Source/WTF/wtf/Atomics.h

/*
 * Copyright (C) 2007-2008, 2010, 2012-2013, 2015 Apple Inc. All rights reserved.
 * Copyright (C) 2007 Justin Haygood (jhaygood@reaktix.com)
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 *
 * 1.  Redistributions of source code must retain the above copyright
 *     notice, this list of conditions and the following disclaimer. 
 * 2.  Redistributions in binary form must reproduce the above copyright
 *     notice, this list of conditions and the following disclaimer in the
 *     documentation and/or other materials provided with the distribution. 
 * 3.  Neither the name of Apple Inc. ("Apple") nor the names of
 *     its contributors may be used to endorse or promote products derived
 *     from this software without specific prior written permission. 
 *
 * THIS SOFTWARE IS PROVIDED BY APPLE AND ITS CONTRIBUTORS "AS IS" AND ANY
 * EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
 * WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
 * DISCLAIMED. IN NO EVENT SHALL APPLE OR ITS CONTRIBUTORS BE LIABLE FOR ANY
 * DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
 * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
 * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
 * ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 *
 *
 * Note: The implementations of InterlockedIncrement and InterlockedDecrement are based
 * on atomic_increment and atomic_exchange_and_add from the Boost C++ Library. The license
 * is virtually identical to the Apple license above but is included here for completeness.
 *
 * Boost Software License - Version 1.0 - August 17th, 2003
 * 
 * Permission is hereby granted, free of charge, to any person or organization
 * obtaining a copy of the software and accompanying documentation covered by
 * this license (the "Software") to use, reproduce, display, distribute,
 * execute, and transmit the Software, and to prepare derivative works of the
 * Software, and to permit third-parties to whom the Software is furnished to
 * do so, all subject to the following:
 * 
 * The copyright notices in the Software and this entire statement, including
 * the above license grant, this restriction and the following disclaimer,
 * must be included in all copies of the Software, in whole or in part, and
 * all derivative works of the Software, unless such copies or derivative
 * works are solely in the form of machine-executable object code generated by
 * a source language processor.
 * 
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE, TITLE AND NON-INFRINGEMENT. IN NO EVENT
 * SHALL THE COPYRIGHT HOLDERS OR ANYONE DISTRIBUTING THE SOFTWARE BE LIABLE
 * FOR ANY DAMAGES OR OTHER LIABILITY, WHETHER IN CONTRACT, TORT OR OTHERWISE,
 * ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 * DEALINGS IN THE SOFTWARE.
 */

#ifndef Atomics_h
#define Atomics_h

#include <atomic>
#include <wtf/StdLibExtras.h>

#if OS(WINDOWS)
#if !COMPILER(GCC_OR_CLANG)
extern "C" void _ReadWriteBarrier(void);
#pragma intrinsic(_ReadWriteBarrier)
#endif
#include <windows.h>
#endif

namespace WTF {

// Atomic wraps around std::atomic with the sole purpose of making the compare_exchange
// operations not alter the expected value. This is more in line with how we typically
// use CAS in our code.
//
// Atomic is a struct without explicitly defined constructors so that it can be
// initialized at compile time.

template<typename T>
struct Atomic {
    // Don't pass a non-default value for the order parameter unless you really know
    // what you are doing and have thought about it very hard. The cost of seq_cst
    // is usually not high enough to justify the risk.

    T load(std::memory_order order = std::memory_order_seq_cst) const { return value.load(order); }

    void store(T desired, std::memory_order order = std::memory_order_seq_cst) { value.store(desired, order); }

    bool compareExchangeWeak(T expected, T desired, std::memory_order order = std::memory_order_seq_cst)
    {
#if OS(WINDOWS)
        // Windows makes strange assertions about the argument to compare_exchange_weak, and anyway,
        // Windows is X86 so seq_cst is cheap.
        order = std::memory_order_seq_cst;
#endif
        T expectedOrActual = expected;
        return value.compare_exchange_weak(expectedOrActual, desired, order);
    }

    bool compareExchangeStrong(T expected, T desired, std::memory_order order = std::memory_order_seq_cst)
    {
#if OS(WINDOWS)
        // See above.
        order = std::memory_order_seq_cst;
#endif
        T expectedOrActual = expected;
        return value.compare_exchange_strong(expectedOrActual, desired, order);
    }

    std::atomic<T> value;
};

// This is a weak CAS function that takes a direct pointer and has no portable fencing guarantees.
template<typename T>
inline bool weakCompareAndSwap(volatile T* location, T expected, T newValue)
{
    return bitwise_cast<Atomic<T>*>(location)->compareExchangeWeak(expected, newValue, std::memory_order_relaxed);
}

// Just a compiler fence. Has no effect on the hardware, but tells the compiler
// not to move things around this call. Should not affect the compiler's ability
// to do things like register allocation and code motion over pure operations.
inline void compilerFence()
{
#if OS(WINDOWS) && !COMPILER(GCC_OR_CLANG)
    _ReadWriteBarrier();
#else
    asm volatile("" ::: "memory");
#endif
}

#if CPU(ARM_THUMB2) || CPU(ARM64)

// Full memory fence. No accesses will float above this, and no accesses will sink
// below it.
inline void armV7_dmb()
{
    asm volatile("dmb sy" ::: "memory");
}

// Like the above, but only affects stores.
inline void armV7_dmb_st()
{
    asm volatile("dmb st" ::: "memory");
}

inline void loadLoadFence() { armV7_dmb(); }
inline void loadStoreFence() { armV7_dmb(); }
inline void storeLoadFence() { armV7_dmb(); }
inline void storeStoreFence() { armV7_dmb_st(); }
inline void memoryBarrierAfterLock() { armV7_dmb(); }
inline void memoryBarrierBeforeUnlock() { armV7_dmb(); }

#elif CPU(X86) || CPU(X86_64)

inline void x86_mfence()
{
#if OS(WINDOWS)
    // I think that this does the equivalent of a dummy interlocked instruction,
    // instead of using the 'mfence' instruction, at least according to MSDN. I
    // know that it is equivalent for our purposes, but it would be good to
    // investigate if that is actually better.
    MemoryBarrier();
#else
    asm volatile("mfence" ::: "memory");
#endif
}

inline void loadLoadFence() { compilerFence(); }
inline void loadStoreFence() { compilerFence(); }
inline void storeLoadFence() { x86_mfence(); }
inline void storeStoreFence() { compilerFence(); }
inline void memoryBarrierAfterLock() { compilerFence(); }
inline void memoryBarrierBeforeUnlock() { compilerFence(); }

#else

inline void loadLoadFence() { compilerFence(); }
inline void loadStoreFence() { compilerFence(); }
inline void storeLoadFence() { compilerFence(); }
inline void storeStoreFence() { compilerFence(); }
inline void memoryBarrierAfterLock() { compilerFence(); }
inline void memoryBarrierBeforeUnlock() { compilerFence(); }

#endif

} // namespace WTF

using WTF::Atomic;

#endif // Atomics_h