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// Copyright 2017 The Dawn 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
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// See the License for the specific language governing permissions and
// limitations under the License.
#include "common/Assert.h"
#include "common/Math.h"
#include <cstddef>
#include <cstdint>
#include <vector>
namespace dawn_native {
// Allocation for command buffers should be fast. To avoid doing an allocation per command
// or to avoid copying commands when reallocing, we use a linear allocator in a growing set
// of large memory blocks. We also use this to have the format to be (u32 commandId, command),
// so that iteration over the commands is easy.
// Usage of the allocator and iterator:
// CommandAllocator allocator;
// DrawCommand* cmd = allocator.Allocate<DrawCommand>(CommandType::Draw);
// // Fill command
// // Repeat allocation and filling commands
// CommandIterator commands(allocator);
// CommandType type;
// while(commands.NextCommandId(&type)) {
// switch(type) {
// case CommandType::Draw:
// DrawCommand* draw = commands.NextCommand<DrawCommand>();
// // Do the draw
// break;
// // other cases
// }
// }
// Note that you need to extract the commands from the CommandAllocator before destroying it
// and must tell the CommandIterator when the allocated commands have been processed for
// deletion.
// These are the lists of blocks, should not be used directly, only through CommandAllocator
// and CommandIterator
struct BlockDef {
size_t size;
uint8_t* block;
using CommandBlocks = std::vector<BlockDef>;
namespace detail {
constexpr uint32_t kEndOfBlock = std::numeric_limits<uint32_t>::max();
constexpr uint32_t kAdditionalData = std::numeric_limits<uint32_t>::max() - 1;
} // namespace detail
class CommandAllocator;
// TODO( prevent copy for both iterator and allocator
class CommandIterator {
CommandIterator(CommandIterator&& other);
CommandIterator& operator=(CommandIterator&& other);
CommandIterator(CommandAllocator&& allocator);
CommandIterator& operator=(CommandAllocator&& allocator);
template <typename E>
bool NextCommandId(E* commandId) {
return NextCommandId(reinterpret_cast<uint32_t*>(commandId));
template <typename T>
T* NextCommand() {
return static_cast<T*>(NextCommand(sizeof(T), alignof(T)));
template <typename T>
T* NextData(size_t count) {
return static_cast<T*>(NextData(sizeof(T) * count, alignof(T)));
// Needs to be called if iteration was stopped early.
void Reset();
void DataWasDestroyed();
bool IsEmpty() const;
DAWN_FORCE_INLINE bool NextCommandId(uint32_t* commandId) {
uint8_t* idPtr = AlignPtr(mCurrentPtr, alignof(uint32_t));
ASSERT(idPtr + sizeof(uint32_t) <=
mBlocks[mCurrentBlock].block + mBlocks[mCurrentBlock].size);
uint32_t id = *reinterpret_cast<uint32_t*>(idPtr);
if (id != detail::kEndOfBlock) {
mCurrentPtr = idPtr + sizeof(uint32_t);
*commandId = id;
return true;
return NextCommandIdInNewBlock(commandId);
bool NextCommandIdInNewBlock(uint32_t* commandId);
DAWN_FORCE_INLINE void* NextCommand(size_t commandSize, size_t commandAlignment) {
uint8_t* commandPtr = AlignPtr(mCurrentPtr, commandAlignment);
ASSERT(commandPtr + sizeof(commandSize) <=
mBlocks[mCurrentBlock].block + mBlocks[mCurrentBlock].size);
mCurrentPtr = commandPtr + commandSize;
return commandPtr;
DAWN_FORCE_INLINE void* NextData(size_t dataSize, size_t dataAlignment) {
uint32_t id;
bool hasId = NextCommandId(&id);
ASSERT(id == detail::kAdditionalData);
return NextCommand(dataSize, dataAlignment);
CommandBlocks mBlocks;
uint8_t* mCurrentPtr = nullptr;
size_t mCurrentBlock = 0;
// Used to avoid a special case for empty iterators.
uint32_t mEndOfBlock = detail::kEndOfBlock;
bool mDataWasDestroyed = false;
class CommandAllocator {
template <typename T, typename E>
T* Allocate(E commandId) {
static_assert(sizeof(E) == sizeof(uint32_t), "");
static_assert(alignof(E) == alignof(uint32_t), "");
static_assert(alignof(T) <= kMaxSupportedAlignment, "");
T* result = reinterpret_cast<T*>(
Allocate(static_cast<uint32_t>(commandId), sizeof(T), alignof(T)));
if (!result) {
return nullptr;
new (result) T;
return result;
template <typename T>
T* AllocateData(size_t count) {
static_assert(alignof(T) <= kMaxSupportedAlignment, "");
T* result = reinterpret_cast<T*>(AllocateData(sizeof(T) * count, alignof(T)));
if (!result) {
return nullptr;
for (size_t i = 0; i < count; i++) {
new (result + i) T;
return result;
// This is used for some internal computations and can be any power of two as long as code
// using the CommandAllocator passes the static_asserts.
static constexpr size_t kMaxSupportedAlignment = 8;
// To avoid checking for overflows at every step of the computations we compute an upper
// bound of the space that will be needed in addition to the command data.
static constexpr size_t kWorstCaseAdditionalSize =
sizeof(uint32_t) + kMaxSupportedAlignment + alignof(uint32_t) + sizeof(uint32_t);
friend CommandIterator;
CommandBlocks&& AcquireBlocks();
DAWN_FORCE_INLINE uint8_t* Allocate(uint32_t commandId,
size_t commandSize,
size_t commandAlignment) {
ASSERT(mCurrentPtr != nullptr);
ASSERT(mEndPtr != nullptr);
ASSERT(commandId != detail::kEndOfBlock);
// It should always be possible to allocate one id, for kEndOfBlock tagging,
ASSERT(IsPtrAligned(mCurrentPtr, alignof(uint32_t)));
ASSERT(mEndPtr >= mCurrentPtr);
ASSERT(static_cast<size_t>(mEndPtr - mCurrentPtr) >= sizeof(uint32_t));
// The memory after the ID will contain the following:
// - the current ID
// - padding to align the command, maximum kMaxSupportedAlignment
// - the command of size commandSize
// - padding to align the next ID, maximum alignof(uint32_t)
// - the next ID of size sizeof(uint32_t)
// This can't overflow because by construction mCurrentPtr always has space for the next
// ID.
size_t remainingSize = static_cast<size_t>(mEndPtr - mCurrentPtr);
// The good case were we have enough space for the command data and upper bound of the
// extra required space.
if ((remainingSize >= kWorstCaseAdditionalSize) &&
(remainingSize - kWorstCaseAdditionalSize >= commandSize)) {
uint32_t* idAlloc = reinterpret_cast<uint32_t*>(mCurrentPtr);
*idAlloc = commandId;
uint8_t* commandAlloc = AlignPtr(mCurrentPtr + sizeof(uint32_t), commandAlignment);
mCurrentPtr = AlignPtr(commandAlloc + commandSize, alignof(uint32_t));
return commandAlloc;
return AllocateInNewBlock(commandId, commandSize, commandAlignment);
uint8_t* AllocateInNewBlock(uint32_t commandId,
size_t commandSize,
size_t commandAlignment);
DAWN_FORCE_INLINE uint8_t* AllocateData(size_t commandSize, size_t commandAlignment) {
return Allocate(detail::kAdditionalData, commandSize, commandAlignment);
bool GetNewBlock(size_t minimumSize);
CommandBlocks mBlocks;
size_t mLastAllocationSize = 2048;
// Pointers to the current range of allocation in the block. Guaranteed to allow for at
// least one uint32_t if not nullptr, so that the special kEndOfBlock command id can always
// be written. Nullptr iff the blocks were moved out.
uint8_t* mCurrentPtr = nullptr;
uint8_t* mEndPtr = nullptr;
// Data used for the block range at initialization so that the first call to Allocate sees
// there is not enough space and calls GetNewBlock. This avoids having to special case the
// initialization in Allocate.
uint32_t mDummyEnum[1] = {0};
} // namespace dawn_native