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// Copyright (c) 2019, the Dart project authors. Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.
#include "platform/assert.h"
#include "platform/globals.h"
// This header defines constants associated with pointer tagging:
// * which bits determine whether or not this is a Smi value or a heap
// pointer;
// * which bits determine whether this is a pointer into a new or an old
// space.
namespace dart {
class RawSmi;
// Dart VM aligns all objects by 2 words in in the old space and misaligns them
// in new space. This allows to distinguish new and old pointers by their bits.
// Note: these bits depend on the word size.
template <intptr_t word_size, intptr_t word_size_log2>
struct ObjectAlignment {
// Alignment offsets are used to determine object age.
static constexpr intptr_t kNewObjectAlignmentOffset = word_size;
static constexpr intptr_t kOldObjectAlignmentOffset = 0;
static constexpr intptr_t kNewObjectBitPosition = word_size_log2;
// Object sizes are aligned to kObjectAlignment.
static constexpr intptr_t kObjectAlignment = 2 * word_size;
static constexpr intptr_t kObjectAlignmentLog2 = word_size_log2 + 1;
static constexpr intptr_t kObjectAlignmentMask = kObjectAlignment - 1;
using HostObjectAlignment = ObjectAlignment<kWordSize, kWordSizeLog2>;
static constexpr intptr_t kNewObjectAlignmentOffset =
static constexpr intptr_t kOldObjectAlignmentOffset =
static constexpr intptr_t kNewObjectBitPosition =
static constexpr intptr_t kObjectAlignment =
static constexpr intptr_t kObjectAlignmentLog2 =
static constexpr intptr_t kObjectAlignmentMask =
// On all targets heap pointers are tagged by set least significant bit.
// To recover address of the actual heap object kHeapObjectTag needs to be
// subtracted from the tagged pointer value.
// Smi-s (small integers) have least significant bit cleared.
// To recover the integer value tagged pointer value needs to be shifted
// right by kSmiTagShift.
enum {
kSmiTag = 0,
kHeapObjectTag = 1,
kSmiTagSize = 1,
kSmiTagMask = 1,
kSmiTagShift = 1,
inline intptr_t ValueFromRawSmi(const RawSmi* raw_value) {
const intptr_t value = reinterpret_cast<intptr_t>(raw_value);
ASSERT((value & kSmiTagMask) == kSmiTag);
return (value >> kSmiTagShift);
} // namespace dart