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// Copyright 2010 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.
// The routines exported by this module are subtle. If you use them, even if
// you get the code right, it will depend on careful reasoning about atomicity
// and memory ordering; it will be less readable, and harder to maintain. If
// you plan to use these routines, you should have a good reason, such as solid
// evidence that performance would otherwise suffer, or there being no
// alternative. You should assume only properties explicitly guaranteed by the
// specifications in this file. You are almost certainly _not_ writing code
// just for the x86; if you assume x86 semantics, x86 hardware bugs and
// implementations on other archtectures will cause your code to break. If you
// do not know what you are doing, avoid these routines, and use a Mutex.
//
// It is incorrect to make direct assignments to/from an atomic variable.
// You should use one of the Load or Store routines. The Relaxed versions
// are provided when no fences are needed:
// Relaxed_Store()
// Relaxed_Load()
// Although there are currently no compiler enforcement, you are encouraged
// to use these.
//
#ifndef V8_BASE_ATOMICOPS_H_
#define V8_BASE_ATOMICOPS_H_
#include <stdint.h>
#include <atomic>
// Small C++ header which defines implementation specific macros used to
// identify the STL implementation.
// - libc++: captures __config for _LIBCPP_VERSION
// - libstdc++: captures bits/c++config.h for __GLIBCXX__
#include <cstddef>
#include "src/base/base-export.h"
#include "src/base/build_config.h"
#include "src/base/macros.h"
#if defined(V8_OS_STARBOARD)
#include "starboard/atomic.h"
#endif // V8_OS_STARBOARD
namespace v8 {
namespace base {
#ifdef V8_OS_STARBOARD
using Atomic8 = SbAtomic8;
using Atomic16 = int16_t;
using Atomic32 = SbAtomic32;
#if SB_IS_64_BIT
using Atomic64 = SbAtomic64;
#endif
#else
using Atomic8 = char;
using Atomic16 = int16_t;
using Atomic32 = int32_t;
#if defined(V8_HOST_ARCH_64_BIT)
// We need to be able to go between Atomic64 and AtomicWord implicitly. This
// means Atomic64 and AtomicWord should be the same type on 64-bit.
#if defined(__ILP32__)
using Atomic64 = int64_t;
#else
using Atomic64 = intptr_t;
#endif // defined(__ILP32__)
#endif // defined(V8_HOST_ARCH_64_BIT)
#endif // V8_OS_STARBOARD
// Use AtomicWord for a machine-sized pointer. It will use the Atomic32 or
// Atomic64 routines below, depending on your architecture.
#if defined(V8_OS_STARBOARD)
using AtomicWord = SbAtomicPtr;
#else
using AtomicWord = intptr_t;
#endif
namespace helper {
template <typename T>
volatile std::atomic<T>* to_std_atomic(volatile T* ptr) {
return reinterpret_cast<volatile std::atomic<T>*>(ptr);
}
template <typename T>
volatile const std::atomic<T>* to_std_atomic_const(volatile const T* ptr) {
return reinterpret_cast<volatile const std::atomic<T>*>(ptr);
}
} // namespace helper
inline void SeqCst_MemoryFence() {
std::atomic_thread_fence(std::memory_order_seq_cst);
}
// Atomically execute:
// result = *ptr;
// if (result == old_value)
// *ptr = new_value;
// return result;
//
// I.e. replace |*ptr| with |new_value| if |*ptr| used to be |old_value|.
// Always return the value of |*ptr| before the operation.
// Acquire, Relaxed, Release correspond to standard C++ memory orders.
inline Atomic8 Relaxed_CompareAndSwap(volatile Atomic8* ptr, Atomic8 old_value,
Atomic8 new_value) {
std::atomic_compare_exchange_strong_explicit(
helper::to_std_atomic(ptr), &old_value, new_value,
std::memory_order_relaxed, std::memory_order_relaxed);
return old_value;
}
inline Atomic16 Relaxed_CompareAndSwap(volatile Atomic16* ptr,
Atomic16 old_value, Atomic16 new_value) {
std::atomic_compare_exchange_strong_explicit(
helper::to_std_atomic(ptr), &old_value, new_value,
std::memory_order_relaxed, std::memory_order_relaxed);
return old_value;
}
inline Atomic32 Relaxed_CompareAndSwap(volatile Atomic32* ptr,
Atomic32 old_value, Atomic32 new_value) {
std::atomic_compare_exchange_strong_explicit(
helper::to_std_atomic(ptr), &old_value, new_value,
std::memory_order_relaxed, std::memory_order_relaxed);
return old_value;
}
inline Atomic32 Relaxed_AtomicExchange(volatile Atomic32* ptr,
Atomic32 new_value) {
return std::atomic_exchange_explicit(helper::to_std_atomic(ptr), new_value,
std::memory_order_relaxed);
}
inline Atomic32 Relaxed_AtomicIncrement(volatile Atomic32* ptr,
Atomic32 increment) {
return increment + std::atomic_fetch_add_explicit(helper::to_std_atomic(ptr),
increment,
std::memory_order_relaxed);
}
inline Atomic32 Acquire_CompareAndSwap(volatile Atomic32* ptr,
Atomic32 old_value, Atomic32 new_value) {
atomic_compare_exchange_strong_explicit(
helper::to_std_atomic(ptr), &old_value, new_value,
std::memory_order_acquire, std::memory_order_acquire);
return old_value;
}
inline Atomic8 Release_CompareAndSwap(volatile Atomic8* ptr, Atomic8 old_value,
Atomic8 new_value) {
bool result = atomic_compare_exchange_strong_explicit(
helper::to_std_atomic(ptr), &old_value, new_value,
std::memory_order_release, std::memory_order_relaxed);
USE(result); // Make gcc compiler happy.
return old_value;
}
inline Atomic32 Release_CompareAndSwap(volatile Atomic32* ptr,
Atomic32 old_value, Atomic32 new_value) {
atomic_compare_exchange_strong_explicit(
helper::to_std_atomic(ptr), &old_value, new_value,
std::memory_order_release, std::memory_order_relaxed);
return old_value;
}
inline Atomic32 AcquireRelease_CompareAndSwap(volatile Atomic32* ptr,
Atomic32 old_value,
Atomic32 new_value) {
atomic_compare_exchange_strong_explicit(
helper::to_std_atomic(ptr), &old_value, new_value,
std::memory_order_acq_rel, std::memory_order_acquire);
return old_value;
}
inline void Relaxed_Store(volatile Atomic8* ptr, Atomic8 value) {
std::atomic_store_explicit(helper::to_std_atomic(ptr), value,
std::memory_order_relaxed);
}
inline void Relaxed_Store(volatile Atomic16* ptr, Atomic16 value) {
std::atomic_store_explicit(helper::to_std_atomic(ptr), value,
std::memory_order_relaxed);
}
inline void Relaxed_Store(volatile Atomic32* ptr, Atomic32 value) {
std::atomic_store_explicit(helper::to_std_atomic(ptr), value,
std::memory_order_relaxed);
}
inline void Release_Store(volatile Atomic8* ptr, Atomic8 value) {
std::atomic_store_explicit(helper::to_std_atomic(ptr), value,
std::memory_order_release);
}
inline void Release_Store(volatile Atomic32* ptr, Atomic32 value) {
std::atomic_store_explicit(helper::to_std_atomic(ptr), value,
std::memory_order_release);
}
inline Atomic8 Relaxed_Load(volatile const Atomic8* ptr) {
return std::atomic_load_explicit(helper::to_std_atomic_const(ptr),
std::memory_order_relaxed);
}
inline Atomic16 Relaxed_Load(volatile const Atomic16* ptr) {
return std::atomic_load_explicit(helper::to_std_atomic_const(ptr),
std::memory_order_relaxed);
}
inline Atomic32 Relaxed_Load(volatile const Atomic32* ptr) {
return std::atomic_load_explicit(helper::to_std_atomic_const(ptr),
std::memory_order_relaxed);
}
inline Atomic8 Acquire_Load(volatile const Atomic8* ptr) {
return std::atomic_load_explicit(helper::to_std_atomic_const(ptr),
std::memory_order_acquire);
}
inline Atomic32 Acquire_Load(volatile const Atomic32* ptr) {
return std::atomic_load_explicit(helper::to_std_atomic_const(ptr),
std::memory_order_acquire);
}
#if defined(V8_HOST_ARCH_64_BIT)
inline Atomic64 Relaxed_CompareAndSwap(volatile Atomic64* ptr,
Atomic64 old_value, Atomic64 new_value) {
std::atomic_compare_exchange_strong_explicit(
helper::to_std_atomic(ptr), &old_value, new_value,
std::memory_order_relaxed, std::memory_order_relaxed);
return old_value;
}
inline Atomic64 Relaxed_AtomicExchange(volatile Atomic64* ptr,
Atomic64 new_value) {
return std::atomic_exchange_explicit(helper::to_std_atomic(ptr), new_value,
std::memory_order_relaxed);
}
inline Atomic64 Relaxed_AtomicIncrement(volatile Atomic64* ptr,
Atomic64 increment) {
return increment + std::atomic_fetch_add_explicit(helper::to_std_atomic(ptr),
increment,
std::memory_order_relaxed);
}
inline Atomic64 Acquire_CompareAndSwap(volatile Atomic64* ptr,
Atomic64 old_value, Atomic64 new_value) {
std::atomic_compare_exchange_strong_explicit(
helper::to_std_atomic(ptr), &old_value, new_value,
std::memory_order_acquire, std::memory_order_acquire);
return old_value;
}
inline Atomic64 Release_CompareAndSwap(volatile Atomic64* ptr,
Atomic64 old_value, Atomic64 new_value) {
std::atomic_compare_exchange_strong_explicit(
helper::to_std_atomic(ptr), &old_value, new_value,
std::memory_order_release, std::memory_order_relaxed);
return old_value;
}
inline Atomic64 AcquireRelease_CompareAndSwap(volatile Atomic64* ptr,
Atomic64 old_value,
Atomic64 new_value) {
std::atomic_compare_exchange_strong_explicit(
helper::to_std_atomic(ptr), &old_value, new_value,
std::memory_order_acq_rel, std::memory_order_acquire);
return old_value;
}
inline void Relaxed_Store(volatile Atomic64* ptr, Atomic64 value) {
std::atomic_store_explicit(helper::to_std_atomic(ptr), value,
std::memory_order_relaxed);
}
inline void Release_Store(volatile Atomic64* ptr, Atomic64 value) {
std::atomic_store_explicit(helper::to_std_atomic(ptr), value,
std::memory_order_release);
}
inline Atomic64 Relaxed_Load(volatile const Atomic64* ptr) {
return std::atomic_load_explicit(helper::to_std_atomic_const(ptr),
std::memory_order_relaxed);
}
inline Atomic64 Acquire_Load(volatile const Atomic64* ptr) {
return std::atomic_load_explicit(helper::to_std_atomic_const(ptr),
std::memory_order_acquire);
}
#endif // defined(V8_HOST_ARCH_64_BIT)
inline void Relaxed_Memcpy(volatile Atomic8* dst, volatile const Atomic8* src,
size_t bytes) {
constexpr size_t kAtomicWordSize = sizeof(AtomicWord);
while (bytes > 0 &&
!IsAligned(reinterpret_cast<uintptr_t>(dst), kAtomicWordSize)) {
Relaxed_Store(dst++, Relaxed_Load(src++));
--bytes;
}
if (IsAligned(reinterpret_cast<uintptr_t>(src), kAtomicWordSize) &&
IsAligned(reinterpret_cast<uintptr_t>(dst), kAtomicWordSize)) {
while (bytes >= kAtomicWordSize) {
Relaxed_Store(
reinterpret_cast<volatile AtomicWord*>(dst),
Relaxed_Load(reinterpret_cast<const volatile AtomicWord*>(src)));
dst += kAtomicWordSize;
src += kAtomicWordSize;
bytes -= kAtomicWordSize;
}
}
while (bytes > 0) {
Relaxed_Store(dst++, Relaxed_Load(src++));
--bytes;
}
}
inline void Relaxed_Memmove(volatile Atomic8* dst, volatile const Atomic8* src,
size_t bytes) {
// Use Relaxed_Memcpy if copying forwards is safe. This is the case if there
// is no overlap, or {dst} lies before {src}.
// This single check checks for both:
if (reinterpret_cast<uintptr_t>(dst) - reinterpret_cast<uintptr_t>(src) >=
bytes) {
Relaxed_Memcpy(dst, src, bytes);
return;
}
// Otherwise copy backwards.
dst += bytes;
src += bytes;
constexpr size_t kAtomicWordSize = sizeof(AtomicWord);
while (bytes > 0 &&
!IsAligned(reinterpret_cast<uintptr_t>(dst), kAtomicWordSize)) {
Relaxed_Store(--dst, Relaxed_Load(--src));
--bytes;
}
if (IsAligned(reinterpret_cast<uintptr_t>(src), kAtomicWordSize) &&
IsAligned(reinterpret_cast<uintptr_t>(dst), kAtomicWordSize)) {
while (bytes >= kAtomicWordSize) {
dst -= kAtomicWordSize;
src -= kAtomicWordSize;
bytes -= kAtomicWordSize;
Relaxed_Store(
reinterpret_cast<volatile AtomicWord*>(dst),
Relaxed_Load(reinterpret_cast<const volatile AtomicWord*>(src)));
}
}
while (bytes > 0) {
Relaxed_Store(--dst, Relaxed_Load(--src));
--bytes;
}
}
} // namespace base
} // namespace v8
// On some platforms we need additional declarations to make
// AtomicWord compatible with our other Atomic* types.
#if defined(V8_OS_MACOSX) || defined(V8_OS_OPENBSD) || defined(V8_OS_AIX)
#include "src/base/atomicops_internals_atomicword_compat.h"
#endif
#endif // V8_BASE_ATOMICOPS_H_
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