoison();
   rep_.set_tag(static_cast<int8_t>(n << 1));
   SmallMemmove<true>(rep_.as_chars(), data, n);
   poison();
 }

 void CopyInlineToString(absl::Nonnull<std::string*> dst) const {
   assert(!is_tree());
   // As Cord can store only 15 bytes it is smaller than std::string's
   // small string optimization buffer size. Therefore we will always trigger
   // the fast assign short path.
   //
   // Copying with a size equal to the maximum allows more efficient, wider
   // stores to be used and no branching.
   dst->assign(rep_.SanitizerSafeCopy().as_chars(), kMaxInline);
   // After the copy we then change the size and put in a 0 byte.
   dst->erase(inline_size());
 }

 void copy_max_inline_to(char* dst) const {
   assert(!is_tree());
   memcpy(dst, rep_.SanitizerSafeCopy().as_chars(), kMaxInline);
 }

 // Initialize this instance to holding the tree value `rep`,
 // initializing the cordz_info to null, i.e.: 'not profiled'.
 void make_tree(CordRep* rep) {
   unpoison();
   rep_.make_tree(rep);
 }

 // Set the tree value of this instance to 'rep`.
 // Requires the current instance to already hold a tree value.
 // Does not affect the value of cordz_info.
 void set_tree(CordRep* rep) {
   assert(is_tree());
   rep_.set_tree(rep);
 }

 // Returns the size of the inlined character data inside this instance.
 // Requires the current instance to hold inline data.
 size_t inline_size() const { return rep_.inline_size(); }

 // Sets the size of the inlined character data inside this instance.
 // Requires `size` to be <= kMaxInline.
 // See the documentation on 'as_chars()' for more information and examples.
 void set_inline_size(size_t size) {
   unpoison();
   rep_.set_inline_size(size);
   poison();
 }

 // Compares 'this' inlined data  with rhs. The comparison is a straightforward
 // lexicographic comparison. `Compare()` returns values as follows:
 //
 //   -1  'this' InlineData instance is smaller
 //    0  the InlineData instances are equal
 //    1  'this' InlineData instance larger
 int Compare(const InlineData& rhs) const {
   return Compare(rep_.SanitizerSafeCopy(), rhs.rep_.SanitizerSafeCopy());
 }

private:
 struct Rep {
   // See cordz_info_t for forced alignment and size of `cordz_info` details.
   struct AsTree {
     explicit constexpr AsTree(absl::cord_internal::CordRep* tree)
         : rep(tree) {}
     cordz_info_t cordz_info = kNullCordzInfo;
     absl::cord_internal::CordRep* rep;
   };

   explicit Rep(DefaultInitType) {}
   constexpr Rep() : data{0} {}
   constexpr Rep(const Rep&) = default;
   constexpr Rep& operator=(const Rep&) = default;

   explicit constexpr Rep(CordRep* rep) : as_tree(rep) {}

   explicit constexpr Rep(absl::string_view chars)
       : data{static_cast<char>((chars.size() << 1)),
              GetOrNull(chars, 0),
              GetOrNull(chars, 1),
              GetOrNull(chars, 2),
              GetOrNull(chars, 3),
              GetOrNull(chars, 4),
              GetOrNull(chars, 5),
              GetOrNull(chars, 6),
              GetOrNull(chars, 7),
              GetOrNull(chars, 8),
              GetOrNull(chars, 9),
              GetOrNull(chars, 10),
              GetOrNull(chars, 11),
              GetOrNull(chars, 12),
              GetOrNull(chars, 13),
              GetOrNull(chars, 14)} {}

#ifdef ABSL_INTERNAL_CORD_HAVE_SANITIZER
   // Break compiler optimization for cases when value is allocated on the
   // stack. Compiler assumes that the the variable is fully accessible
   // regardless of our poisoning.
   // Missing report: https://github.com/llvm/llvm-project/issues/100640
   const Rep* self() const {
     const Rep* volatile ptr = this;
     return ptr;
   }
   Rep* self() {
     Rep* volatile ptr = this;
     return ptr;
   }
#else
   constexpr const Rep* self() const { return this; }
   constexpr Rep* self() { return this; }
#endif

   // Disable sanitizer as we must always be able to read `tag`.
   ABSL_CORD_INTERNAL_NO_SANITIZE
   int8_t tag() const { return reinterpret_cast<const int8_t*>(this)[0]; }
   void set_tag(int8_t rhs) { reinterpret_cast<int8_t*>(self())[0] = rhs; }

   char* as_chars() { return self()->data + 1; }
   const char* as_chars() const { return self()->data + 1; }

   bool is_tree() const { return (self()->tag() & 1) != 0; }

   size_t inline_size() const {
     ABSL_ASSERT(!self()->is_tree());
     return static_cast<size_t>(self()->tag()) >> 1;
   }

   void set_inline_size(size_t size) {
     ABSL_ASSERT(size <= kMaxInline);
     self()->set_tag(static_cast<int8_t>(size << 1));
   }

   CordRep* tree() const { return self()->as_tree.rep; }
   void set_tree(CordRep* rhs) { self()->as_tree.rep = rhs; }

   cordz_info_t cordz_info() const { return self()->as_tree.cordz_info; }
   void set_cordz_info(cordz_info_t rhs) { self()->as_tree.cordz_info = rhs; }

   void make_tree(CordRep* tree) {
     self()->as_tree.rep = tree;
     self()->as_tree.cordz_info = kNullCordzInfo;
   }

#ifdef ABSL_INTERNAL_CORD_HAVE_SANITIZER
   constexpr Rep SanitizerSafeCopy() const {
     if (!absl::is_constant_evaluated()) {
       Rep res;
       if (is_tree()) {
         res = *this;
       } else {
         res.set_tag(tag());
         memcpy(res.as_chars(), as_chars(), inline_size());
       }
       return res;
     } else {
       return *this;
     }
   }
#else
   constexpr const Rep& SanitizerSafeCopy() const { return *this; }
#endif

   // If the data has length <= kMaxInline, we store it in `data`, and
   // store the size in the first char of `data` shifted left + 1.
   // Else we store it in a tree and store a pointer to that tree in
   // `as_tree.rep` with a tagged pointer to make `tag() & 1` non zero.
   union {
     char data[kMaxInline + 1];
     AsTree as_tree;
   };

   // TODO(b/145829486): see swap(InlineData, InlineData) for more info.
   inline void SwapValue(Rep rhs, Rep& refrhs) {
     memcpy(&refrhs, this, sizeof(*this));
     memcpy(this, &rhs, sizeof(*this));
   }
 };

 // Private implementation of `Compare()`
 static inline int Compare(const Rep& lhs, const Rep& rhs) {
   uint64_t x, y;
   memcpy(&x, lhs.as_chars(), sizeof(x));
   memcpy(&y, rhs.as_chars(), sizeof(y));
   if (x == y) {
     memcpy(&x, lhs.as_chars() + 7, sizeof(x));
     memcpy(&y, rhs.as_chars() + 7, sizeof(y));
     if (x == y) {
       if (lhs.inline_size() == rhs.inline_size()) return 0;
       return lhs.inline_size() < rhs.inline_size() ? -1 : 1;
     }
   }
   x = absl::big_endian::FromHost64(x);
   y = absl::big_endian::FromHost64(y);
   return x < y ? -1 : 1;
 }

 Rep rep_;
};

static_assert(sizeof(InlineData) == kMaxInline + 1, "");

#ifdef ABSL_INTERNAL_CORD_HAVE_SANITIZER

constexpr InlineData::InlineData(const InlineData& rhs) noexcept
   : rep_(rhs.rep_.SanitizerSafeCopy()) {
 poison();
}

inline InlineData& InlineData::operator=(const InlineData& rhs) noexcept {
 unpoison();
 rep_ = rhs.rep_.SanitizerSafeCopy();
 poison();
 return *this;
}

constexpr void InlineData::poison_this() {
 if (!absl::is_constant_evaluated()) {
   container_internal::SanitizerPoisonObject(this);
 }
}

constexpr void InlineData::unpoison() {
 if (!absl::is_constant_evaluated()) {
   container_internal::SanitizerUnpoisonObject(this);
 }
}

constexpr void InlineData::poison() {
 if (!absl::is_constant_evaluated()) {
   if (is_tree()) {
     container_internal::SanitizerUnpoisonObject(this);
   } else if (const size_t size = inline_size()) {
     if (size < kMaxInline) {
       const char* end = rep_.as_chars() + size;
       container_internal::SanitizerPoisonMemoryRegion(end, kMaxInline - size);
     }
   } else {
     container_internal::SanitizerPoisonObject(this);
   }
 }
}

#else  // ABSL_INTERNAL_CORD_HAVE_SANITIZER

constexpr InlineData::InlineData(const InlineData&) noexcept = default;
inline InlineData& InlineData::operator=(const InlineData&) noexcept = default;

constexpr void InlineData::poison_this() {}
constexpr void InlineData::unpoison() {}
constexpr void InlineData::poison() {}

#endif  // ABSL_INTERNAL_CORD_HAVE_SANITIZER

inline CordRepSubstring* CordRep::substring() {
 assert(IsSubstring());
 return static_cast<CordRepSubstring*>(this);
}

inline const CordRepSubstring* CordRep::substring() const {
 assert(IsSubstring());
 return static_cast<const CordRepSubstring*>(this);
}

inline CordRepExternal* CordRep::external() {
 assert(IsExternal());
 return static_cast<CordRepExternal*>(this);
}

inline const CordRepExternal* CordRep::external() const {
 assert(IsExternal());
 return static_cast<const CordRepExternal*>(this);
}

inline CordRep* CordRep::Ref(CordRep* rep) {
 // ABSL_ASSUME is a workaround for
 // https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105585
 ABSL_ASSUME(rep != nullptr);
 rep->refcount.Increment();
 return rep;
}

inline void CordRep::Unref(CordRep* rep) {
 assert(rep != nullptr);
 // Expect refcount to be 0. Avoiding the cost of an atomic decrement should
 // typically outweigh the cost of an extra branch checking for ref == 1.
 if (ABSL_PREDICT_FALSE(!rep->refcount.DecrementExpectHighRefcount())) {
   Destroy(rep);
 }
}

inline void swap(InlineData& lhs, InlineData& rhs) noexcept {
 lhs.unpoison();
 rhs.unpoison();
 // TODO(b/145829486): `std::swap(lhs.rep_, rhs.rep_)` results in bad codegen
 // on clang, spilling the temporary swap value on the stack. Since `Rep` is
 // trivial, we can make clang DTRT by calling a hand-rolled `SwapValue` where
 // we pass `rhs` both by value (register allocated) and by reference. The IR
 // then folds and inlines correctly into an optimized swap without spill.
 lhs.rep_.SwapValue(rhs.rep_, rhs.rep_);
 rhs.poison();
 lhs.poison();
}

}  // namespace cord_internal

ABSL_NAMESPACE_END
}  // namespace absl
#endif  // ABSL_STRINGS_INTERNAL_CORD_INTERNAL_H_