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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
//
// http://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.
#include "dawn_native/BuddyAllocator.h"
#include "common/Assert.h"
#include "common/Math.h"
namespace dawn::native {
BuddyAllocator::BuddyAllocator(uint64_t maxSize) : mMaxBlockSize(maxSize) {
ASSERT(IsPowerOfTwo(maxSize));
mFreeLists.resize(Log2(mMaxBlockSize) + 1);
// Insert the level0 free block.
mRoot = new BuddyBlock(maxSize, /*offset*/ 0);
mFreeLists[0] = {mRoot};
}
BuddyAllocator::~BuddyAllocator() {
if (mRoot) {
DeleteBlock(mRoot);
}
}
uint64_t BuddyAllocator::ComputeTotalNumOfFreeBlocksForTesting() const {
return ComputeNumOfFreeBlocks(mRoot);
}
uint64_t BuddyAllocator::ComputeNumOfFreeBlocks(BuddyBlock* block) const {
if (block->mState == BlockState::Free) {
return 1;
} else if (block->mState == BlockState::Split) {
return ComputeNumOfFreeBlocks(block->split.pLeft) +
ComputeNumOfFreeBlocks(block->split.pLeft->pBuddy);
}
return 0;
}
uint32_t BuddyAllocator::ComputeLevelFromBlockSize(uint64_t blockSize) const {
// Every level in the buddy system can be indexed by order-n where n = log2(blockSize).
// However, mFreeList zero-indexed by level.
// For example, blockSize=4 is Level1 if MAX_BLOCK is 8.
return Log2(mMaxBlockSize) - Log2(blockSize);
}
uint64_t BuddyAllocator::GetNextFreeAlignedBlock(size_t allocationBlockLevel,
uint64_t alignment) const {
ASSERT(IsPowerOfTwo(alignment));
// The current level is the level that corresponds to the allocation size. The free list may
// not contain a block at that level until a larger one gets allocated (and splits).
// Continue to go up the tree until such a larger block exists.
//
// Even if the block exists at the level, it cannot be used if it's offset is unaligned.
// When the alignment is also a power-of-two, we simply use the next free block whose size
// is greater than or equal to the alignment value.
//
// After one 8-byte allocation:
//
// Level --------------------------------
// 0 32 | S |
// --------------------------------
// 1 16 | S | F2 | S - split
// -------------------------------- F - free
// 2 8 | Aa | F1 | | A - allocated
// --------------------------------
//
// Allocate(size=8, alignment=8) will be satisfied by using F1.
// Allocate(size=8, alignment=4) will be satified by using F1.
// Allocate(size=8, alignment=16) will be satisified by using F2.
//
for (size_t ii = 0; ii <= allocationBlockLevel; ++ii) {
size_t currLevel = allocationBlockLevel - ii;
BuddyBlock* freeBlock = mFreeLists[currLevel].head;
if (freeBlock && (freeBlock->mOffset % alignment == 0)) {
return currLevel;
}
}
return kInvalidOffset; // No free block exists at any level.
}
// Inserts existing free block into the free-list.
// Called by allocate upon splitting to insert a child block into a free-list.
// Note: Always insert into the head of the free-list. As when a larger free block at a lower
// level was split, there were no smaller free blocks at a higher level to allocate.
void BuddyAllocator::InsertFreeBlock(BuddyBlock* block, size_t level) {
ASSERT(block->mState == BlockState::Free);
// Inserted block is now the front (no prev).
block->free.pPrev = nullptr;
// Old head is now the inserted block's next.
block->free.pNext = mFreeLists[level].head;
// Block already in HEAD position (ex. right child was inserted first).
if (mFreeLists[level].head != nullptr) {
// Old head's previous is the inserted block.
mFreeLists[level].head->free.pPrev = block;
}
mFreeLists[level].head = block;
}
void BuddyAllocator::RemoveFreeBlock(BuddyBlock* block, size_t level) {
ASSERT(block->mState == BlockState::Free);
if (mFreeLists[level].head == block) {
// Block is in HEAD position.
mFreeLists[level].head = mFreeLists[level].head->free.pNext;
} else {
// Block is after HEAD position.
BuddyBlock* pPrev = block->free.pPrev;
BuddyBlock* pNext = block->free.pNext;
ASSERT(pPrev != nullptr);
ASSERT(pPrev->mState == BlockState::Free);
pPrev->free.pNext = pNext;
if (pNext != nullptr) {
ASSERT(pNext->mState == BlockState::Free);
pNext->free.pPrev = pPrev;
}
}
}
uint64_t BuddyAllocator::Allocate(uint64_t allocationSize, uint64_t alignment) {
if (allocationSize == 0 || allocationSize > mMaxBlockSize) {
return kInvalidOffset;
}
// Compute the level
const uint32_t allocationSizeToLevel = ComputeLevelFromBlockSize(allocationSize);
ASSERT(allocationSizeToLevel < mFreeLists.size());
uint64_t currBlockLevel = GetNextFreeAlignedBlock(allocationSizeToLevel, alignment);
// Error when no free blocks exist (allocator is full)
if (currBlockLevel == kInvalidOffset) {
return kInvalidOffset;
}
// Split free blocks level-by-level.
// Terminate when the current block level is equal to the computed level of the requested
// allocation.
BuddyBlock* currBlock = mFreeLists[currBlockLevel].head;
for (; currBlockLevel < allocationSizeToLevel; currBlockLevel++) {
ASSERT(currBlock->mState == BlockState::Free);
// Remove curr block (about to be split).
RemoveFreeBlock(currBlock, currBlockLevel);
// Create two free child blocks (the buddies).
const uint64_t nextLevelSize = currBlock->mSize / 2;
BuddyBlock* leftChildBlock = new BuddyBlock(nextLevelSize, currBlock->mOffset);
BuddyBlock* rightChildBlock =
new BuddyBlock(nextLevelSize, currBlock->mOffset + nextLevelSize);
// Remember the parent to merge these back upon de-allocation.
rightChildBlock->pParent = currBlock;
leftChildBlock->pParent = currBlock;
// Make them buddies.
leftChildBlock->pBuddy = rightChildBlock;
rightChildBlock->pBuddy = leftChildBlock;
// Insert the children back into the free list into the next level.
// The free list does not require a specific order. However, an order is specified as
// it's ideal to allocate lower addresses first by having the leftmost child in HEAD.
InsertFreeBlock(rightChildBlock, currBlockLevel + 1);
InsertFreeBlock(leftChildBlock, currBlockLevel + 1);
// Curr block is now split.
currBlock->mState = BlockState::Split;
currBlock->split.pLeft = leftChildBlock;
// Decend down into the next level.
currBlock = leftChildBlock;
}
// Remove curr block from free-list (now allocated).
RemoveFreeBlock(currBlock, currBlockLevel);
currBlock->mState = BlockState::Allocated;
return currBlock->mOffset;
}
void BuddyAllocator::Deallocate(uint64_t offset) {
BuddyBlock* curr = mRoot;
// TODO(crbug.com/dawn/827): Optimize de-allocation.
// Passing allocationSize directly will avoid the following level-by-level search;
// however, it requires the size information to be stored outside the allocator.
// Search for the free block node that corresponds to the block offset.
size_t currBlockLevel = 0;
while (curr->mState == BlockState::Split) {
if (offset < curr->split.pLeft->pBuddy->mOffset) {
curr = curr->split.pLeft;
} else {
curr = curr->split.pLeft->pBuddy;
}
currBlockLevel++;
}
ASSERT(curr->mState == BlockState::Allocated);
// Ensure the block is at the correct level
ASSERT(currBlockLevel == ComputeLevelFromBlockSize(curr->mSize));
// Mark curr free so we can merge.
curr->mState = BlockState::Free;
// Merge the buddies (LevelN-to-Level0).
while (currBlockLevel > 0 && curr->pBuddy->mState == BlockState::Free) {
// Remove the buddy.
RemoveFreeBlock(curr->pBuddy, currBlockLevel);
BuddyBlock* parent = curr->pParent;
// The buddies were inserted in a specific order but
// could be deleted in any order.
DeleteBlock(curr->pBuddy);
DeleteBlock(curr);
// Parent is now free.
parent->mState = BlockState::Free;
// Ascend up to the next level (parent block).
curr = parent;
currBlockLevel--;
}
InsertFreeBlock(curr, currBlockLevel);
}
// Helper which deletes a block in the tree recursively (post-order).
void BuddyAllocator::DeleteBlock(BuddyBlock* block) {
ASSERT(block != nullptr);
if (block->mState == BlockState::Split) {
// Delete the pair in same order we inserted.
DeleteBlock(block->split.pLeft->pBuddy);
DeleteBlock(block->split.pLeft);
}
delete block;
}
} // namespace dawn::native