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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 INCLUDE_CPPGC_INTERNAL_MEMBER_STORAGE_H_
#define INCLUDE_CPPGC_INTERNAL_MEMBER_STORAGE_H_
#include <atomic>
#include <cstddef>
#include <type_traits>
#include "cppgc/internal/api-constants.h"
#include "cppgc/internal/caged-heap.h"
#include "cppgc/internal/logging.h"
#include "cppgc/sentinel-pointer.h"
#include "v8config.h" // NOLINT(build/include_directory)
namespace cppgc {
namespace internal {
enum class WriteBarrierSlotType {
kCompressed,
kUncompressed,
};
#if defined(CPPGC_POINTER_COMPRESSION)
#if defined(__clang__)
// Attribute const allows the compiler to assume that CageBaseGlobal::g_base_
// doesn't change (e.g. across calls) and thereby avoid redundant loads.
#define CPPGC_CONST __attribute__((const))
#define CPPGC_REQUIRE_CONSTANT_INIT \
__attribute__((require_constant_initialization))
#else // defined(__clang__)
#define CPPGC_CONST
#define CPPGC_REQUIRE_CONSTANT_INIT
#endif // defined(__clang__)
class V8_EXPORT CageBaseGlobal final {
public:
V8_INLINE CPPGC_CONST static uintptr_t Get() {
CPPGC_DCHECK(IsBaseConsistent());
return g_base_.base;
}
V8_INLINE CPPGC_CONST static bool IsSet() {
CPPGC_DCHECK(IsBaseConsistent());
return (g_base_.base & ~kLowerHalfWordMask) != 0;
}
private:
// We keep the lower halfword as ones to speed up decompression.
static constexpr uintptr_t kLowerHalfWordMask =
(api_constants::kCagedHeapReservationAlignment - 1);
static union alignas(api_constants::kCachelineSize) Base {
uintptr_t base;
char cache_line[api_constants::kCachelineSize];
} g_base_ CPPGC_REQUIRE_CONSTANT_INIT;
CageBaseGlobal() = delete;
V8_INLINE static bool IsBaseConsistent() {
return kLowerHalfWordMask == (g_base_.base & kLowerHalfWordMask);
}
friend class CageBaseGlobalUpdater;
};
#undef CPPGC_REQUIRE_CONSTANT_INIT
#undef CPPGC_CONST
class V8_TRIVIAL_ABI CompressedPointer final {
public:
struct AtomicInitializerTag {};
using IntegralType = uint32_t;
static constexpr auto kWriteBarrierSlotType =
WriteBarrierSlotType::kCompressed;
V8_INLINE CompressedPointer() : value_(0u) {}
V8_INLINE explicit CompressedPointer(const void* value,
AtomicInitializerTag) {
StoreAtomic(value);
}
V8_INLINE explicit CompressedPointer(const void* ptr)
: value_(Compress(ptr)) {}
V8_INLINE explicit CompressedPointer(std::nullptr_t) : value_(0u) {}
V8_INLINE explicit CompressedPointer(SentinelPointer)
: value_(kCompressedSentinel) {}
V8_INLINE const void* Load() const { return Decompress(value_); }
V8_INLINE const void* LoadAtomic() const {
return Decompress(
reinterpret_cast<const std::atomic<IntegralType>&>(value_).load(
std::memory_order_relaxed));
}
V8_INLINE void Store(const void* ptr) { value_ = Compress(ptr); }
V8_INLINE void StoreAtomic(const void* value) {
reinterpret_cast<std::atomic<IntegralType>&>(value_).store(
Compress(value), std::memory_order_relaxed);
}
V8_INLINE void Clear() { value_ = 0u; }
V8_INLINE bool IsCleared() const { return !value_; }
V8_INLINE bool IsSentinel() const { return value_ == kCompressedSentinel; }
V8_INLINE uint32_t GetAsInteger() const { return value_; }
V8_INLINE friend bool operator==(CompressedPointer a, CompressedPointer b) {
return a.value_ == b.value_;
}
V8_INLINE friend bool operator!=(CompressedPointer a, CompressedPointer b) {
return a.value_ != b.value_;
}
V8_INLINE friend bool operator<(CompressedPointer a, CompressedPointer b) {
return a.value_ < b.value_;
}
V8_INLINE friend bool operator<=(CompressedPointer a, CompressedPointer b) {
return a.value_ <= b.value_;
}
V8_INLINE friend bool operator>(CompressedPointer a, CompressedPointer b) {
return a.value_ > b.value_;
}
V8_INLINE friend bool operator>=(CompressedPointer a, CompressedPointer b) {
return a.value_ >= b.value_;
}
static V8_INLINE IntegralType Compress(const void* ptr) {
static_assert(SentinelPointer::kSentinelValue ==
1 << api_constants::kPointerCompressionShift,
"The compression scheme relies on the sentinel encoded as 1 "
"<< kPointerCompressionShift");
static constexpr size_t kGigaCageMask =
~(api_constants::kCagedHeapReservationAlignment - 1);
static constexpr size_t kPointerCompressionShiftMask =
(1 << api_constants::kPointerCompressionShift) - 1;
CPPGC_DCHECK(CageBaseGlobal::IsSet());
const uintptr_t base = CageBaseGlobal::Get();
CPPGC_DCHECK(!ptr || ptr == kSentinelPointer ||
(base & kGigaCageMask) ==
(reinterpret_cast<uintptr_t>(ptr) & kGigaCageMask));
CPPGC_DCHECK(
(reinterpret_cast<uintptr_t>(ptr) & kPointerCompressionShiftMask) == 0);
const auto uptr = reinterpret_cast<uintptr_t>(ptr);
// Shift the pointer and truncate.
auto compressed = static_cast<IntegralType>(
uptr >> api_constants::kPointerCompressionShift);
// Normal compressed pointers must have the MSB set. This is guaranteed by
// the cage alignment.
CPPGC_DCHECK((!compressed || compressed == kCompressedSentinel) ||
(compressed & (1 << 31)));
return compressed;
}
static V8_INLINE void* Decompress(IntegralType ptr) {
CPPGC_DCHECK(CageBaseGlobal::IsSet());
const uintptr_t base = CageBaseGlobal::Get();
return Decompress(ptr, base);
}
static V8_INLINE void* Decompress(IntegralType ptr, uintptr_t base) {
CPPGC_DCHECK(CageBaseGlobal::IsSet());
CPPGC_DCHECK(base == CageBaseGlobal::Get());
// Sign-extend compressed pointer to full width. This ensure that normal
// pointers have only 1s in the base part of the address. It's also
// important to shift the unsigned value, as otherwise it would result in
// undefined behavior.
const uint64_t mask = static_cast<uint64_t>(static_cast<int32_t>(ptr))
<< api_constants::kPointerCompressionShift;
// Set the base part of the address for normal compressed pointers. Note
// that nullptr and the sentinel value do not have 1s in the base part and
// remain as-is in this operation.
return reinterpret_cast<void*>(mask & base);
}
// For a given memory `address`, this method iterates all possible pointers
// that can be reasonably recovered with the current compression scheme and
// passes them to `callback`.
template <typename Callback>
static V8_INLINE void VisitPossiblePointers(const void* address,
Callback callback);
private:
static constexpr IntegralType kCompressedSentinel =
SentinelPointer::kSentinelValue >>
api_constants::kPointerCompressionShift;
// All constructors initialize `value_`. Do not add a default value here as it
// results in a non-atomic write on some builds, even when the atomic version
// of the constructor is used.
IntegralType value_;
};
template <typename Callback>
// static
void CompressedPointer::VisitPossiblePointers(const void* address,
Callback callback) {
const uintptr_t base = CageBaseGlobal::Get();
CPPGC_DCHECK(base);
// We may have random compressed pointers on stack (e.g. due to inlined
// collections). These could be present in both halfwords.
const uint32_t compressed_low =
static_cast<uint32_t>(reinterpret_cast<uintptr_t>(address));
callback(CompressedPointer::Decompress(compressed_low, base));
const uint32_t compressed_high = static_cast<uint32_t>(
reinterpret_cast<uintptr_t>(address) >> (sizeof(uint32_t) * CHAR_BIT));
callback(CompressedPointer::Decompress(compressed_high, base));
// Iterate possible intermediate values, see `Decompress()`. The intermediate
// value of decompressing is a 64-bit value where 35 bits are the offset. We
// don't assume sign extension is stored and recover that part.
//
// Note that this case conveniently also recovers the full pointer.
static constexpr uintptr_t kBitForIntermediateValue =
(sizeof(uint32_t) * CHAR_BIT) + api_constants::kPointerCompressionShift;
static constexpr uintptr_t kSignExtensionMask =
~((uintptr_t{1} << kBitForIntermediateValue) - 1);
const uintptr_t intermediate_sign_extended =
reinterpret_cast<uintptr_t>(address) | kSignExtensionMask;
callback(reinterpret_cast<void*>(intermediate_sign_extended & base));
}
#endif // defined(CPPGC_POINTER_COMPRESSION)
class V8_TRIVIAL_ABI RawPointer final {
public:
struct AtomicInitializerTag {};
using IntegralType = uintptr_t;
static constexpr auto kWriteBarrierSlotType =
WriteBarrierSlotType::kUncompressed;
V8_INLINE RawPointer() : ptr_(nullptr) {}
V8_INLINE explicit RawPointer(const void* ptr, AtomicInitializerTag) {
StoreAtomic(ptr);
}
V8_INLINE explicit RawPointer(const void* ptr) : ptr_(ptr) {}
V8_INLINE const void* Load() const { return ptr_; }
V8_INLINE const void* LoadAtomic() const {
return reinterpret_cast<const std::atomic<const void*>&>(ptr_).load(
std::memory_order_relaxed);
}
V8_INLINE void Store(const void* ptr) { ptr_ = ptr; }
V8_INLINE void StoreAtomic(const void* ptr) {
reinterpret_cast<std::atomic<const void*>&>(ptr_).store(
ptr, std::memory_order_relaxed);
}
V8_INLINE void Clear() { ptr_ = nullptr; }
V8_INLINE bool IsCleared() const { return !ptr_; }
V8_INLINE bool IsSentinel() const { return ptr_ == kSentinelPointer; }
V8_INLINE uintptr_t GetAsInteger() const {
return reinterpret_cast<uintptr_t>(ptr_);
}
V8_INLINE friend bool operator==(RawPointer a, RawPointer b) {
return a.ptr_ == b.ptr_;
}
V8_INLINE friend bool operator!=(RawPointer a, RawPointer b) {
return a.ptr_ != b.ptr_;
}
V8_INLINE friend bool operator<(RawPointer a, RawPointer b) {
return a.ptr_ < b.ptr_;
}
V8_INLINE friend bool operator<=(RawPointer a, RawPointer b) {
return a.ptr_ <= b.ptr_;
}
V8_INLINE friend bool operator>(RawPointer a, RawPointer b) {
return a.ptr_ > b.ptr_;
}
V8_INLINE friend bool operator>=(RawPointer a, RawPointer b) {
return a.ptr_ >= b.ptr_;
}
template <typename Callback>
static V8_INLINE void VisitPossiblePointers(const void* address,
Callback callback) {
// Pass along the full pointer.
return callback(const_cast<void*>(address));
}
private:
// All constructors initialize `ptr_`. Do not add a default value here as it
// results in a non-atomic write on some builds, even when the atomic version
// of the constructor is used.
const void* ptr_;
};
#if defined(CPPGC_POINTER_COMPRESSION)
using DefaultMemberStorage = CompressedPointer;
#else // !defined(CPPGC_POINTER_COMPRESSION)
using DefaultMemberStorage = RawPointer;
#endif // !defined(CPPGC_POINTER_COMPRESSION)
} // namespace internal
} // namespace cppgc
#endif // INCLUDE_CPPGC_INTERNAL_MEMBER_STORAGE_H_
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