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// Copyright 2019 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 <cstddef>
#include <cstdint>
#include <limits>
#include <vector>
namespace dawn::native {
// Buddy allocator uses the buddy memory allocation technique to satisfy an allocation request.
// Memory is split into halves until just large enough to fit to the request. This
// requires the allocation size to be a power-of-two value. The allocator "allocates" a block by
// returning the starting offset whose size is guaranteed to be greater than or equal to the
// allocation size. To deallocate, the same offset is used to find the corresponding block.
// Internally, it manages a free list to track free blocks in a full binary tree.
// Every index in the free list corresponds to a level in the tree. That level also determines
// the size of the block to be used to satisfy the request. The first level (index=0) represents
// the root whose size is also called the max block size.
class BuddyAllocator {
BuddyAllocator(uint64_t maxSize);
// Required methods.
uint64_t Allocate(uint64_t allocationSize, uint64_t alignment = 1);
void Deallocate(uint64_t offset);
// For testing purposes only.
uint64_t ComputeTotalNumOfFreeBlocksForTesting() const;
static constexpr uint64_t kInvalidOffset = std::numeric_limits<uint64_t>::max();
uint32_t ComputeLevelFromBlockSize(uint64_t blockSize) const;
uint64_t GetNextFreeAlignedBlock(size_t allocationBlockLevel, uint64_t alignment) const;
enum class BlockState { Free, Split, Allocated };
struct BuddyBlock {
BuddyBlock(uint64_t size, uint64_t offset)
: mOffset(offset), mSize(size), mState(BlockState::Free) {
free.pPrev = nullptr;
free.pNext = nullptr;
uint64_t mOffset;
uint64_t mSize;
// Pointer to this block's buddy, iff parent is split.
// Used to quickly merge buddy blocks upon de-allocate.
BuddyBlock* pBuddy = nullptr;
BuddyBlock* pParent = nullptr;
// Track whether this block has been split or not.
BlockState mState;
struct FreeLinks {
BuddyBlock* pPrev;
BuddyBlock* pNext;
struct SplitLink {
BuddyBlock* pLeft;
union {
// Used upon allocation.
// Avoids searching for the next free block.
FreeLinks free;
// Used upon de-allocation.
// Had this block split upon allocation, it and it's buddy is to be deleted.
SplitLink split;
void InsertFreeBlock(BuddyBlock* block, size_t level);
void RemoveFreeBlock(BuddyBlock* block, size_t level);
void DeleteBlock(BuddyBlock* block);
uint64_t ComputeNumOfFreeBlocks(BuddyBlock* block) const;
// Keep track the head and tail (for faster insertion/removal).
struct BlockList {
BuddyBlock* head = nullptr; // First free block in level.
// TODO( Track the tail.
BuddyBlock* mRoot = nullptr; // Used to deallocate non-free blocks.
uint64_t mMaxBlockSize = 0;
// List of linked-lists of free blocks where the index is a level that
// corresponds to a power-of-two sized block.
std::vector<BlockList> mFreeLists;
} // namespace dawn::native