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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.
#ifndef RUNTIME_VM_POINTER_TAGGING_H_
#define RUNTIME_VM_POINTER_TAGGING_H_
#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 {
// 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;
// Discriminate between true and false based on the alignment bit.
static constexpr intptr_t kBoolValueBitPosition = kObjectAlignmentLog2;
static constexpr intptr_t kBoolValueMask = 1 << kBoolValueBitPosition;
// Discriminate between bool and null based on bit after the alignment bit.
static constexpr intptr_t kBoolVsNullBitPosition = kObjectAlignmentLog2 + 1;
static constexpr intptr_t kBoolVsNullMask = 1 << kBoolVsNullBitPosition;
static constexpr intptr_t kTrueOffsetFromNull = kObjectAlignment * 2;
static constexpr intptr_t kFalseOffsetFromNull = kObjectAlignment * 3;
};
using HostObjectAlignment = ObjectAlignment<kWordSize, kWordSizeLog2>;
static constexpr intptr_t kNewObjectAlignmentOffset =
HostObjectAlignment::kNewObjectAlignmentOffset;
static constexpr intptr_t kOldObjectAlignmentOffset =
HostObjectAlignment::kOldObjectAlignmentOffset;
static constexpr intptr_t kNewObjectBitPosition =
HostObjectAlignment::kNewObjectBitPosition;
static constexpr intptr_t kObjectAlignment =
HostObjectAlignment::kObjectAlignment;
static constexpr intptr_t kObjectAlignmentLog2 =
HostObjectAlignment::kObjectAlignmentLog2;
static constexpr intptr_t kObjectAlignmentMask =
HostObjectAlignment::kObjectAlignmentMask;
static constexpr intptr_t kBoolValueBitPosition =
HostObjectAlignment::kBoolValueBitPosition;
static constexpr intptr_t kBoolValueMask = HostObjectAlignment::kBoolValueMask;
static constexpr intptr_t kBoolVsNullBitPosition =
HostObjectAlignment::kBoolVsNullBitPosition;
static constexpr intptr_t kBoolVsNullMask =
HostObjectAlignment::kBoolVsNullMask;
static constexpr intptr_t kTrueOffsetFromNull =
HostObjectAlignment::kTrueOffsetFromNull;
static constexpr intptr_t kFalseOffsetFromNull =
HostObjectAlignment::kFalseOffsetFromNull;
static constexpr intptr_t kObjectStartAlignment = 64;
COMPILE_ASSERT(kObjectStartAlignment >= kObjectAlignment);
COMPILE_ASSERT(kObjectStartAlignment >= 2 * kBoolValueMask);
COMPILE_ASSERT(kObjectStartAlignment >= 2 * kBoolVsNullMask);
// 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,
};
#if !defined(DART_COMPRESSED_POINTERS)
static constexpr uintptr_t kHeapBaseMask = 0;
#else
static constexpr uintptr_t kHeapBaseMask = ~(4LL * GB - 1);
#endif
} // namespace dart
#endif // RUNTIME_VM_POINTER_TAGGING_H_