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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_OBJECT_START_BITMAP_H_
#define V8_HEAP_OBJECT_START_BITMAP_H_
#include <limits.h>
#include <stdint.h>
#include <array>
#include "include/v8-internal.h"
#include "src/base/bits.h"
#include "src/base/macros.h"
#include "src/common/globals.h"
namespace v8 {
namespace internal {
static constexpr size_t kAllocationGranularity = kTaggedSize;
static constexpr size_t kAllocationMask = kAllocationGranularity - 1;
static const int kPageSize = 1 << kPageSizeBits;
// A bitmap for recording object starts. Objects have to be allocated at
// minimum granularity of kGranularity.
//
// Depends on internals such as:
// - kPageSize
// - kAllocationGranularity
//
// ObjectStartBitmap does not support concurrent access and is used only by the
// main thread.
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(size_t offset = 0);
// Finds an object header based on a maybe_inner_ptr. Will search for an
// object start in decreasing address order.
//
// This must only be used when there exists at least one entry in the bitmap.
inline Address FindBasePtr(Address maybe_inner_ptr) const;
inline void SetBit(Address);
inline void ClearBit(Address);
inline bool CheckBit(Address) 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:
inline void store(size_t cell_index, uint32_t value);
inline uint32_t load(size_t cell_index) const;
inline Address offset() const;
static constexpr size_t kBitsPerCell = sizeof(uint32_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(Address, size_t*, size_t*) const;
inline Address StartIndexToAddress(size_t object_start_index) const;
size_t offset_;
std::array<uint32_t, kReservedForBitmap> object_start_bit_map_;
};
ObjectStartBitmap::ObjectStartBitmap(size_t offset) : offset_(offset) {
Clear();
}
Address ObjectStartBitmap::FindBasePtr(Address maybe_inner_ptr) const {
DCHECK_LE(offset(), maybe_inner_ptr);
size_t object_offset = maybe_inner_ptr - 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;
// check if maybe_inner_ptr is the base pointer
uint32_t byte = load(cell_index) & ((1 << (bit + 1)) - 1);
while (!byte && cell_index) {
DCHECK_LT(0u, cell_index);
byte = load(--cell_index);
}
const int leading_zeroes = v8::base::bits::CountLeadingZeros(byte);
if (leading_zeroes == kBitsPerCell) {
return kNullAddress;
}
object_start_number =
(cell_index * kBitsPerCell) + (kBitsPerCell - 1) - leading_zeroes;
Address base_ptr = StartIndexToAddress(object_start_number);
return base_ptr;
}
void ObjectStartBitmap::SetBit(Address base_ptr) {
size_t cell_index, object_bit;
ObjectStartIndexAndBit(base_ptr, &cell_index, &object_bit);
store(cell_index,
static_cast<uint32_t>(load(cell_index) | (1 << object_bit)));
}
void ObjectStartBitmap::ClearBit(Address base_ptr) {
size_t cell_index, object_bit;
ObjectStartIndexAndBit(base_ptr, &cell_index, &object_bit);
store(cell_index,
static_cast<uint32_t>(load(cell_index) & ~(1 << object_bit)));
}
bool ObjectStartBitmap::CheckBit(Address base_ptr) const {
size_t cell_index, object_bit;
ObjectStartIndexAndBit(base_ptr, &cell_index, &object_bit);
return load(cell_index) & (1 << object_bit);
}
void ObjectStartBitmap::store(size_t cell_index, uint32_t value) {
object_start_bit_map_[cell_index] = value;
return;
}
uint32_t ObjectStartBitmap::load(size_t cell_index) const {
return object_start_bit_map_[cell_index];
}
Address ObjectStartBitmap::offset() const { return offset_; }
void ObjectStartBitmap::ObjectStartIndexAndBit(Address base_ptr,
size_t* cell_index,
size_t* bit) const {
const size_t object_offset = base_ptr - 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;
}
Address ObjectStartBitmap::StartIndexToAddress(
size_t object_start_index) const {
return offset() + (kAllocationGranularity * object_start_index);
}
template <typename Callback>
inline void ObjectStartBitmap::Iterate(Callback callback) const {
for (size_t cell_index = 0; cell_index < kReservedForBitmap; cell_index++) {
uint32_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 = StartIndexToAddress(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);
}
} // namespace internal
} // namespace v8
#endif // V8_HEAP_OBJECT_START_BITMAP_H_
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