// 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_