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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.
#ifndef V8_HEAP_CPPGC_OBJECT_START_BITMAP_H_
#define V8_HEAP_CPPGC_OBJECT_START_BITMAP_H_
#include <limits.h>
#include <stdint.h>
#include <array>
#include "include/cppgc/internal/write-barrier.h"
#include "src/base/atomic-utils.h"
#include "src/base/bits.h"
#include "src/base/macros.h"
#include "src/heap/cppgc/globals.h"
#include "src/heap/cppgc/heap-object-header.h"
namespace cppgc {
namespace internal {
// A bitmap for recording object starts. Objects have to be allocated at
// minimum granularity of kGranularity.
//
// Depends on internals such as:
// - kBlinkPageSize
// - kAllocationGranularity
//
// ObjectStartBitmap supports concurrent reads from multiple threads but
// only a single mutator thread can write to it.
class V8_EXPORT_PRIVATE ObjectStartBitmap {
public:
// Granularity of addresses added to the bitmap.
static constexpr size_t Granularity() { return kAllocationGranularity; }
// Maximum number of entries in the bitmap.
static constexpr size_t MaxEntries() {
return kReservedForBitmap * kBitsPerCell;
}
explicit inline ObjectStartBitmap(Address offset);
// Finds an object header based on a
// address_maybe_pointing_to_the_middle_of_object. Will search for an object
// start in decreasing address order.
template <AccessMode = AccessMode::kNonAtomic>
inline HeapObjectHeader* FindHeader(
ConstAddress address_maybe_pointing_to_the_middle_of_object) const;
template <AccessMode = AccessMode::kNonAtomic>
inline void SetBit(ConstAddress);
template <AccessMode = AccessMode::kNonAtomic>
inline void ClearBit(ConstAddress);
template <AccessMode = AccessMode::kNonAtomic>
inline bool CheckBit(ConstAddress) const;
// Iterates all object starts recorded in the bitmap.
//
// The callback is of type
// void(Address)
// and is passed the object start address as parameter.
template <typename Callback>
inline void Iterate(Callback) const;
// Clear the object start bitmap.
inline void Clear();
private:
template <AccessMode = AccessMode::kNonAtomic>
inline void store(size_t cell_index, uint8_t value);
template <AccessMode = AccessMode::kNonAtomic>
inline uint8_t load(size_t cell_index) const;
static constexpr size_t kBitsPerCell = sizeof(uint8_t) * CHAR_BIT;
static constexpr size_t kCellMask = kBitsPerCell - 1;
static constexpr size_t kBitmapSize =
(kPageSize + ((kBitsPerCell * kAllocationGranularity) - 1)) /
(kBitsPerCell * kAllocationGranularity);
static constexpr size_t kReservedForBitmap =
((kBitmapSize + kAllocationMask) & ~kAllocationMask);
inline void ObjectStartIndexAndBit(ConstAddress, size_t*, size_t*) const;
const Address offset_;
// The bitmap contains a bit for every kGranularity aligned address on a
// a NormalPage, i.e., for a page of size kBlinkPageSize.
std::array<uint8_t, kReservedForBitmap> object_start_bit_map_;
};
ObjectStartBitmap::ObjectStartBitmap(Address offset) : offset_(offset) {
Clear();
}
template <AccessMode mode>
HeapObjectHeader* ObjectStartBitmap::FindHeader(
ConstAddress address_maybe_pointing_to_the_middle_of_object) const {
DCHECK_LE(offset_, address_maybe_pointing_to_the_middle_of_object);
size_t object_offset =
address_maybe_pointing_to_the_middle_of_object - offset_;
size_t object_start_number = object_offset / kAllocationGranularity;
size_t cell_index = object_start_number / kBitsPerCell;
DCHECK_GT(object_start_bit_map_.size(), cell_index);
const size_t bit = object_start_number & kCellMask;
uint8_t byte = load<mode>(cell_index) & ((1 << (bit + 1)) - 1);
while (!byte && cell_index) {
DCHECK_LT(0u, cell_index);
byte = load<mode>(--cell_index);
}
const int leading_zeroes = v8::base::bits::CountLeadingZeros(byte);
object_start_number =
(cell_index * kBitsPerCell) + (kBitsPerCell - 1) - leading_zeroes;
object_offset = object_start_number * kAllocationGranularity;
return reinterpret_cast<HeapObjectHeader*>(object_offset + offset_);
}
template <AccessMode mode>
void ObjectStartBitmap::SetBit(ConstAddress header_address) {
size_t cell_index, object_bit;
ObjectStartIndexAndBit(header_address, &cell_index, &object_bit);
// Only a single mutator thread can write to the bitmap, so no need for CAS.
store<mode>(cell_index,
static_cast<uint8_t>(load(cell_index) | (1 << object_bit)));
}
template <AccessMode mode>
void ObjectStartBitmap::ClearBit(ConstAddress header_address) {
size_t cell_index, object_bit;
ObjectStartIndexAndBit(header_address, &cell_index, &object_bit);
store<mode>(cell_index,
static_cast<uint8_t>(load(cell_index) & ~(1 << object_bit)));
}
template <AccessMode mode>
bool ObjectStartBitmap::CheckBit(ConstAddress header_address) const {
size_t cell_index, object_bit;
ObjectStartIndexAndBit(header_address, &cell_index, &object_bit);
return load<mode>(cell_index) & (1 << object_bit);
}
template <AccessMode mode>
void ObjectStartBitmap::store(size_t cell_index, uint8_t value) {
if (mode == AccessMode::kNonAtomic) {
object_start_bit_map_[cell_index] = value;
return;
}
v8::base::AsAtomicPtr(&object_start_bit_map_[cell_index])
->store(value, std::memory_order_release);
}
template <AccessMode mode>
uint8_t ObjectStartBitmap::load(size_t cell_index) const {
if (mode == AccessMode::kNonAtomic) {
return object_start_bit_map_[cell_index];
}
return v8::base::AsAtomicPtr(&object_start_bit_map_[cell_index])
->load(std::memory_order_acquire);
}
void ObjectStartBitmap::ObjectStartIndexAndBit(ConstAddress header_address,
size_t* cell_index,
size_t* bit) const {
const size_t object_offset = header_address - offset_;
DCHECK(!(object_offset & kAllocationMask));
const size_t object_start_number = object_offset / kAllocationGranularity;
*cell_index = object_start_number / kBitsPerCell;
DCHECK_GT(kBitmapSize, *cell_index);
*bit = object_start_number & kCellMask;
}
template <typename Callback>
inline void ObjectStartBitmap::Iterate(Callback callback) const {
for (size_t cell_index = 0; cell_index < kReservedForBitmap; cell_index++) {
if (!object_start_bit_map_[cell_index]) continue;
uint8_t value = object_start_bit_map_[cell_index];
while (value) {
const int trailing_zeroes = v8::base::bits::CountTrailingZeros(value);
const size_t object_start_number =
(cell_index * kBitsPerCell) + trailing_zeroes;
const Address object_address =
offset_ + (kAllocationGranularity * object_start_number);
callback(object_address);
// Clear current object bit in temporary value to advance iteration.
value &= ~(1 << (object_start_number & kCellMask));
}
}
}
void ObjectStartBitmap::Clear() {
std::fill(object_start_bit_map_.begin(), object_start_bit_map_.end(), 0);
}
// A platform aware version of ObjectStartBitmap to provide platform specific
// optimizations (e.g. Use non-atomic stores on ARMv7 when not marking).
class V8_EXPORT_PRIVATE PlatformAwareObjectStartBitmap
: public ObjectStartBitmap {
public:
explicit inline PlatformAwareObjectStartBitmap(Address offset);
template <AccessMode = AccessMode::kNonAtomic>
inline void SetBit(ConstAddress);
template <AccessMode = AccessMode::kNonAtomic>
inline void ClearBit(ConstAddress);
private:
template <AccessMode>
static bool ShouldForceNonAtomic();
};
PlatformAwareObjectStartBitmap::PlatformAwareObjectStartBitmap(Address offset)
: ObjectStartBitmap(offset) {}
// static
template <AccessMode mode>
bool PlatformAwareObjectStartBitmap::ShouldForceNonAtomic() {
#if defined(V8_TARGET_ARCH_ARM)
// Use non-atomic accesses on ARMv7 when marking is not active.
if (mode == AccessMode::kAtomic) {
if (V8_LIKELY(!WriteBarrier::IsAnyIncrementalOrConcurrentMarking()))
return true;
}
#endif // defined(V8_TARGET_ARCH_ARM)
return false;
}
template <AccessMode mode>
void PlatformAwareObjectStartBitmap::SetBit(ConstAddress header_address) {
if (ShouldForceNonAtomic<mode>()) {
ObjectStartBitmap::SetBit<AccessMode::kNonAtomic>(header_address);
return;
}
ObjectStartBitmap::SetBit<mode>(header_address);
}
template <AccessMode mode>
void PlatformAwareObjectStartBitmap::ClearBit(ConstAddress header_address) {
if (ShouldForceNonAtomic<mode>()) {
ObjectStartBitmap::ClearBit<AccessMode::kNonAtomic>(header_address);
return;
}
ObjectStartBitmap::ClearBit<mode>(header_address);
}
} // namespace internal
} // namespace cppgc
#endif // V8_HEAP_CPPGC_OBJECT_START_BITMAP_H_
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