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// Copyright 2021 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_LINEAR_ALLOCATION_AREA_H_
#define V8_HEAP_LINEAR_ALLOCATION_AREA_H_
// This header file is included outside of src/heap/.
// Avoid including src/heap/ internals.
#include "include/v8-internal.h"
#include "src/common/checks.h"
namespace v8 {
namespace internal {
// A linear allocation area to allocate objects from.
//
// Invariant that must hold at all times:
// start <= top <= limit
class LinearAllocationArea final {
public:
LinearAllocationArea() = default;
LinearAllocationArea(Address top, Address limit)
: start_(top), top_(top), limit_(limit) {
Verify();
}
void Reset(Address top, Address limit) {
start_ = top;
top_ = top;
limit_ = limit;
Verify();
}
void ResetStart() { start_ = top_; }
V8_INLINE bool CanIncrementTop(size_t bytes) const {
Verify();
return (top_ + bytes) <= limit_;
}
V8_INLINE Address IncrementTop(size_t bytes) {
Address old_top = top_;
top_ += bytes;
Verify();
return old_top;
}
V8_INLINE bool DecrementTopIfAdjacent(Address new_top, size_t bytes) {
Verify();
if ((new_top + bytes) == top_) {
top_ = new_top;
if (start_ > top_) {
ResetStart();
}
Verify();
return true;
}
return false;
}
V8_INLINE bool MergeIfAdjacent(LinearAllocationArea& other) {
Verify();
other.Verify();
if (top_ == other.limit_) {
top_ = other.top_;
start_ = other.start_;
other.Reset(kNullAddress, kNullAddress);
Verify();
return true;
}
return false;
}
V8_INLINE void SetLimit(Address limit) {
limit_ = limit;
Verify();
}
V8_INLINE Address start() const {
Verify();
return start_;
}
V8_INLINE Address top() const {
Verify();
return top_;
}
V8_INLINE Address limit() const {
Verify();
return limit_;
}
const Address* top_address() const { return &top_; }
Address* top_address() { return &top_; }
const Address* limit_address() const { return &limit_; }
Address* limit_address() { return &limit_; }
void Verify() const {
#ifdef DEBUG
SLOW_DCHECK(start_ <= top_);
SLOW_DCHECK(top_ <= limit_);
if (V8_COMPRESS_POINTERS_8GB_BOOL) {
SLOW_DCHECK(IsAligned(top_, kObjectAlignment8GbHeap));
} else {
SLOW_DCHECK(IsAligned(top_, kObjectAlignment));
}
#endif // DEBUG
}
static constexpr int kSize = 3 * kSystemPointerSize;
private:
// The start of the LAB. Initially coincides with `top_`. As top is moved
// ahead, the area [start_, top_[ denotes a range of new objects. This range
// is reset with `ResetStart()`.
Address start_ = kNullAddress;
// The top of the LAB that is used for allocation.
Address top_ = kNullAddress;
// Limit of the LAB the denotes the end of the valid range for allocation.
Address limit_ = kNullAddress;
};
static_assert(sizeof(LinearAllocationArea) == LinearAllocationArea::kSize,
"LinearAllocationArea's size must be small because it "
"is included in IsolateData.");
} // namespace internal
} // namespace v8
#endif // V8_HEAP_LINEAR_ALLOCATION_AREA_H_
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