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// Copyright 2022 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_COMMON_PTR_COMPR_H_
#define V8_COMMON_PTR_COMPR_H_
#include "src/base/memory.h"
#include "src/common/globals.h"
namespace v8::internal {
// This is just a collection of compression scheme related functions. Having
// such a class allows plugging different decompression scheme in certain
// places by introducing another CompressionScheme class with a customized
// implementation. This is useful, for example, for Code::code
// field (see InstructionStreamSlot).
class V8HeapCompressionScheme {
public:
V8_INLINE static Address GetPtrComprCageBaseAddress(Address on_heap_addr);
V8_INLINE static Address GetPtrComprCageBaseAddress(
PtrComprCageBase cage_base);
// Compresses full-pointer representation of a tagged value to on-heap
// representation.
// Must only be used for compressing object pointers since this function
// assumes that we deal with a valid address inside the pointer compression
// cage.
V8_INLINE static Tagged_t CompressObject(Address tagged);
// Compress a potentially invalid pointer.
V8_INLINE static Tagged_t CompressAny(Address tagged);
// Decompresses smi value.
V8_INLINE static Address DecompressTaggedSigned(Tagged_t raw_value);
// Decompresses any tagged value, preserving both weak- and smi- tags.
template <typename TOnHeapAddress>
V8_INLINE static Address DecompressTagged(TOnHeapAddress on_heap_addr,
Tagged_t raw_value);
// Given a 64bit raw value, found on the stack, calls the callback function
// with all possible pointers that may be "contained" in compressed form in
// this value, either as complete compressed pointers or as intermediate
// (half-computed) results.
template <typename ProcessPointerCallback>
V8_INLINE static void ProcessIntermediatePointers(
PtrComprCageBase cage_base, Address raw_value,
ProcessPointerCallback callback);
// Process-wide cage base value used for decompression.
V8_INLINE static void InitBase(Address base);
V8_INLINE static Address base();
private:
// These non-inlined accessors to base_ field are used in component builds
// where cross-component access to thread local variables is not allowed.
static V8_EXPORT_PRIVATE Address base_non_inlined();
static V8_EXPORT_PRIVATE void set_base_non_inlined(Address base);
#ifdef V8_COMPRESS_POINTERS_IN_SHARED_CAGE
static V8_EXPORT_PRIVATE uintptr_t base_ V8_CONSTINIT;
#else
static thread_local uintptr_t base_ V8_CONSTINIT;
#endif // V8_COMPRESS_POINTERS_IN_SHARED_CAGE
};
#ifdef V8_EXTERNAL_CODE_SPACE
// Compression scheme used for fields containing InstructionStream objects
// (namely for the Code::code field). Same as
// V8HeapCompressionScheme but with a different base value.
class ExternalCodeCompressionScheme {
public:
V8_INLINE static Address PrepareCageBaseAddress(Address on_heap_addr);
// Note that this compression scheme doesn't allow reconstruction of the cage
// base value from any arbitrary value, thus the cage base has to be passed
// explicitly to the decompression functions.
static Address GetPtrComprCageBaseAddress(Address on_heap_addr) = delete;
V8_INLINE static Address GetPtrComprCageBaseAddress(
PtrComprCageBase cage_base);
// Compresses full-pointer representation of a tagged value to on-heap
// representation.
// Must only be used for compressing object pointers (incl. SMI) since this
// function assumes pointers to be inside the pointer compression cage.
V8_INLINE static Tagged_t CompressObject(Address tagged);
// Compress anything that does not follow the above requirements (e.g. a maybe
// object, or a marker bit pattern).
V8_INLINE static Tagged_t CompressAny(Address tagged);
// Decompresses smi value.
V8_INLINE static Address DecompressTaggedSigned(Tagged_t raw_value);
// Decompresses any tagged value, preserving both weak- and smi- tags.
template <typename TOnHeapAddress>
V8_INLINE static Address DecompressTagged(TOnHeapAddress on_heap_addr,
Tagged_t raw_value);
// Process-wide cage base value used for decompression.
V8_INLINE static void InitBase(Address base);
V8_INLINE static Address base();
private:
// These non-inlined accessors to base_ field are used in component builds
// where cross-component access to thread local variables is not allowed.
static Address base_non_inlined();
static void set_base_non_inlined(Address base);
#ifdef V8_COMPRESS_POINTERS_IN_SHARED_CAGE
static V8_EXPORT_PRIVATE uintptr_t base_ V8_CONSTINIT;
#else
static thread_local uintptr_t base_ V8_CONSTINIT;
#endif // V8_COMPRESS_POINTERS_IN_SHARED_CAGE
};
#endif // V8_EXTERNAL_CODE_SPACE
// Accessors for fields that may be unaligned due to pointer compression.
template <typename V>
static inline V ReadMaybeUnalignedValue(Address p) {
// Pointer compression causes types larger than kTaggedSize to be unaligned.
#ifdef V8_COMPRESS_POINTERS
constexpr bool v8_pointer_compression_unaligned = sizeof(V) > kTaggedSize;
#else
constexpr bool v8_pointer_compression_unaligned = false;
#endif
// Bug(v8:8875) Double fields may be unaligned.
constexpr bool unaligned_double_field =
std::is_same<V, double>::value && kDoubleSize > kTaggedSize;
if (unaligned_double_field || v8_pointer_compression_unaligned) {
return base::ReadUnalignedValue<V>(p);
} else {
return base::Memory<V>(p);
}
}
template <typename V>
static inline void WriteMaybeUnalignedValue(Address p, V value) {
// Pointer compression causes types larger than kTaggedSize to be unaligned.
#ifdef V8_COMPRESS_POINTERS
constexpr bool v8_pointer_compression_unaligned = sizeof(V) > kTaggedSize;
#else
constexpr bool v8_pointer_compression_unaligned = false;
#endif
// Bug(v8:8875) Double fields may be unaligned.
constexpr bool unaligned_double_field =
std::is_same<V, double>::value && kDoubleSize > kTaggedSize;
if (unaligned_double_field || v8_pointer_compression_unaligned) {
base::WriteUnalignedValue<V>(p, value);
} else {
base::Memory<V>(p) = value;
}
}
// When multi-cage pointer compression mode is enabled this scope object
// saves current cage's base values and sets them according to given Isolate.
// For all other configurations this scope object is a no-op.
class PtrComprCageAccessScope final {
public:
#ifdef V8_COMPRESS_POINTERS_IN_ISOLATE_CAGE
V8_INLINE explicit PtrComprCageAccessScope(Isolate* isolate);
V8_INLINE ~PtrComprCageAccessScope();
#else
V8_INLINE explicit PtrComprCageAccessScope(Isolate* isolate) {}
V8_INLINE ~PtrComprCageAccessScope() {}
#endif
private:
#ifdef V8_COMPRESS_POINTERS_IN_ISOLATE_CAGE
const Address cage_base_;
#ifdef V8_EXTERNAL_CODE_SPACE
// In case this configuration is necessary the code cage base must be saved too.
#error Multi-cage pointer compression with external code space is not supported
#endif // V8_EXTERNAL_CODE_SPACE
#endif // V8_COMPRESS_POINTERS_IN_ISOLATE_CAGE
};
} // namespace v8::internal
#endif // V8_COMMON_PTR_COMPR_H_
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