gecko-dev/mfbt/BufferList.h

436 lines
12 KiB
C++

/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* vim: set ts=8 sts=2 et sw=2 tw=80: */
/* 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/. */
#ifndef mozilla_BufferList_h
#define mozilla_BufferList_h
#include <algorithm>
#include "mozilla/AllocPolicy.h"
#include "mozilla/Move.h"
#include "mozilla/Types.h"
#include "mozilla/TypeTraits.h"
#include "mozilla/Vector.h"
#include <string.h>
// BufferList represents a sequence of buffers of data. A BufferList can choose
// to own its buffers or not. The class handles writing to the buffers,
// iterating over them, and reading data out. Unlike SegmentedVector, the
// buffers may be of unequal size. Like SegmentedVector, BufferList is a nice
// way to avoid large contiguous allocations (which can trigger OOMs).
namespace mozilla {
template<typename AllocPolicy>
class BufferList : private AllocPolicy
{
// Each buffer in a BufferList has a size and a capacity. The first mSize
// bytes are initialized and the remaining |mCapacity - mSize| bytes are free.
struct Segment
{
char* mData;
size_t mSize;
size_t mCapacity;
Segment(char* aData, size_t aSize, size_t aCapacity)
: mData(aData),
mSize(aSize),
mCapacity(aCapacity)
{
}
Segment(const Segment&) = delete;
Segment& operator=(const Segment&) = delete;
Segment(Segment&&) = default;
Segment& operator=(Segment&&) = default;
char* Start() const { return mData; }
char* End() const { return mData + mSize; }
};
public:
// For the convenience of callers, all segments are required to be a multiple
// of 8 bytes in capacity. Also, every buffer except the last one is required
// to be full (i.e., size == capacity). Therefore, a byte at offset N within
// the BufferList and stored in memory at an address A will satisfy
// (N % Align == A % Align) if Align == 2, 4, or 8.
//
// NB: FlattenBytes can create non-full segments in the middle of the
// list. However, it ensures that these buffers are 8-byte aligned, so the
// offset invariant is not violated.
static const size_t kSegmentAlignment = 8;
// Allocate a BufferList. The BufferList will free all its buffers when it is
// destroyed. An initial buffer of size aInitialSize and capacity
// aInitialCapacity is allocated automatically. This data will be contiguous
// an can be accessed via |Start()|. Subsequent buffers will be allocated with
// capacity aStandardCapacity.
BufferList(size_t aInitialSize,
size_t aInitialCapacity,
size_t aStandardCapacity,
AllocPolicy aAP = AllocPolicy())
: AllocPolicy(aAP),
mOwning(true),
mSize(0),
mStandardCapacity(aStandardCapacity)
{
MOZ_ASSERT(aInitialCapacity % kSegmentAlignment == 0);
MOZ_ASSERT(aStandardCapacity % kSegmentAlignment == 0);
if (aInitialCapacity) {
AllocateSegment(aInitialSize, aInitialCapacity);
}
}
BufferList(const BufferList& aOther) = delete;
BufferList(BufferList&& aOther)
: mOwning(aOther.mOwning),
mSegments(Move(aOther.mSegments)),
mSize(aOther.mSize),
mStandardCapacity(aOther.mStandardCapacity)
{
aOther.mSegments.clear();
aOther.mSize = 0;
}
BufferList& operator=(const BufferList& aOther) = delete;
BufferList& operator=(BufferList&& aOther)
{
Clear();
mOwning = aOther.mOwning;
mSegments = Move(aOther.mSegments);
mSize = aOther.mSize;
aOther.mSegments.clear();
aOther.mSize = 0;
return *this;
}
~BufferList() { Clear(); }
// Returns the sum of the sizes of all the buffers.
size_t Size() const { return mSize; }
void Clear()
{
if (mOwning) {
for (Segment& segment : mSegments) {
this->free_(segment.mData);
}
}
mSegments.clear();
mSize = 0;
}
// Iterates over bytes in the segments. You can advance it by as many bytes as
// you choose.
class IterImpl
{
// Invariants:
// (0) mSegment <= bufferList.mSegments.size()
// (1) mData <= mDataEnd
// (2) If mSegment is not the last segment, mData < mDataEnd
uintptr_t mSegment;
char* mData;
char* mDataEnd;
friend class BufferList;
public:
explicit IterImpl(const BufferList& aBuffers)
: mSegment(0),
mData(nullptr),
mDataEnd(nullptr)
{
if (!aBuffers.mSegments.empty()) {
mData = aBuffers.mSegments[0].Start();
mDataEnd = aBuffers.mSegments[0].End();
}
}
// Returns a pointer to the raw data. It is valid to access up to
// RemainingInSegment bytes of this buffer.
char* Data() const
{
MOZ_RELEASE_ASSERT(!Done());
return mData;
}
// Returns true if the memory in the range [Data(), Data() + aBytes) is all
// part of one contiguous buffer.
bool HasRoomFor(size_t aBytes) const
{
MOZ_RELEASE_ASSERT(mData <= mDataEnd);
return size_t(mDataEnd - mData) >= aBytes;
}
// Returns the maximum value aBytes for which HasRoomFor(aBytes) will be
// true.
size_t RemainingInSegment() const
{
MOZ_RELEASE_ASSERT(mData <= mDataEnd);
return mDataEnd - mData;
}
// Advances the iterator by aBytes bytes. aBytes must be less than
// RemainingInSegment(). If advancing by aBytes takes the iterator to the
// end of a buffer, it will be moved to the beginning of the next buffer
// unless it is the last buffer.
void Advance(const BufferList& aBuffers, size_t aBytes)
{
const Segment& segment = aBuffers.mSegments[mSegment];
MOZ_RELEASE_ASSERT(segment.Start() <= mData);
MOZ_RELEASE_ASSERT(mData <= mDataEnd);
MOZ_RELEASE_ASSERT(mDataEnd == segment.End());
MOZ_RELEASE_ASSERT(HasRoomFor(aBytes));
mData += aBytes;
if (mData == mDataEnd && mSegment + 1 < aBuffers.mSegments.length()) {
mSegment++;
const Segment& nextSegment = aBuffers.mSegments[mSegment];
mData = nextSegment.Start();
mDataEnd = nextSegment.End();
MOZ_RELEASE_ASSERT(mData < mDataEnd);
}
}
// Advance the iterator by aBytes, possibly crossing segments. This function
// returns false if it runs out of buffers to advance through. Otherwise it
// returns true.
bool AdvanceAcrossSegments(const BufferList& aBuffers, size_t aBytes)
{
size_t bytes = aBytes;
while (bytes) {
size_t toAdvance = std::min(bytes, RemainingInSegment());
if (!toAdvance) {
return false;
}
Advance(aBuffers, toAdvance);
bytes -= toAdvance;
}
return true;
}
// Returns true when the iterator reaches the end of the BufferList.
bool Done() const
{
return mData == mDataEnd;
}
};
// Special convenience method that returns Iter().Data().
char* Start() { return mSegments[0].mData; }
IterImpl Iter() const { return IterImpl(*this); }
// Copies aSize bytes from aData into the BufferList. The storage for these
// bytes may be split across multiple buffers. Size() is increased by aSize.
inline bool WriteBytes(const char* aData, size_t aSize);
// Copies possibly non-contiguous byte range starting at aIter into
// aData. aIter is advanced by aSize bytes. Returns false if it runs out of
// data before aSize.
inline bool ReadBytes(IterImpl& aIter, char* aData, size_t aSize) const;
// FlattenBytes reconfigures the BufferList so that data in the range
// [aIter, aIter + aSize) is stored contiguously. A pointer to this data is
// returned in aOutData. Returns false if not enough data is available. All
// other iterators are invalidated by this method.
//
// This method requires aIter and aSize to be 8-byte aligned.
inline bool FlattenBytes(IterImpl& aIter, const char** aOutData, size_t aSize);
// Return a new BufferList that shares storage with this BufferList. The new
// BufferList is read-only. It allows iteration over aSize bytes starting at
// aIter. Borrow can fail, in which case *aSuccess will be false upon
// return. The borrowed BufferList can use a different AllocPolicy than the
// original one. However, it is not responsible for freeing buffers, so the
// AllocPolicy is only used for the buffer vector.
template<typename BorrowingAllocPolicy>
BufferList<BorrowingAllocPolicy> Borrow(IterImpl& aIter, size_t aSize, bool* aSuccess,
BorrowingAllocPolicy aAP = BorrowingAllocPolicy());
private:
explicit BufferList(AllocPolicy aAP)
: AllocPolicy(aAP),
mOwning(false),
mSize(0),
mStandardCapacity(0)
{
}
void* AllocateSegment(size_t aSize, size_t aCapacity)
{
MOZ_RELEASE_ASSERT(mOwning);
char* data = this->template pod_malloc<char>(aCapacity);
if (!data) {
return nullptr;
}
if (!mSegments.append(Segment(data, aSize, aCapacity))) {
this->free_(data);
return nullptr;
}
mSize += aSize;
return data;
}
bool mOwning;
Vector<Segment, 1, AllocPolicy> mSegments;
size_t mSize;
size_t mStandardCapacity;
};
template<typename AllocPolicy>
bool
BufferList<AllocPolicy>::WriteBytes(const char* aData, size_t aSize)
{
MOZ_RELEASE_ASSERT(mOwning);
MOZ_RELEASE_ASSERT(mStandardCapacity);
size_t copied = 0;
size_t remaining = aSize;
if (!mSegments.empty()) {
Segment& lastSegment = mSegments.back();
size_t toCopy = std::min(aSize, lastSegment.mCapacity - lastSegment.mSize);
memcpy(lastSegment.mData + lastSegment.mSize, aData, toCopy);
lastSegment.mSize += toCopy;
mSize += toCopy;
copied += toCopy;
remaining -= toCopy;
}
while (remaining) {
size_t toCopy = std::min(remaining, mStandardCapacity);
void* data = AllocateSegment(toCopy, mStandardCapacity);
if (!data) {
return false;
}
memcpy(data, aData + copied, toCopy);
copied += toCopy;
remaining -= toCopy;
}
return true;
}
template<typename AllocPolicy>
bool
BufferList<AllocPolicy>::ReadBytes(IterImpl& aIter, char* aData, size_t aSize) const
{
size_t copied = 0;
size_t remaining = aSize;
while (remaining) {
size_t toCopy = std::min(aIter.RemainingInSegment(), remaining);
if (!toCopy) {
// We've run out of data in the last segment.
return false;
}
memcpy(aData + copied, aIter.Data(), toCopy);
copied += toCopy;
remaining -= toCopy;
aIter.Advance(*this, toCopy);
}
return true;
}
template<typename AllocPolicy>
bool
BufferList<AllocPolicy>::FlattenBytes(IterImpl& aIter, const char** aOutData, size_t aSize)
{
MOZ_RELEASE_ASSERT(aSize);
MOZ_RELEASE_ASSERT(mOwning);
if (aIter.HasRoomFor(aSize)) {
// If the data is already contiguous, just return a pointer.
*aOutData = aIter.Data();
aIter.Advance(*this, aSize);
return true;
}
// This buffer will become the new contiguous segment.
char* buffer = this->template pod_malloc<char>(Size());
if (!buffer) {
return false;
}
size_t copied = 0;
size_t offset;
bool found = false;
for (size_t i = 0; i < mSegments.length(); i++) {
Segment& segment = mSegments[i];
memcpy(buffer + copied, segment.Start(), segment.mSize);
if (i == aIter.mSegment) {
offset = copied + (aIter.mData - segment.Start());
// Do we have aSize bytes after aIter?
if (Size() - offset >= aSize) {
found = true;
*aOutData = buffer + offset;
aIter.mSegment = 0;
aIter.mData = buffer + offset + aSize;
aIter.mDataEnd = buffer + Size();
}
}
this->free_(segment.mData);
copied += segment.mSize;
}
mSegments.clear();
mSegments.infallibleAppend(Segment(buffer, Size(), Size()));
if (!found) {
aIter.mSegment = 0;
aIter.mData = Start();
aIter.mDataEnd = Start() + Size();
}
return found;
}
template<typename AllocPolicy> template<typename BorrowingAllocPolicy>
BufferList<BorrowingAllocPolicy>
BufferList<AllocPolicy>::Borrow(IterImpl& aIter, size_t aSize, bool* aSuccess,
BorrowingAllocPolicy aAP)
{
BufferList<BorrowingAllocPolicy> result(aAP);
size_t size = aSize;
while (size) {
size_t toAdvance = std::min(size, aIter.RemainingInSegment());
if (!toAdvance || !result.mSegments.append(Segment(aIter.mData, toAdvance, toAdvance))) {
*aSuccess = false;
return result;
}
aIter.Advance(*this, toAdvance);
size -= toAdvance;
}
result.mSize = aSize;
*aSuccess = true;
return result;
}
} // namespace mozilla
#endif /* mozilla_BufferList_h */