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/*
* Copyright (C) 2007-2017 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.
*
* THIS SOFTWARE IS PROVIDED BY APPLE INC. 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 INC. 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.
*/
#pragma once
#include <atomic>
#include <wtf/FastMalloc.h>
#include <wtf/StdLibExtras.h>
#if OS(WINDOWS)
#if !COMPILER(GCC_COMPATIBLE)
extern "C" void _ReadWriteBarrier(void);
#pragma intrinsic(_ReadWriteBarrier)
#endif
#include <windows.h>
#include <intrin.h>
#endif
namespace WTF {
ALWAYS_INLINE bool hasFence(std::memory_order order)
{
return order != std::memory_order_relaxed;
}
// 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 {
WTF_MAKE_STRUCT_FAST_ALLOCATED;
// 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.
ALWAYS_INLINE T load(std::memory_order order = std::memory_order_seq_cst) const { return value.load(order); }
ALWAYS_INLINE T loadRelaxed() const { return load(std::memory_order_relaxed); }
// This is a load that simultaneously does a full fence - neither loads nor stores will move
// above or below it.
ALWAYS_INLINE T loadFullyFenced() const
{
Atomic<T>* ptr = const_cast<Atomic<T>*>(this);
return ptr->exchangeAdd(T());
}
ALWAYS_INLINE void store(T desired, std::memory_order order = std::memory_order_seq_cst) { value.store(desired, order); }
ALWAYS_INLINE void storeRelaxed(T desired) { store(desired, std::memory_order_relaxed); }
// This is a store that simultaneously does a full fence - neither loads nor stores will move
// above or below it.
ALWAYS_INLINE void storeFullyFenced(T desired)
{
exchange(desired);
}
ALWAYS_INLINE bool compareExchangeWeak(T expected, T desired, std::memory_order order = std::memory_order_seq_cst)
{
T expectedOrActual = expected;
return value.compare_exchange_weak(expectedOrActual, desired, order);
}
ALWAYS_INLINE bool compareExchangeWeakRelaxed(T expected, T desired)
{
return compareExchangeWeak(expected, desired, std::memory_order_relaxed);
}
ALWAYS_INLINE bool compareExchangeWeak(T expected, T desired, std::memory_order order_success, std::memory_order order_failure)
{
T expectedOrActual = expected;
return value.compare_exchange_weak(expectedOrActual, desired, order_success, order_failure);
}
// WARNING: This does not have strong fencing guarantees when it fails. For example, stores could
// sink below it in that case.
ALWAYS_INLINE T compareExchangeStrong(T expected, T desired, std::memory_order order = std::memory_order_seq_cst)
{
T expectedOrActual = expected;
value.compare_exchange_strong(expectedOrActual, desired, order);
return expectedOrActual;
}
ALWAYS_INLINE T compareExchangeStrong(T expected, T desired, std::memory_order order_success, std::memory_order order_failure)
{
T expectedOrActual = expected;
value.compare_exchange_strong(expectedOrActual, desired, order_success, order_failure);
return expectedOrActual;
}
template<typename U>
ALWAYS_INLINE T exchangeAdd(U operand, std::memory_order order = std::memory_order_seq_cst) { return value.fetch_add(operand, order); }
template<typename U>
ALWAYS_INLINE T exchangeAnd(U operand, std::memory_order order = std::memory_order_seq_cst) { return value.fetch_and(operand, order); }
template<typename U>
ALWAYS_INLINE T exchangeOr(U operand, std::memory_order order = std::memory_order_seq_cst) { return value.fetch_or(operand, order); }
template<typename U>
ALWAYS_INLINE T exchangeSub(U operand, std::memory_order order = std::memory_order_seq_cst) { return value.fetch_sub(operand, order); }
template<typename U>
ALWAYS_INLINE T exchangeXor(U operand, std::memory_order order = std::memory_order_seq_cst) { return value.fetch_xor(operand, order); }
ALWAYS_INLINE T exchange(T newValue, std::memory_order order = std::memory_order_seq_cst) { return value.exchange(newValue, order); }
template<typename Func>
ALWAYS_INLINE bool transaction(const Func& func, std::memory_order order = std::memory_order_seq_cst)
{
for (;;) {
T oldValue = load(std::memory_order_relaxed);
T newValue = oldValue;
if (!func(newValue))
return false;
if (compareExchangeWeak(oldValue, newValue, order))
return true;
}
}
template<typename Func>
ALWAYS_INLINE bool transactionRelaxed(const Func& func)
{
return transaction(func, std::memory_order_relaxed);
}
Atomic() = default;
constexpr Atomic(T initial)
: value(std::forward<T>(initial))
{
}
std::atomic<T> value;
};
template<typename T>
inline T atomicLoad(T* location, std::memory_order order = std::memory_order_seq_cst)
{
return bitwise_cast<Atomic<T>*>(location)->load(order);
}
template<typename T>
inline T atomicLoadFullyFenced(T* location)
{
return bitwise_cast<Atomic<T>*>(location)->loadFullyFenced();
}
template<typename T>
inline void atomicStore(T* location, T newValue, std::memory_order order = std::memory_order_seq_cst)
{
bitwise_cast<Atomic<T>*>(location)->store(newValue, order);
}
template<typename T>
inline void atomicStoreFullyFenced(T* location, T newValue)
{
bitwise_cast<Atomic<T>*>(location)->storeFullyFenced(newValue);
}
template<typename T>
inline bool atomicCompareExchangeWeak(T* location, T expected, T newValue, std::memory_order order = std::memory_order_seq_cst)
{
return bitwise_cast<Atomic<T>*>(location)->compareExchangeWeak(expected, newValue, order);
}
template<typename T>
inline bool atomicCompareExchangeWeakRelaxed(T* location, T expected, T newValue)
{
return bitwise_cast<Atomic<T>*>(location)->compareExchangeWeakRelaxed(expected, newValue);
}
template<typename T>
inline T atomicCompareExchangeStrong(T* location, T expected, T newValue, std::memory_order order = std::memory_order_seq_cst)
{
return bitwise_cast<Atomic<T>*>(location)->compareExchangeStrong(expected, newValue, order);
}
template<typename T, typename U>
inline T atomicExchangeAdd(T* location, U operand, std::memory_order order = std::memory_order_seq_cst)
{
return bitwise_cast<Atomic<T>*>(location)->exchangeAdd(operand, order);
}
template<typename T, typename U>
inline T atomicExchangeAnd(T* location, U operand, std::memory_order order = std::memory_order_seq_cst)
{
return bitwise_cast<Atomic<T>*>(location)->exchangeAnd(operand, order);
}
template<typename T, typename U>
inline T atomicExchangeOr(T* location, U operand, std::memory_order order = std::memory_order_seq_cst)
{
return bitwise_cast<Atomic<T>*>(location)->exchangeOr(operand, order);
}
template<typename T, typename U>
inline T atomicExchangeSub(T* location, U operand, std::memory_order order = std::memory_order_seq_cst)
{
return bitwise_cast<Atomic<T>*>(location)->exchangeSub(operand, order);
}
template<typename T, typename U>
inline T atomicExchangeXor(T* location, U operand, std::memory_order order = std::memory_order_seq_cst)
{
return bitwise_cast<Atomic<T>*>(location)->exchangeXor(operand, order);
}
template<typename T>
inline T atomicExchange(T* location, T newValue, std::memory_order order = std::memory_order_seq_cst)
{
return bitwise_cast<Atomic<T>*>(location)->exchange(newValue, order);
}
// 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_COMPATIBLE)
_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 arm_dmb()
{
asm volatile("dmb ish" ::: "memory");
}
// Like the above, but only affects stores.
inline void arm_dmb_st()
{
asm volatile("dmb ishst" ::: "memory");
}
inline void arm_isb()
{
asm volatile("isb" ::: "memory");
}
inline void loadLoadFence() { arm_dmb(); }
inline void loadStoreFence() { arm_dmb(); }
inline void storeLoadFence() { arm_dmb(); }
inline void storeStoreFence() { arm_dmb_st(); }
inline void crossModifyingCodeFence() { arm_isb(); }
#elif CPU(X86) || CPU(X86_64)
inline void x86_ortop()
{
#if OS(WINDOWS)
MemoryBarrier();
#elif CPU(X86_64)
// This has acqrel semantics and is much cheaper than mfence. For exampe, in the JSC GC, using
// mfence as a store-load fence was a 9% slow-down on Octane/splay while using this was neutral.
asm volatile("lock; orl $0, (%%rsp)" ::: "memory");
#else
asm volatile("lock; orl $0, (%%esp)" ::: "memory");
#endif
}
inline void x86_cpuid()
{
#if OS(WINDOWS)
int info[4];
__cpuid(info, 0);
#else
intptr_t a = 0, b, c, d;
asm volatile(
"cpuid"
: "+a"(a), "=b"(b), "=c"(c), "=d"(d)
:
: "memory");
#endif
}
inline void loadLoadFence() { compilerFence(); }
inline void loadStoreFence() { compilerFence(); }
inline void storeLoadFence() { x86_ortop(); }
inline void storeStoreFence() { compilerFence(); }
inline void crossModifyingCodeFence() { x86_cpuid(); }
#else
inline void loadLoadFence() { std::atomic_thread_fence(std::memory_order_seq_cst); }
inline void loadStoreFence() { std::atomic_thread_fence(std::memory_order_seq_cst); }
inline void storeLoadFence() { std::atomic_thread_fence(std::memory_order_seq_cst); }
inline void storeStoreFence() { std::atomic_thread_fence(std::memory_order_seq_cst); }
inline void crossModifyingCodeFence() { std::atomic_thread_fence(std::memory_order_seq_cst); } // Probably not strong enough.
#endif
#if CPU(ARM64) || CPU(X86) || CPU(X86_64)
// Use this fence if you want a fence between loads that are already depdendent.
inline void dependentLoadLoadFence() { compilerFence(); }
#else
inline void dependentLoadLoadFence() { loadLoadFence(); }
#endif
template<typename T>
T opaque(T pointer)
{
#if !OS(WINDOWS)
asm volatile("" : "+r"(pointer) ::);
#endif
return pointer;
}
typedef unsigned InternalDependencyType;
inline InternalDependencyType opaqueMixture()
{
return 0;
}
template<typename... Arguments, typename T>
inline InternalDependencyType opaqueMixture(T value, Arguments... arguments)
{
union {
InternalDependencyType copy;
T value;
} u;
u.copy = 0;
u.value = value;
return opaqueMixture(arguments...) + u.copy;
}
class Dependency {
WTF_MAKE_FAST_ALLOCATED;
public:
Dependency()
: m_value(0)
{
}
// On TSO architectures, this is a load-load fence and the value it returns is not meaningful (it's
// zero). The load-load fence is usually just a compiler fence. On ARM, this is a self-xor that
// produces zero, but it's concealed from the compiler. The CPU understands this dummy op to be a
// phantom dependency.
template<typename... Arguments>
static Dependency fence(Arguments... arguments)
{
InternalDependencyType input = opaqueMixture(arguments...);
InternalDependencyType output;
#if CPU(ARM64)
// Create a magical zero value through inline assembly, whose computation
// isn't visible to the optimizer. This zero is then usable as an offset in
// further address computations: adding zero does nothing, but the compiler
// doesn't know it. It's magical because it creates an address dependency
// from the load of `location` to the uses of the dependency, which triggers
// the ARM ISA's address dependency rule, a.k.a. the mythical C++ consume
// ordering. This forces weak memory order CPUs to observe `location` and
// dependent loads in their store order without the reader using a barrier
// or an acquire load.
asm("eor %w[out], %w[in], %w[in]"
: [out] "=r"(output)
: [in] "r"(input));
#elif CPU(ARM)
asm("eor %[out], %[in], %[in]"
: [out] "=r"(output)
: [in] "r"(input));
#else
// No dependency is needed for this architecture.
loadLoadFence();
output = 0;
UNUSED_PARAM(input);
#endif
Dependency result;
result.m_value = output;
return result;
}
// This function exists as a helper to aid in not making mistakes when doing a load
// and fencing on the result of the load. A couple examples of where things can go
// wrong, and how this function helps:
//
// Consider this program:
// ```
// a = load(p1)
// b = load(p2)
// if (a != b) return;
// d = Dependency::fence(b)
// ```
// When consuming the d dependency, the compiler can prove that a and b are the same
// value, and end up replacing the dependency on whatever register is allocated for `a`
// instead of being over `b`, leading to the dependency being on load(p1) instead of
// load(p2). We fix this by splitting the value feeding into the fence and the value
// being used:
// b' = load(p2)
// Dependency::fence(b')
// b = opaque(b')
// b' feeds into the fence, and b will be the value compared. Crucially, the compiler can't
// prove that b == b'.
//
// Let's consider another use case. Imagine you end up with a program like this (perhaps
// after some inlining or various optimizations):
// a = load(p1)
// b = load(p2)
// if (a != b) return;
// c = load(p2)
// d = Dependency::fence(c)
// Similar to the first test, the compiler can prove a and b are the same, allowing it to
// prove that c == a == b, allowing it to potentially have the dependency be on the wrong
// value, similar to above. The fix here is to obscure the pointer we're loading from from
// the compiler.
template<typename T>
static Dependency loadAndFence(const T* pointer, T& output)
{
#if CPU(ARM64) || CPU(ARM)
T value = *opaque(pointer);
Dependency dependency = Dependency::fence(value);
output = opaque(value);
return dependency;
#else
T value = *pointer;
Dependency dependency = Dependency::fence(value);
output = value;
return dependency;
#endif
}
// On TSO architectures, this just returns the pointer you pass it. On ARM, this produces a new
// pointer that is dependent on this dependency and the input pointer.
template<typename T>
T* consume(T* pointer)
{
#if CPU(ARM64) || CPU(ARM)
return bitwise_cast<T*>(bitwise_cast<char*>(pointer) + m_value);
#else
UNUSED_PARAM(m_value);
return pointer;
#endif
}
private:
InternalDependencyType m_value;
};
template<typename InputType, typename ValueType>
struct InputAndValue {
WTF_MAKE_STRUCT_FAST_ALLOCATED;
InputAndValue() { }
InputAndValue(InputType input, ValueType value)
: input(input)
, value(value)
{
}
InputType input;
ValueType value;
};
template<typename InputType, typename ValueType>
InputAndValue<InputType, ValueType> inputAndValue(InputType input, ValueType value)
{
return InputAndValue<InputType, ValueType>(input, value);
}
template<typename T, typename Func>
ALWAYS_INLINE T& ensurePointer(Atomic<T*>& pointer, const Func& func)
{
for (;;) {
T* oldValue = pointer.load(std::memory_order_relaxed);
if (oldValue) {
// On all sensible CPUs, we get an implicit dependency-based load-load barrier when
// loading this.
return *oldValue;
}
T* newValue = func();
if (pointer.compareExchangeWeak(oldValue, newValue))
return *newValue;
delete newValue;
}
}
} // namespace WTF
using WTF::Atomic;
using WTF::Dependency;
using WTF::InputAndValue;
using WTF::inputAndValue;
using WTF::ensurePointer;
using WTF::opaqueMixture;