chromium/v8/src/objects/compressed-slots.h

// Copyright 2019 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_OBJECTS_COMPRESSED_SLOTS_H_
#define V8_OBJECTS_COMPRESSED_SLOTS_H_

#include "include/v8config.h"
#include "src/common/globals.h"
#include "src/common/ptr-compr.h"
#include "src/objects/slots.h"
#include "src/objects/tagged-field.h"

namespace v8::internal {

#ifdef V8_COMPRESS_POINTERS

// A CompressedObjectSlot instance describes a kTaggedSize-sized field ("slot")
// holding a compressed tagged pointer (smi or heap object).
// Its address() is the address of the slot.
// The slot's contents can be read and written using operator* and store().
class CompressedObjectSlot : public SlotBase<CompressedObjectSlot, Tagged_t> { … };

// A CompressedMaybeObjectSlot instance describes a kTaggedSize-sized field
// ("slot") holding a possibly-weak compressed tagged pointer
// (think: Tagged<MaybeObject>).
// Its address() is the address of the slot.
// The slot's contents can be read and written using operator* and store().
class CompressedMaybeObjectSlot
    : public SlotBase<CompressedMaybeObjectSlot, Tagged_t> { … };

// A CompressedHeapObjectSlot instance describes a kTaggedSize-sized field
// ("slot") holding a weak or strong compressed pointer to a heap object (think:
// Tagged<HeapObjectReference>).
// Its address() is the address of the slot.
// The slot's contents can be read and written using operator* and store().
// In case it is known that that slot contains a strong heap object pointer,
// ToHeapObject() can be used to retrieve that heap object.
class CompressedHeapObjectSlot
    : public SlotBase<CompressedHeapObjectSlot, Tagged_t> { … };

// An OffHeapCompressedObjectSlot instance describes a kTaggedSize-sized field
// ("slot") holding a compressed tagged pointer (smi or heap object).
// Unlike CompressedObjectSlot, it does not assume that the slot is on the heap,
// and so does not provide an operator* with implicit Isolate* calculation.
// Its address() is the address of the slot.
// The slot's contents can be read and written using load() and store().
template <typename CompressionScheme>
class OffHeapCompressedObjectSlot
    : public SlotBase<OffHeapCompressedObjectSlot<CompressionScheme>,
                      Tagged_t> { … };

#endif  // V8_COMPRESS_POINTERS

}  // namespace v8::internal

#endif  // V8_OBJECTS_COMPRESSED_SLOTS_H_