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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_OBJECTS_TAGGED_INDEX_H_
#define V8_OBJECTS_TAGGED_INDEX_H_
#include "src/common/globals.h"
#include "src/objects/heap-object.h"
// Has to be the last include (doesn't have include guards):
#include "src/objects/object-macros.h"
namespace v8 {
namespace internal {
// TaggedIndex represents integer values that can be stored in 31 bits.
// The on 32-bit architectures ptr_ value has the following format:
// [31 bit signed int] 0
// The on 64-bit architectures ptr_ value has the following format:
// [32 bits of sign-extended lower part][31 bit signed int] 0
// Thus, on 32-bit architectures TaggedIndex is exactly the same as Smi but
// on 64-bit architectures TaggedIndex differs from Smi in the following
// aspects:
// 1) TaggedIndex payload is always 31 bit independent of the Smi payload size
// 2) TaggedIndex is always properly sign-extended independent of whether
// pointer compression is enabled or not. In the former case, upper 32 bits
// of a Smi value may contain 0 or sign or isolate root value.
//
// Given the above constraints TaggedIndex has the following properties:
// 1) it still looks like a Smi from GC point of view and therefore it's safe
// to pass TaggedIndex values to runtime functions or builtins on the stack
// 2) since the TaggedIndex values are already properly sign-extended it's
// safe to use them as indices in offset-computation functions.
class TaggedIndex : public Object {
public:
// This replaces the OBJECT_CONSTRUCTORS macro, because TaggedIndex are
// special in that we want them to be constexprs.
constexpr TaggedIndex() : Object() {}
explicit constexpr TaggedIndex(Address ptr) : Object(ptr) {
CONSTEXPR_DCHECK(HAS_SMI_TAG(ptr));
}
// Returns the integer value.
inline intptr_t value() const {
// Truncate and shift down (requires >> to be sign extending).
return static_cast<intptr_t>(ptr()) >> kSmiTagSize;
}
// Convert a value to a TaggedIndex object.
static inline TaggedIndex FromIntptr(intptr_t value) {
CONSTEXPR_DCHECK(TaggedIndex::IsValid(value));
return TaggedIndex((static_cast<Address>(value) << kSmiTagSize) | kSmiTag);
}
// Returns whether value can be represented in a TaggedIndex.
static inline bool constexpr IsValid(intptr_t value) {
return kMinValue <= value && value <= kMaxValue;
}
DECL_CAST(TaggedIndex)
// Dispatched behavior.
DECL_VERIFIER(TaggedIndex)
STATIC_ASSERT(kSmiTagSize == 1);
static constexpr int kTaggedValueSize = 31;
static constexpr intptr_t kMinValue =
static_cast<intptr_t>(kUintptrAllBitsSet << (kTaggedValueSize - 1));
static constexpr intptr_t kMaxValue = -(kMinValue + 1);
};
CAST_ACCESSOR(TaggedIndex)
} // namespace internal
} // namespace v8
#include "src/objects/object-macros-undef.h"
#endif // V8_OBJECTS_TAGGED_INDEX_H_
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