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cord_buffer.h (22786B)

---

// 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.
//
// -----------------------------------------------------------------------------
// File: cord_buffer.h
// -----------------------------------------------------------------------------
//
// This file defines an `absl::CordBuffer` data structure to hold data for
// eventual inclusion within an existing `Cord` data structure. Cord buffers are
// useful for building large Cords that may require custom allocation of its
// associated memory.
//
#ifndef ABSL_STRINGS_CORD_BUFFER_H_
#define ABSL_STRINGS_CORD_BUFFER_H_

#include <algorithm>
#include <cassert>
#include <cstddef>
#include <cstdint>
#include <memory>
#include <utility>

#include "absl/base/config.h"
#include "absl/base/macros.h"
#include "absl/numeric/bits.h"
#include "absl/strings/internal/cord_internal.h"
#include "absl/strings/internal/cord_rep_flat.h"
#include "absl/types/span.h"

namespace absl {
ABSL_NAMESPACE_BEGIN

class Cord;
class CordBufferTestPeer;

// CordBuffer
//
// CordBuffer manages memory buffers for purposes such as zero-copy APIs as well
// as applications building cords with large data requiring granular control
// over the allocation and size of cord data. For example, a function creating
// a cord of random data could use a CordBuffer as follows:
//
//   absl::Cord CreateRandomCord(size_t length) {
//     absl::Cord cord;
//     while (length > 0) {
//       CordBuffer buffer = CordBuffer::CreateWithDefaultLimit(length);
//       absl::Span<char> data = buffer.available_up_to(length);
//       FillRandomValues(data.data(), data.size());
//       buffer.IncreaseLengthBy(data.size());
//       cord.Append(std::move(buffer));
//       length -= data.size();
//     }
//     return cord;
//   }
//
// CordBuffer instances are by default limited to a capacity of `kDefaultLimit`
// bytes. `kDefaultLimit` is currently just under 4KiB, but this default may
// change in the future and/or for specific architectures. The default limit is
// aimed to provide a good trade-off between performance and memory overhead.
// Smaller buffers typically incur more compute cost while larger buffers are
// more CPU efficient but create significant memory overhead because of such
// allocations being less granular. Using larger buffers may also increase the
// risk of memory fragmentation.
//
// Applications create a buffer using one of the `CreateWithDefaultLimit()` or
// `CreateWithCustomLimit()` methods. The returned instance will have a non-zero
// capacity and a zero length. Applications use the `data()` method to set the
// contents of the managed memory, and once done filling the buffer, use the
// `IncreaseLengthBy()` or 'SetLength()' method to specify the length of the
// initialized data before adding the buffer to a Cord.
//
// The `CreateWithCustomLimit()` method is intended for applications needing
// larger buffers than the default memory limit, allowing the allocation of up
// to a capacity of `kCustomLimit` bytes minus some minimum internal overhead.
// The usage of `CreateWithCustomLimit()` should be limited to only those use
// cases where the distribution of the input is relatively well known, and/or
// where the trade-off between the efficiency gains outweigh the risk of memory
// fragmentation. See the documentation for `CreateWithCustomLimit()` for more
// information on using larger custom limits.
//
// The capacity of a `CordBuffer` returned by one of the `Create` methods may
// be larger than the requested capacity due to rounding, alignment and
// granularity of the memory allocator. Applications should use the `capacity`
// method to obtain the effective capacity of the returned instance as
// demonstrated in the provided example above.
//
// CordBuffer is a move-only class. All references into the managed memory are
// invalidated when an instance is moved into either another CordBuffer instance
// or a Cord. Writing to a location obtained by a previous call to `data()`
// after an instance was moved will lead to undefined behavior.
//
// A `moved from` CordBuffer instance will have a valid, but empty state.
// CordBuffer is thread compatible.
class CordBuffer {
public:
 // kDefaultLimit
 //
 // Default capacity limits of allocated CordBuffers.
 // See the class comments for more information on allocation limits.
 static constexpr size_t kDefaultLimit = cord_internal::kMaxFlatLength;

 // kCustomLimit
 //
 // Maximum size for CreateWithCustomLimit() allocated buffers.
 // Note that the effective capacity may be slightly less
 // because of internal overhead of internal cord buffers.
 static constexpr size_t kCustomLimit = 64U << 10;

 // Constructors, Destructors and Assignment Operators

 // Creates an empty CordBuffer.
 CordBuffer() = default;

 // Destroys this CordBuffer instance and, if not empty, releases any memory
 // managed by this instance, invalidating previously returned references.
 ~CordBuffer();

 // CordBuffer is move-only
 CordBuffer(CordBuffer&& rhs) noexcept;
 CordBuffer& operator=(CordBuffer&&) noexcept;
 CordBuffer(const CordBuffer&) = delete;
 CordBuffer& operator=(const CordBuffer&) = delete;

 // CordBuffer::MaximumPayload()
 //
 // Returns the guaranteed maximum payload for a CordBuffer returned by the
 // `CreateWithDefaultLimit()` method. While small, each internal buffer inside
 // a Cord incurs an overhead to manage the length, type and reference count
 // for the buffer managed inside the cord tree. Applications can use this
 // method to get approximate number of buffers required for a given byte
 // size, etc.
 //
 // For example:
 //   const size_t payload = absl::CordBuffer::MaximumPayload();
 //   const size_t buffer_count = (total_size + payload - 1) / payload;
 //   buffers.reserve(buffer_count);
 static constexpr size_t MaximumPayload();

 // Overload to the above `MaximumPayload()` except that it returns the
 // maximum payload for a CordBuffer returned by the `CreateWithCustomLimit()`
 // method given the provided `block_size`.
 static constexpr size_t MaximumPayload(size_t block_size);

 // CordBuffer::CreateWithDefaultLimit()
 //
 // Creates a CordBuffer instance of the desired `capacity`, capped at the
 // default limit `kDefaultLimit`. The returned buffer has a guaranteed
 // capacity of at least `min(kDefaultLimit, capacity)`. See the class comments
 // for more information on buffer capacities and intended usage.
 static CordBuffer CreateWithDefaultLimit(size_t capacity);

 // CordBuffer::CreateWithCustomLimit()
 //
 // Creates a CordBuffer instance of the desired `capacity` rounded to an
 // appropriate power of 2 size less than, or equal to `block_size`.
 // Requires `block_size` to be a power of 2.
 //
 // If `capacity` is less than or equal to `kDefaultLimit`, then this method
 // behaves identical to `CreateWithDefaultLimit`, which means that the caller
 // is guaranteed to get a buffer of at least the requested capacity.
 //
 // If `capacity` is greater than or equal to `block_size`, then this method
 // returns a buffer with an `allocated size` of `block_size` bytes. Otherwise,
 // this methods returns a buffer with a suitable smaller power of 2 block size
 // to satisfy the request. The actual size depends on a number of factors, and
 // is typically (but not necessarily) the highest or second highest power of 2
 // value less than or equal to `capacity`.
 //
 // The 'allocated size' includes a small amount of overhead required for
 // internal state, which is currently 13 bytes on 64-bit platforms. For
 // example: a buffer created with `block_size` and `capacity' set to 8KiB
 // will have an allocated size of 8KiB, and an effective internal `capacity`
 // of 8KiB - 13 = 8179 bytes.
 //
 // To demonstrate this in practice, let's assume we want to read data from
 // somewhat larger files using approximately 64KiB buffers:
 //
 //   absl::Cord ReadFromFile(int fd, size_t n) {
 //     absl::Cord cord;
 //     while (n > 0) {
 //       CordBuffer buffer = CordBuffer::CreateWithCustomLimit(64 << 10, n);
 //       absl::Span<char> data = buffer.available_up_to(n);
 //       ReadFileDataOrDie(fd, data.data(), data.size());
 //       buffer.IncreaseLengthBy(data.size());
 //       cord.Append(std::move(buffer));
 //       n -= data.size();
 //     }
 //     return cord;
 //   }
 //
 // If we'd use this function to read a file of 659KiB, we may get the
 // following pattern of allocated cord buffer sizes:
 //
 //   CreateWithCustomLimit(64KiB, 674816) --> ~64KiB (65523)
 //   CreateWithCustomLimit(64KiB, 674816) --> ~64KiB (65523)
 //   ...
 //   CreateWithCustomLimit(64KiB,  19586) --> ~16KiB (16371)
 //   CreateWithCustomLimit(64KiB,   3215) -->   3215 (at least 3215)
 //
 // The reason the method returns a 16K buffer instead of a roughly 19K buffer
 // is to reduce memory overhead and fragmentation risks. Using carefully
 // chosen power of 2 values reduces the entropy of allocated memory sizes.
 //
 // Additionally, let's assume we'd use the above function on files that are
 // generally smaller than 64K. If we'd use 'precise' sized buffers for such
 // files, than we'd get a very wide distribution of allocated memory sizes
 // rounded to 4K page sizes, and we'd end up with a lot of unused capacity.
 //
 // In general, application should only use custom sizes if the data they are
 // consuming or storing is expected to be many times the chosen block size,
 // and be based on objective data and performance metrics. For example, a
 // compress function may work faster and consume less CPU when using larger
 // buffers. Such an application should pick a size offering a reasonable
 // trade-off between expected data size, compute savings with larger buffers,
 // and the cost or fragmentation effect of larger buffers.
 // Applications must pick a reasonable spot on that curve, and make sure their
 // data meets their expectations in size distributions such as "mostly large".
 static CordBuffer CreateWithCustomLimit(size_t block_size, size_t capacity);

 // CordBuffer::available()
 //
 // Returns the span delineating the available capacity in this buffer
 // which is defined as `{ data() + length(), capacity() - length() }`.
 absl::Span<char> available();

 // CordBuffer::available_up_to()
 //
 // Returns the span delineating the available capacity in this buffer limited
 // to `size` bytes. This is equivalent to `available().subspan(0, size)`.
 absl::Span<char> available_up_to(size_t size);

 // CordBuffer::data()
 //
 // Returns a non-null reference to the data managed by this instance.
 // Applications are allowed to write up to `capacity` bytes of instance data.
 // CordBuffer data is uninitialized by default. Reading data from an instance
 // that has not yet been initialized will lead to undefined behavior.
 char* data();
 const char* data() const;

 // CordBuffer::length()
 //
 // Returns the length of this instance. The default length of a CordBuffer is
 // 0, indicating an 'empty' CordBuffer. Applications must specify the length
 // of the data in a CordBuffer before adding it to a Cord.
 size_t length() const;

 // CordBuffer::capacity()
 //
 // Returns the capacity of this instance. All instances have a non-zero
 // capacity: default and `moved from` instances have a small internal buffer.
 size_t capacity() const;

 // CordBuffer::IncreaseLengthBy()
 //
 // Increases the length of this buffer by the specified 'n' bytes.
 // Applications must make sure all data in this buffer up to the new length
 // has been initialized before adding a CordBuffer to a Cord: failure to do so
 // will lead to undefined behavior.  Requires `length() + n <= capacity()`.
 // Typically, applications will use 'available_up_to()` to get a span of the
 // desired capacity, and use `span.size()` to increase the length as in:
 //   absl::Span<char> span = buffer.available_up_to(desired);
 //   buffer.IncreaseLengthBy(span.size());
 //   memcpy(span.data(), src, span.size());
 //   etc...
 void IncreaseLengthBy(size_t n);

 // CordBuffer::SetLength()
 //
 // Sets the data length of this instance. Applications must make sure all data
 // of the specified length has been initialized before adding a CordBuffer to
 // a Cord: failure to do so will lead to undefined behavior.
 // Setting the length to a small value or zero does not release any memory
 // held by this CordBuffer instance. Requires `length <= capacity()`.
 // Applications should preferably use the `IncreaseLengthBy()` method above
 // in combination with the 'available()` or `available_up_to()` methods.
 void SetLength(size_t length);

private:
 // Make sure we don't accidentally over promise.
 static_assert(kCustomLimit <= cord_internal::kMaxLargeFlatSize, "");

 // Assume the cost of an 'uprounded' allocation to CeilPow2(size) versus
 // the cost of allocating at least 1 extra flat <= 4KB:
 // - Flat overhead = 13 bytes
 // - Btree amortized cost / node =~ 13 bytes
 // - 64 byte granularity of tcmalloc at 4K =~ 32 byte average
 // CPU cost and efficiency requires we should at least 'save' something by
 // splitting, as a poor man's measure, we say the slop needs to be
 // at least double the cost offset to make it worth splitting: ~128 bytes.
 static constexpr size_t kMaxPageSlop = 128;

 // Overhead for allocation a flat.
 static constexpr size_t kOverhead = cord_internal::kFlatOverhead;

 using CordRepFlat = cord_internal::CordRepFlat;

 // `Rep` is the internal data representation of a CordBuffer. The internal
 // representation has an internal small size optimization similar to
 // std::string (SSO).
 struct Rep {
   // Inline SSO size of a CordBuffer
   static constexpr size_t kInlineCapacity = sizeof(intptr_t) * 2 - 1;

   // Creates a default instance with kInlineCapacity.
   Rep() : short_rep{} {}

   // Creates an instance managing an allocated non zero CordRep.
   explicit Rep(cord_internal::CordRepFlat* rep) : long_rep{rep} {
     assert(rep != nullptr);
   }

   // Returns true if this instance manages the SSO internal buffer.
   bool is_short() const {
     constexpr size_t offset = offsetof(Short, raw_size);
     return (reinterpret_cast<const char*>(this)[offset] & 1) != 0;
   }

   // Returns the available area of the internal SSO data
   absl::Span<char> short_available() {
     const size_t length = short_length();
     return absl::Span<char>(short_rep.data + length,
                             kInlineCapacity - length);
   }

   // Returns the available area of the internal SSO data
   absl::Span<char> long_available() const {
     assert(!is_short());
     const size_t length = long_rep.rep->length;
     return absl::Span<char>(long_rep.rep->Data() + length,
                             long_rep.rep->Capacity() - length);
   }

   // Returns the length of the internal SSO data.
   size_t short_length() const {
     assert(is_short());
     return static_cast<size_t>(short_rep.raw_size >> 1);
   }

   // Sets the length of the internal SSO data.
   // Disregards any previously set CordRep instance.
   void set_short_length(size_t length) {
     short_rep.raw_size = static_cast<char>((length << 1) + 1);
   }

   // Adds `n` to the current short length.
   void add_short_length(size_t n) {
     assert(is_short());
     short_rep.raw_size += static_cast<char>(n << 1);
   }

   // Returns reference to the internal SSO data buffer.
   char* data() {
     assert(is_short());
     return short_rep.data;
   }
   const char* data() const {
     assert(is_short());
     return short_rep.data;
   }

   // Returns a pointer the external CordRep managed by this instance.
   cord_internal::CordRepFlat* rep() const {
     assert(!is_short());
     return long_rep.rep;
   }

   // The internal representation takes advantage of the fact that allocated
   // memory is always on an even address, and uses the least significant bit
   // of the first or last byte (depending on endianness) as the inline size
   // indicator overlapping with the least significant byte of the CordRep*.
#if defined(ABSL_IS_BIG_ENDIAN)
   struct Long {
     explicit Long(cord_internal::CordRepFlat* rep_arg) : rep(rep_arg) {}
     void* padding;
     cord_internal::CordRepFlat* rep;
   };
   struct Short {
     char data[sizeof(Long) - 1];
     char raw_size = 1;
   };
#else
   struct Long {
     explicit Long(cord_internal::CordRepFlat* rep_arg) : rep(rep_arg) {}
     cord_internal::CordRepFlat* rep;
     void* padding;
   };
   struct Short {
     char raw_size = 1;
     char data[sizeof(Long) - 1];
   };
#endif

   union {
     Long long_rep;
     Short short_rep;
   };
 };

 // Power2 functions
 static bool IsPow2(size_t size) { return absl::has_single_bit(size); }
 static size_t Log2Floor(size_t size) {
   return static_cast<size_t>(absl::bit_width(size) - 1);
 }
 static size_t Log2Ceil(size_t size) {
   return static_cast<size_t>(absl::bit_width(size - 1));
 }

 // Implementation of `CreateWithCustomLimit()`.
 // This implementation allows for future memory allocation hints to
 // be passed down into the CordRepFlat allocation function.
 template <typename... AllocationHints>
 static CordBuffer CreateWithCustomLimitImpl(size_t block_size,
                                             size_t capacity,
                                             AllocationHints... hints);

 // Consumes the value contained in this instance and resets the instance.
 // This method returns a non-null Cordrep* if the current instances manages a
 // CordRep*, and resets the instance to an empty SSO instance. If the current
 // instance is an SSO instance, then this method returns nullptr and sets
 // `short_value` to the inlined data value. In either case, the current
 // instance length is reset to zero.
 // This method is intended to be used by Cord internal functions only.
 cord_internal::CordRep* ConsumeValue(absl::string_view& short_value) {
   cord_internal::CordRep* rep = nullptr;
   if (rep_.is_short()) {
     short_value = absl::string_view(rep_.data(), rep_.short_length());
   } else {
     rep = rep_.rep();
   }
   rep_.set_short_length(0);
   return rep;
 }

 // Internal constructor.
 explicit CordBuffer(cord_internal::CordRepFlat* rep) : rep_(rep) {
   assert(rep != nullptr);
 }

 Rep rep_;

 friend class Cord;
 friend class CordBufferTestPeer;
};

inline constexpr size_t CordBuffer::MaximumPayload() {
 return cord_internal::kMaxFlatLength;
}

inline constexpr size_t CordBuffer::MaximumPayload(size_t block_size) {
 return (std::min)(kCustomLimit, block_size) - cord_internal::kFlatOverhead;
}

inline CordBuffer CordBuffer::CreateWithDefaultLimit(size_t capacity) {
 if (capacity > Rep::kInlineCapacity) {
   auto* rep = cord_internal::CordRepFlat::New(capacity);
   rep->length = 0;
   return CordBuffer(rep);
 }
 return CordBuffer();
}

template <typename... AllocationHints>
inline CordBuffer CordBuffer::CreateWithCustomLimitImpl(
   size_t block_size, size_t capacity, AllocationHints... hints) {
 assert(IsPow2(block_size));
 capacity = (std::min)(capacity, kCustomLimit);
 block_size = (std::min)(block_size, kCustomLimit);
 if (capacity + kOverhead >= block_size) {
   capacity = block_size;
 } else if (capacity <= kDefaultLimit) {
   capacity = capacity + kOverhead;
 } else if (!IsPow2(capacity)) {
   // Check if rounded up to next power 2 is a good enough fit
   // with limited waste making it an acceptable direct fit.
   const size_t rounded_up = size_t{1} << Log2Ceil(capacity);
   const size_t slop = rounded_up - capacity;
   if (slop >= kOverhead && slop <= kMaxPageSlop + kOverhead) {
     capacity = rounded_up;
   } else {
     // Round down to highest power of 2 <= capacity.
     // Consider a more aggressive step down if that may reduce the
     // risk of fragmentation where 'people are holding it wrong'.
     const size_t rounded_down = size_t{1} << Log2Floor(capacity);
     capacity = rounded_down;
   }
 }
 const size_t length = capacity - kOverhead;
 auto* rep = CordRepFlat::New(CordRepFlat::Large(), length, hints...);
 rep->length = 0;
 return CordBuffer(rep);
}

inline CordBuffer CordBuffer::CreateWithCustomLimit(size_t block_size,
                                                   size_t capacity) {
 return CreateWithCustomLimitImpl(block_size, capacity);
}

inline CordBuffer::~CordBuffer() {
 if (!rep_.is_short()) {
   cord_internal::CordRepFlat::Delete(rep_.rep());
 }
}

inline CordBuffer::CordBuffer(CordBuffer&& rhs) noexcept : rep_(rhs.rep_) {
 rhs.rep_.set_short_length(0);
}

inline CordBuffer& CordBuffer::operator=(CordBuffer&& rhs) noexcept {
 if (!rep_.is_short()) cord_internal::CordRepFlat::Delete(rep_.rep());
 rep_ = rhs.rep_;
 rhs.rep_.set_short_length(0);
 return *this;
}

inline absl::Span<char> CordBuffer::available() {
 return rep_.is_short() ? rep_.short_available() : rep_.long_available();
}

inline absl::Span<char> CordBuffer::available_up_to(size_t size) {
 return available().subspan(0, size);
}

inline char* CordBuffer::data() {
 return rep_.is_short() ? rep_.data() : rep_.rep()->Data();
}

inline const char* CordBuffer::data() const {
 return rep_.is_short() ? rep_.data() : rep_.rep()->Data();
}

inline size_t CordBuffer::capacity() const {
 return rep_.is_short() ? Rep::kInlineCapacity : rep_.rep()->Capacity();
}

inline size_t CordBuffer::length() const {
 return rep_.is_short() ? rep_.short_length() : rep_.rep()->length;
}

inline void CordBuffer::SetLength(size_t length) {
 ABSL_HARDENING_ASSERT(length <= capacity());
 if (rep_.is_short()) {
   rep_.set_short_length(length);
 } else {
   rep_.rep()->length = length;
 }
}

inline void CordBuffer::IncreaseLengthBy(size_t n) {
 ABSL_HARDENING_ASSERT(n <= capacity() && length() + n <= capacity());
 if (rep_.is_short()) {
   rep_.add_short_length(n);
 } else {
   rep_.rep()->length += n;
 }
}

ABSL_NAMESPACE_END
}  // namespace absl

#endif  // ABSL_STRINGS_CORD_BUFFER_H_