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// Copyright 2020 the V8 project authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
#include "src/heap/cppgc/free-list.h"
#include <algorithm>
#include "include/cppgc/internal/logging.h"
#include "src/base/bits.h"
#include "src/heap/cppgc/globals.h"
#include "src/heap/cppgc/heap-object-header-inl.h"
#include "src/heap/cppgc/sanitizers.h"
namespace cppgc {
namespace internal {
namespace {
uint32_t BucketIndexForSize(uint32_t size) {
return v8::base::bits::WhichPowerOfTwo(
v8::base::bits::RoundDownToPowerOfTwo32(size));
}
} // namespace
class FreeList::Entry : public HeapObjectHeader {
public:
explicit Entry(size_t size) : HeapObjectHeader(size, kFreeListGCInfoIndex) {
static_assert(sizeof(Entry) == kFreeListEntrySize, "Sizes must match");
}
Entry* Next() const { return next_; }
void SetNext(Entry* next) { next_ = next; }
void Link(Entry** previous_next) {
next_ = *previous_next;
*previous_next = this;
}
void Unlink(Entry** previous_next) {
*previous_next = next_;
next_ = nullptr;
}
private:
Entry* next_ = nullptr;
};
FreeList::FreeList() { Clear(); }
FreeList::FreeList(FreeList&& other) V8_NOEXCEPT
: free_list_heads_(std::move(other.free_list_heads_)),
free_list_tails_(std::move(other.free_list_tails_)),
biggest_free_list_index_(std::move(other.biggest_free_list_index_)) {
other.Clear();
}
FreeList& FreeList::operator=(FreeList&& other) V8_NOEXCEPT {
Clear();
Append(std::move(other));
DCHECK(other.IsEmpty());
return *this;
}
void FreeList::Add(FreeList::Block block) {
const size_t size = block.size;
DCHECK_GT(kPageSize, size);
DCHECK_LE(sizeof(HeapObjectHeader), size);
if (block.size < sizeof(Entry)) {
// Create wasted entry. This can happen when an almost emptied linear
// allocation buffer is returned to the freelist.
new (block.address) HeapObjectHeader(size, kFreeListGCInfoIndex);
return;
}
// Make sure the freelist header is writable.
SET_MEMORY_ACCESIBLE(block.address, sizeof(Entry));
Entry* entry = new (block.address) Entry(size);
const size_t index = BucketIndexForSize(static_cast<uint32_t>(size));
entry->Link(&free_list_heads_[index]);
biggest_free_list_index_ = std::max(biggest_free_list_index_, index);
if (!entry->Next()) {
free_list_tails_[index] = entry;
}
}
void FreeList::Append(FreeList&& other) {
#if DEBUG
const size_t expected_size = Size() + other.Size();
#endif
// Newly created entries get added to the head.
for (size_t index = 0; index < free_list_tails_.size(); ++index) {
Entry* other_tail = other.free_list_tails_[index];
Entry*& this_head = this->free_list_heads_[index];
if (other_tail) {
other_tail->SetNext(this_head);
if (!this_head) {
this->free_list_tails_[index] = other_tail;
}
this_head = other.free_list_heads_[index];
other.free_list_heads_[index] = nullptr;
other.free_list_tails_[index] = nullptr;
}
}
biggest_free_list_index_ =
std::max(biggest_free_list_index_, other.biggest_free_list_index_);
other.biggest_free_list_index_ = 0;
#if DEBUG
DCHECK_EQ(expected_size, Size());
#endif
DCHECK(other.IsEmpty());
}
FreeList::Block FreeList::Allocate(size_t allocation_size) {
// Try reusing a block from the largest bin. The underlying reasoning
// being that we want to amortize this slow allocation call by carving
// off as a large a free block as possible in one go; a block that will
// service this block and let following allocations be serviced quickly
// by bump allocation.
// bucket_size represents minimal size of entries in a bucket.
size_t bucket_size = static_cast<size_t>(1) << biggest_free_list_index_;
size_t index = biggest_free_list_index_;
for (; index > 0; --index, bucket_size >>= 1) {
DCHECK(IsConsistent(index));
Entry* entry = free_list_heads_[index];
if (allocation_size > bucket_size) {
// Final bucket candidate; check initial entry if it is able
// to service this allocation. Do not perform a linear scan,
// as it is considered too costly.
if (!entry || entry->GetSize() < allocation_size) break;
}
if (entry) {
if (!entry->Next()) {
DCHECK_EQ(entry, free_list_tails_[index]);
free_list_tails_[index] = nullptr;
}
entry->Unlink(&free_list_heads_[index]);
biggest_free_list_index_ = index;
return {entry, entry->GetSize()};
}
}
biggest_free_list_index_ = index;
return {nullptr, 0u};
}
void FreeList::Clear() {
std::fill(free_list_heads_.begin(), free_list_heads_.end(), nullptr);
std::fill(free_list_tails_.begin(), free_list_tails_.end(), nullptr);
biggest_free_list_index_ = 0;
}
size_t FreeList::Size() const {
size_t size = 0;
for (auto* entry : free_list_heads_) {
while (entry) {
size += entry->GetSize();
entry = entry->Next();
}
}
return size;
}
bool FreeList::IsEmpty() const {
return std::all_of(free_list_heads_.cbegin(), free_list_heads_.cend(),
[](const auto* entry) { return !entry; });
}
bool FreeList::Contains(Block block) const {
for (Entry* list : free_list_heads_) {
for (Entry* entry = list; entry; entry = entry->Next()) {
if (entry <= block.address &&
(reinterpret_cast<Address>(block.address) + block.size <=
reinterpret_cast<Address>(entry) + entry->GetSize()))
return true;
}
}
return false;
}
bool FreeList::IsConsistent(size_t index) const {
// Check that freelist head and tail pointers are consistent, i.e.
// - either both are nulls (no entries in the bucket);
// - or both are non-nulls and the tail points to the end.
return (!free_list_heads_[index] && !free_list_tails_[index]) ||
(free_list_heads_[index] && free_list_tails_[index] &&
!free_list_tails_[index]->Next());
}
} // namespace internal
} // namespace cppgc
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