Revision d07004f4c117182086c2e2682de542ecd8f2b59f authored by Robert O'Callahan on 18 December 2013, 05:37:24 UTC, committed by Robert O'Callahan on 18 December 2013, 05:37:24 UTC
1 parent 6ac4414
Atomics.h
/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* vim: set ts=8 sts=2 et sw=2 tw=80: */
/* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
/*
* Implements (almost always) lock-free atomic operations. The operations here
* are a subset of that which can be found in C++11's <atomic> header, with a
* different API to enforce consistent memory ordering constraints.
*
* Anyone caught using |volatile| for inter-thread memory safety needs to be
* sent a copy of this header and the C++11 standard.
*/
#ifndef mozilla_Atomics_h
#define mozilla_Atomics_h
#include "mozilla/Assertions.h"
#include "mozilla/Attributes.h"
#include "mozilla/Compiler.h"
#include "mozilla/TypeTraits.h"
#include <stdint.h>
/*
* Our minimum deployment target on clang/OS X is OS X 10.6, whose SDK
* does not have <atomic>. So be sure to check for <atomic> support
* along with C++0x support.
*/
#if defined(__clang__) || defined(__GNUC__)
/*
* Clang doesn't like <atomic> from libstdc++ before 4.7 due to the
* loose typing of the atomic builtins. GCC 4.5 and 4.6 lacks inline
* definitions for unspecialized std::atomic and causes linking errors.
* Therefore, we require at least 4.7.0 for using libstdc++.
*/
# if MOZ_USING_LIBSTDCXX && MOZ_LIBSTDCXX_VERSION_AT_LEAST(4, 7, 0)
# define MOZ_HAVE_CXX11_ATOMICS
# elif MOZ_USING_LIBCXX
# define MOZ_HAVE_CXX11_ATOMICS
# endif
#elif defined(_MSC_VER) && _MSC_VER >= 1700
# define MOZ_HAVE_CXX11_ATOMICS
#endif
namespace mozilla {
/**
* An enum of memory ordering possibilities for atomics.
*
* Memory ordering is the observable state of distinct values in memory.
* (It's a separate concept from atomicity, which concerns whether an
* operation can ever be observed in an intermediate state. Don't
* conflate the two!) Given a sequence of operations in source code on
* memory, it is *not* always the case that, at all times and on all
* cores, those operations will appear to have occurred in that exact
* sequence. First, the compiler might reorder that sequence, if it
* thinks another ordering will be more efficient. Second, the CPU may
* not expose so consistent a view of memory. CPUs will often perform
* their own instruction reordering, above and beyond that performed by
* the compiler. And each core has its own memory caches, and accesses
* (reads and writes both) to "memory" may only resolve to out-of-date
* cache entries -- not to the "most recently" performed operation in
* some global sense. Any access to a value that may be used by
* multiple threads, potentially across multiple cores, must therefore
* have a memory ordering imposed on it, for all code on all
* threads/cores to have a sufficiently coherent worldview.
*
* http://gcc.gnu.org/wiki/Atomic/GCCMM/AtomicSync and
* http://en.cppreference.com/w/cpp/atomic/memory_order go into more
* detail on all this, including examples of how each mode works.
*
* Note that for simplicity and practicality, not all of the modes in
* C++11 are supported. The missing C++11 modes are either subsumed by
* the modes we provide below, or not relevant for the CPUs we support
* in Gecko. These three modes are confusing enough as it is!
*/
enum MemoryOrdering {
/*
* Relaxed ordering is the simplest memory ordering: none at all.
* When the result of a write is observed, nothing may be inferred
* about other memory. Writes ostensibly performed "before" on the
* writing thread may not yet be visible. Writes performed "after" on
* the writing thread may already be visible, if the compiler or CPU
* reordered them. (The latter can happen if reads and/or writes get
* held up in per-processor caches.) Relaxed ordering means
* operations can always use cached values (as long as the actual
* updates to atomic values actually occur, correctly, eventually), so
* it's usually the fastest sort of atomic access. For this reason,
* *it's also the most dangerous kind of access*.
*
* Relaxed ordering is good for things like process-wide statistics
* counters that don't need to be consistent with anything else, so
* long as updates themselves are atomic. (And so long as any
* observations of that value can tolerate being out-of-date -- if you
* need some sort of up-to-date value, you need some sort of other
* synchronizing operation.) It's *not* good for locks, mutexes,
* reference counts, etc. that mediate access to other memory, or must
* be observably consistent with other memory.
*
* x86 architectures don't take advantage of the optimization
* opportunities that relaxed ordering permits. Thus it's possible
* that using relaxed ordering will "work" on x86 but fail elsewhere
* (ARM, say, which *does* implement non-sequentially-consistent
* relaxed ordering semantics). Be extra-careful using relaxed
* ordering if you can't easily test non-x86 architectures!
*/
Relaxed,
/*
* When an atomic value is updated with ReleaseAcquire ordering, and
* that new value is observed with ReleaseAcquire ordering, prior
* writes (atomic or not) are also observable. What ReleaseAcquire
* *doesn't* give you is any observable ordering guarantees for
* ReleaseAcquire-ordered operations on different objects. For
* example, if there are two cores that each perform ReleaseAcquire
* operations on separate objects, each core may or may not observe
* the operations made by the other core. The only way the cores can
* be synchronized with ReleaseAcquire is if they both
* ReleaseAcquire-access the same object. This implies that you can't
* necessarily describe some global total ordering of ReleaseAcquire
* operations.
*
* ReleaseAcquire ordering is good for (as the name implies) atomic
* operations on values controlling ownership of things: reference
* counts, mutexes, and the like. However, if you are thinking about
* using these to implement your own locks or mutexes, you should take
* a good, hard look at actual lock or mutex primitives first.
*/
ReleaseAcquire,
/*
* When an atomic value is updated with SequentiallyConsistent
* ordering, all writes observable when the update is observed, just
* as with ReleaseAcquire ordering. But, furthermore, a global total
* ordering of SequentiallyConsistent operations *can* be described.
* For example, if two cores perform SequentiallyConsistent operations
* on separate objects, one core will observably perform its update
* (and all previous operations will have completed), then the other
* core will observably perform its update (and all previous
* operations will have completed). (Although those previous
* operations aren't themselves ordered -- they could be intermixed,
* or ordered if they occur on atomic values with ordering
* requirements.) SequentiallyConsistent is the *simplest and safest*
* ordering of atomic operations -- it's always as if one operation
* happens, then another, then another, in some order -- and every
* core observes updates to happen in that single order. Because it
* has the most synchronization requirements, operations ordered this
* way also tend to be slowest.
*
* SequentiallyConsistent ordering can be desirable when multiple
* threads observe objects, and they all have to agree on the
* observable order of changes to them. People expect
* SequentiallyConsistent ordering, even if they shouldn't, when
* writing code, atomic or otherwise. SequentiallyConsistent is also
* the ordering of choice when designing lockless data structures. If
* you don't know what order to use, use this one.
*/
SequentiallyConsistent,
};
} // namespace mozilla
// Build up the underlying intrinsics.
#ifdef MOZ_HAVE_CXX11_ATOMICS
# include <atomic>
namespace mozilla {
namespace detail {
/*
* We provide CompareExchangeFailureOrder to work around a bug in some
* versions of GCC's <atomic> header. See bug 898491.
*/
template<MemoryOrdering Order> struct AtomicOrderConstraints;
template<>
struct AtomicOrderConstraints<Relaxed>
{
static const std::memory_order AtomicRMWOrder = std::memory_order_relaxed;
static const std::memory_order LoadOrder = std::memory_order_relaxed;
static const std::memory_order StoreOrder = std::memory_order_relaxed;
static const std::memory_order CompareExchangeFailureOrder =
std::memory_order_relaxed;
};
template<>
struct AtomicOrderConstraints<ReleaseAcquire>
{
static const std::memory_order AtomicRMWOrder = std::memory_order_acq_rel;
static const std::memory_order LoadOrder = std::memory_order_acquire;
static const std::memory_order StoreOrder = std::memory_order_release;
static const std::memory_order CompareExchangeFailureOrder =
std::memory_order_acquire;
};
template<>
struct AtomicOrderConstraints<SequentiallyConsistent>
{
static const std::memory_order AtomicRMWOrder = std::memory_order_seq_cst;
static const std::memory_order LoadOrder = std::memory_order_seq_cst;
static const std::memory_order StoreOrder = std::memory_order_seq_cst;
static const std::memory_order CompareExchangeFailureOrder =
std::memory_order_seq_cst;
};
template<typename T, MemoryOrdering Order>
struct IntrinsicBase
{
typedef std::atomic<T> ValueType;
typedef AtomicOrderConstraints<Order> OrderedOp;
};
template<typename T, MemoryOrdering Order>
struct IntrinsicMemoryOps : public IntrinsicBase<T, Order>
{
typedef IntrinsicBase<T, Order> Base;
static T load(const typename Base::ValueType& ptr) {
return ptr.load(Base::OrderedOp::LoadOrder);
}
static void store(typename Base::ValueType& ptr, T val) {
ptr.store(val, Base::OrderedOp::StoreOrder);
}
static T exchange(typename Base::ValueType& ptr, T val) {
return ptr.exchange(val, Base::OrderedOp::AtomicRMWOrder);
}
static bool compareExchange(typename Base::ValueType& ptr, T oldVal, T newVal) {
return ptr.compare_exchange_strong(oldVal, newVal,
Base::OrderedOp::AtomicRMWOrder,
Base::OrderedOp::CompareExchangeFailureOrder);
}
};
template<typename T, MemoryOrdering Order>
struct IntrinsicAddSub : public IntrinsicBase<T, Order>
{
typedef IntrinsicBase<T, Order> Base;
static T add(typename Base::ValueType& ptr, T val) {
return ptr.fetch_add(val, Base::OrderedOp::AtomicRMWOrder);
}
static T sub(typename Base::ValueType& ptr, T val) {
return ptr.fetch_sub(val, Base::OrderedOp::AtomicRMWOrder);
}
};
template<typename T, MemoryOrdering Order>
struct IntrinsicAddSub<T*, Order> : public IntrinsicBase<T*, Order>
{
typedef IntrinsicBase<T*, Order> Base;
static T* add(typename Base::ValueType& ptr, ptrdiff_t val) {
return ptr.fetch_add(fixupAddend(val), Base::OrderedOp::AtomicRMWOrder);
}
static T* sub(typename Base::ValueType& ptr, ptrdiff_t val) {
return ptr.fetch_sub(fixupAddend(val), Base::OrderedOp::AtomicRMWOrder);
}
private:
/*
* GCC 4.6's <atomic> header has a bug where adding X to an
* atomic<T*> is not the same as adding X to a T*. Hence the need
* for this function to provide the correct addend.
*/
static ptrdiff_t fixupAddend(ptrdiff_t val) {
#if defined(__clang__) || defined(_MSC_VER)
return val;
#elif defined(__GNUC__) && MOZ_GCC_VERSION_AT_LEAST(4, 6, 0) && \
!MOZ_GCC_VERSION_AT_LEAST(4, 7, 0)
return val * sizeof(T);
#else
return val;
#endif
}
};
template<typename T, MemoryOrdering Order>
struct IntrinsicIncDec : public IntrinsicAddSub<T, Order>
{
typedef IntrinsicBase<T, Order> Base;
static T inc(typename Base::ValueType& ptr) {
return IntrinsicAddSub<T, Order>::add(ptr, 1);
}
static T dec(typename Base::ValueType& ptr) {
return IntrinsicAddSub<T, Order>::sub(ptr, 1);
}
};
template<typename T, MemoryOrdering Order>
struct AtomicIntrinsics : public IntrinsicMemoryOps<T, Order>,
public IntrinsicIncDec<T, Order>
{
typedef IntrinsicBase<T, Order> Base;
static T or_(typename Base::ValueType& ptr, T val) {
return ptr.fetch_or(val, Base::OrderedOp::AtomicRMWOrder);
}
static T xor_(typename Base::ValueType& ptr, T val) {
return ptr.fetch_xor(val, Base::OrderedOp::AtomicRMWOrder);
}
static T and_(typename Base::ValueType& ptr, T val) {
return ptr.fetch_and(val, Base::OrderedOp::AtomicRMWOrder);
}
};
template<typename T, MemoryOrdering Order>
struct AtomicIntrinsics<T*, Order>
: public IntrinsicMemoryOps<T*, Order>, public IntrinsicIncDec<T*, Order>
{
};
} // namespace detail
} // namespace mozilla
#elif defined(__GNUC__)
namespace mozilla {
namespace detail {
/*
* The __sync_* family of intrinsics is documented here:
*
* http://gcc.gnu.org/onlinedocs/gcc-4.6.4/gcc/Atomic-Builtins.html
*
* While these intrinsics are deprecated in favor of the newer __atomic_*
* family of intrincs:
*
* http://gcc.gnu.org/onlinedocs/gcc-4.7.3/gcc/_005f_005fatomic-Builtins.html
*
* any GCC version that supports the __atomic_* intrinsics will also support
* the <atomic> header and so will be handled above. We provide a version of
* atomics using the __sync_* intrinsics to support older versions of GCC.
*
* All __sync_* intrinsics that we use below act as full memory barriers, for
* both compiler and hardware reordering, except for __sync_lock_test_and_set,
* which is a only an acquire barrier. When we call __sync_lock_test_and_set,
* we add a barrier above it as appropriate.
*/
template<MemoryOrdering Order> struct Barrier;
/*
* Some processors (in particular, x86) don't require quite so many calls to
* __sync_sychronize as our specializations of Barrier produce. If
* performance turns out to be an issue, defining these specializations
* on a per-processor basis would be a good first tuning step.
*/
template<>
struct Barrier<Relaxed>
{
static void beforeLoad() {}
static void afterLoad() {}
static void beforeStore() {}
static void afterStore() {}
};
template<>
struct Barrier<ReleaseAcquire>
{
static void beforeLoad() {}
static void afterLoad() { __sync_synchronize(); }
static void beforeStore() { __sync_synchronize(); }
static void afterStore() {}
};
template<>
struct Barrier<SequentiallyConsistent>
{
static void beforeLoad() { __sync_synchronize(); }
static void afterLoad() { __sync_synchronize(); }
static void beforeStore() { __sync_synchronize(); }
static void afterStore() { __sync_synchronize(); }
};
template<typename T, MemoryOrdering Order>
struct IntrinsicMemoryOps
{
static T load(const T& ptr) {
Barrier<Order>::beforeLoad();
T val = ptr;
Barrier<Order>::afterLoad();
return val;
}
static void store(T& ptr, T val) {
Barrier<Order>::beforeStore();
ptr = val;
Barrier<Order>::afterStore();
}
static T exchange(T& ptr, T val) {
// __sync_lock_test_and_set is only an acquire barrier; loads and stores
// can't be moved up from after to before it, but they can be moved down
// from before to after it. We may want a stricter ordering, so we need
// an explicit barrier.
Barrier<Order>::beforeStore();
return __sync_lock_test_and_set(&ptr, val);
}
static bool compareExchange(T& ptr, T oldVal, T newVal) {
return __sync_bool_compare_and_swap(&ptr, oldVal, newVal);
}
};
template<typename T>
struct IntrinsicAddSub
{
typedef T ValueType;
static T add(T& ptr, T val) {
return __sync_fetch_and_add(&ptr, val);
}
static T sub(T& ptr, T val) {
return __sync_fetch_and_sub(&ptr, val);
}
};
template<typename T>
struct IntrinsicAddSub<T*>
{
typedef T* ValueType;
/*
* The reinterpret_casts are needed so that
* __sync_fetch_and_{add,sub} will properly type-check.
*
* Also, these functions do not provide standard semantics for
* pointer types, so we need to adjust the addend.
*/
static ValueType add(ValueType& ptr, ptrdiff_t val) {
ValueType amount = reinterpret_cast<ValueType>(val * sizeof(T));
return __sync_fetch_and_add(&ptr, amount);
}
static ValueType sub(ValueType& ptr, ptrdiff_t val) {
ValueType amount = reinterpret_cast<ValueType>(val * sizeof(T));
return __sync_fetch_and_sub(&ptr, amount);
}
};
template<typename T>
struct IntrinsicIncDec : public IntrinsicAddSub<T>
{
static T inc(T& ptr) { return IntrinsicAddSub<T>::add(ptr, 1); }
static T dec(T& ptr) { return IntrinsicAddSub<T>::sub(ptr, 1); }
};
template<typename T, MemoryOrdering Order>
struct AtomicIntrinsics : public IntrinsicMemoryOps<T, Order>,
public IntrinsicIncDec<T>
{
static T or_(T& ptr, T val) {
return __sync_fetch_and_or(&ptr, val);
}
static T xor_(T& ptr, T val) {
return __sync_fetch_and_xor(&ptr, val);
}
static T and_(T& ptr, T val) {
return __sync_fetch_and_and(&ptr, val);
}
};
template<typename T, MemoryOrdering Order>
struct AtomicIntrinsics<T*, Order> : public IntrinsicMemoryOps<T*, Order>,
public IntrinsicIncDec<T*>
{
};
} // namespace detail
} // namespace mozilla
#elif defined(_MSC_VER)
/*
* Windows comes with a full complement of atomic operations.
* Unfortunately, most of those aren't available for Windows XP (even if
* the compiler supports intrinsics for them), which is the oldest
* version of Windows we support. Therefore, we only provide operations
* on 32-bit datatypes for 32-bit Windows versions; for 64-bit Windows
* versions, we support 64-bit datatypes as well.
*
* To avoid namespace pollution issues, we declare whatever functions we
* need ourselves.
*/
extern "C" {
long __cdecl _InterlockedExchangeAdd(long volatile* dst, long value);
long __cdecl _InterlockedOr(long volatile* dst, long value);
long __cdecl _InterlockedXor(long volatile* dst, long value);
long __cdecl _InterlockedAnd(long volatile* dst, long value);
long __cdecl _InterlockedExchange(long volatile *dst, long value);
long __cdecl _InterlockedCompareExchange(long volatile *dst, long newVal, long oldVal);
}
# pragma intrinsic(_InterlockedExchangeAdd)
# pragma intrinsic(_InterlockedOr)
# pragma intrinsic(_InterlockedXor)
# pragma intrinsic(_InterlockedAnd)
# pragma intrinsic(_InterlockedExchange)
# pragma intrinsic(_InterlockedCompareExchange)
namespace mozilla {
namespace detail {
# if !defined(_M_IX86) && !defined(_M_X64)
/*
* The implementations below are optimized for x86ish systems. You
* will have to modify them if you are porting to Windows on a
* different architecture.
*/
# error "Unknown CPU type"
# endif
/*
* The PrimitiveIntrinsics template should define |Type|, the datatype of size
* DataSize upon which we operate, and the following eight functions.
*
* static Type add(Type* ptr, Type val);
* static Type sub(Type* ptr, Type val);
* static Type or_(Type* ptr, Type val);
* static Type xor_(Type* ptr, Type val);
* static Type and_(Type* ptr, Type val);
*
* These functions perform the obvious operation on the value contained in
* |*ptr| combined with |val| and return the value previously stored in
* |*ptr|.
*
* static void store(Type* ptr, Type val);
*
* This function atomically stores |val| into |*ptr| and must provide a full
* memory fence after the store to prevent compiler and hardware instruction
* reordering. It should also act as a compiler barrier to prevent reads and
* writes from moving to after the store.
*
* static Type exchange(Type* ptr, Type val);
*
* This function atomically stores |val| into |*ptr| and returns the previous
* contents of *ptr;
*
* static bool compareExchange(Type* ptr, Type oldVal, Type newVal);
*
* This function atomically performs the following operation:
*
* if (*ptr == oldVal) {
* *ptr = newVal;
* return true;
* } else {
* return false;
* }
*
*/
template<size_t DataSize> struct PrimitiveIntrinsics;
template<>
struct PrimitiveIntrinsics<4>
{
typedef long Type;
static Type add(Type* ptr, Type val) {
return _InterlockedExchangeAdd(ptr, val);
}
static Type sub(Type* ptr, Type val) {
/*
* _InterlockedExchangeSubtract isn't available before Windows 7,
* and we must support Windows XP.
*/
return _InterlockedExchangeAdd(ptr, -val);
}
static Type or_(Type* ptr, Type val) {
return _InterlockedOr(ptr, val);
}
static Type xor_(Type* ptr, Type val) {
return _InterlockedXor(ptr, val);
}
static Type and_(Type* ptr, Type val) {
return _InterlockedAnd(ptr, val);
}
static void store(Type* ptr, Type val) {
_InterlockedExchange(ptr, val);
}
static Type exchange(Type* ptr, Type val) {
return _InterlockedExchange(ptr, val);
}
static bool compareExchange(Type* ptr, Type oldVal, Type newVal) {
return _InterlockedCompareExchange(ptr, newVal, oldVal) == oldVal;
}
};
# if defined(_M_X64)
extern "C" {
long long __cdecl _InterlockedExchangeAdd64(long long volatile* dst,
long long value);
long long __cdecl _InterlockedOr64(long long volatile* dst,
long long value);
long long __cdecl _InterlockedXor64(long long volatile* dst,
long long value);
long long __cdecl _InterlockedAnd64(long long volatile* dst,
long long value);
long long __cdecl _InterlockedExchange64(long long volatile* dst,
long long value);
long long __cdecl _InterlockedCompareExchange64(long long volatile* dst,
long long newVal,
long long oldVal);
}
# pragma intrinsic(_InterlockedExchangeAdd64)
# pragma intrinsic(_InterlockedOr64)
# pragma intrinsic(_InterlockedXor64)
# pragma intrinsic(_InterlockedAnd64)
# pragma intrinsic(_InterlockedExchange64)
# pragma intrinsic(_InterlockedCompareExchange64)
template <>
struct PrimitiveIntrinsics<8>
{
typedef __int64 Type;
static Type add(Type* ptr, Type val) {
return _InterlockedExchangeAdd64(ptr, val);
}
static Type sub(Type* ptr, Type val) {
/*
* There is no _InterlockedExchangeSubtract64.
*/
return _InterlockedExchangeAdd64(ptr, -val);
}
static Type or_(Type* ptr, Type val) {
return _InterlockedOr64(ptr, val);
}
static Type xor_(Type* ptr, Type val) {
return _InterlockedXor64(ptr, val);
}
static Type and_(Type* ptr, Type val) {
return _InterlockedAnd64(ptr, val);
}
static void store(Type* ptr, Type val) {
_InterlockedExchange64(ptr, val);
}
static Type exchange(Type* ptr, Type val) {
return _InterlockedExchange64(ptr, val);
}
static bool compareExchange(Type* ptr, Type oldVal, Type newVal) {
return _InterlockedCompareExchange64(ptr, newVal, oldVal) == oldVal;
}
};
# endif
extern "C" { void _ReadWriteBarrier(); }
# pragma intrinsic(_ReadWriteBarrier)
template<MemoryOrdering Order> struct Barrier;
/*
* We do not provide an afterStore method in Barrier, as Relaxed and
* ReleaseAcquire orderings do not require one, and the required barrier
* for SequentiallyConsistent is handled by PrimitiveIntrinsics.
*/
template<>
struct Barrier<Relaxed>
{
static void beforeLoad() {}
static void afterLoad() {}
static void beforeStore() {}
};
template<>
struct Barrier<ReleaseAcquire>
{
static void beforeLoad() {}
static void afterLoad() { _ReadWriteBarrier(); }
static void beforeStore() { _ReadWriteBarrier(); }
};
template<>
struct Barrier<SequentiallyConsistent>
{
static void beforeLoad() { _ReadWriteBarrier(); }
static void afterLoad() { _ReadWriteBarrier(); }
static void beforeStore() { _ReadWriteBarrier(); }
};
template<typename PrimType, typename T>
struct CastHelper
{
static PrimType toPrimType(T val) { return static_cast<PrimType>(val); }
static T fromPrimType(PrimType val) { return static_cast<T>(val); }
};
template<typename PrimType, typename T>
struct CastHelper<PrimType, T*>
{
static PrimType toPrimType(T* val) { return reinterpret_cast<PrimType>(val); }
static T* fromPrimType(PrimType val) { return reinterpret_cast<T*>(val); }
};
template<typename T>
struct IntrinsicBase
{
typedef T ValueType;
typedef PrimitiveIntrinsics<sizeof(T)> Primitives;
typedef typename Primitives::Type PrimType;
static_assert(sizeof(PrimType) == sizeof(T),
"Selection of PrimitiveIntrinsics was wrong");
typedef CastHelper<PrimType, T> Cast;
};
template<typename T, MemoryOrdering Order>
struct IntrinsicMemoryOps : public IntrinsicBase<T>
{
static ValueType load(const ValueType& ptr) {
Barrier<Order>::beforeLoad();
ValueType val = ptr;
Barrier<Order>::afterLoad();
return val;
}
static void store(ValueType& ptr, ValueType val) {
// For SequentiallyConsistent, Primitives::store() will generate the
// proper memory fence. Everything else just needs a barrier before
// the store.
if (Order == SequentiallyConsistent) {
Primitives::store(reinterpret_cast<PrimType*>(&ptr),
Cast::toPrimType(val));
} else {
Barrier<Order>::beforeStore();
ptr = val;
}
}
static ValueType exchange(ValueType& ptr, ValueType val) {
PrimType oldval =
Primitives::exchange(reinterpret_cast<PrimType*>(&ptr),
Cast::toPrimType(val));
return Cast::fromPrimType(oldval);
}
static bool compareExchange(ValueType& ptr, ValueType oldVal, ValueType newVal) {
return Primitives::compareExchange(reinterpret_cast<PrimType*>(&ptr),
Cast::toPrimType(oldVal),
Cast::toPrimType(newVal));
}
};
template<typename T>
struct IntrinsicApplyHelper : public IntrinsicBase<T>
{
typedef PrimType (*BinaryOp)(PrimType*, PrimType);
typedef PrimType (*UnaryOp)(PrimType*);
static ValueType applyBinaryFunction(BinaryOp op, ValueType& ptr,
ValueType val) {
PrimType* primTypePtr = reinterpret_cast<PrimType*>(&ptr);
PrimType primTypeVal = Cast::toPrimType(val);
return Cast::fromPrimType(op(primTypePtr, primTypeVal));
}
static ValueType applyUnaryFunction(UnaryOp op, ValueType& ptr) {
PrimType* primTypePtr = reinterpret_cast<PrimType*>(&ptr);
return Cast::fromPrimType(op(primTypePtr));
}
};
template<typename T>
struct IntrinsicAddSub : public IntrinsicApplyHelper<T>
{
static ValueType add(ValueType& ptr, ValueType val) {
return applyBinaryFunction(&Primitives::add, ptr, val);
}
static ValueType sub(ValueType& ptr, ValueType val) {
return applyBinaryFunction(&Primitives::sub, ptr, val);
}
};
template<typename T>
struct IntrinsicAddSub<T*> : public IntrinsicApplyHelper<T*>
{
static ValueType add(ValueType& ptr, ptrdiff_t amount) {
return applyBinaryFunction(&Primitives::add, ptr,
(ValueType)(amount * sizeof(ValueType)));
}
static ValueType sub(ValueType& ptr, ptrdiff_t amount) {
return applyBinaryFunction(&Primitives::sub, ptr,
(ValueType)(amount * sizeof(ValueType)));
}
};
template<typename T>
struct IntrinsicIncDec : public IntrinsicAddSub<T>
{
static ValueType inc(ValueType& ptr) { return add(ptr, 1); }
static ValueType dec(ValueType& ptr) { return sub(ptr, 1); }
};
template<typename T, MemoryOrdering Order>
struct AtomicIntrinsics : public IntrinsicMemoryOps<T, Order>,
public IntrinsicIncDec<T>
{
static ValueType or_(ValueType& ptr, T val) {
return applyBinaryFunction(&Primitives::or_, ptr, val);
}
static ValueType xor_(ValueType& ptr, T val) {
return applyBinaryFunction(&Primitives::xor_, ptr, val);
}
static ValueType and_(ValueType& ptr, T val) {
return applyBinaryFunction(&Primitives::and_, ptr, val);
}
};
template<typename T, MemoryOrdering Order>
struct AtomicIntrinsics<T*, Order> : public IntrinsicMemoryOps<T*, Order>,
public IntrinsicIncDec<T*>
{
};
} // namespace detail
} // namespace mozilla
#else
# error "Atomic compiler intrinsics are not supported on your platform"
#endif
namespace mozilla {
namespace detail {
template<typename T, MemoryOrdering Order>
class AtomicBase
{
// We only support 32-bit types on 32-bit Windows, which constrains our
// implementation elsewhere. But we support pointer-sized types everywhere.
static_assert(sizeof(T) == 4 || (sizeof(uintptr_t) == 8 && sizeof(T) == 8),
"mozilla/Atomics.h only supports 32-bit and pointer-sized types");
protected:
typedef typename detail::AtomicIntrinsics<T, Order> Intrinsics;
typename Intrinsics::ValueType mValue;
public:
MOZ_CONSTEXPR AtomicBase() : mValue() {}
MOZ_CONSTEXPR AtomicBase(T aInit) : mValue(aInit) {}
operator T() const { return Intrinsics::load(mValue); }
T operator=(T aValue) {
Intrinsics::store(mValue, aValue);
return aValue;
}
/**
* Performs an atomic swap operation. aValue is stored and the previous
* value of this variable is returned.
*/
T exchange(T aValue) {
return Intrinsics::exchange(mValue, aValue);
}
/**
* Performs an atomic compare-and-swap operation and returns true if it
* succeeded. This is equivalent to atomically doing
*
* if (mValue == aOldValue) {
* mValue = aNewValue;
* return true;
* } else {
* return false;
* }
*/
bool compareExchange(T aOldValue, T aNewValue) {
return Intrinsics::compareExchange(mValue, aOldValue, aNewValue);
}
private:
template<MemoryOrdering AnyOrder>
AtomicBase(const AtomicBase<T, AnyOrder>& aCopy) MOZ_DELETE;
};
template<typename T, MemoryOrdering Order>
class AtomicBaseIncDec : public AtomicBase<T, Order>
{
typedef typename detail::AtomicBase<T, Order> Base;
public:
MOZ_CONSTEXPR AtomicBaseIncDec() : Base() {}
MOZ_CONSTEXPR AtomicBaseIncDec(T aInit) : Base(aInit) {}
using Base::operator=;
T operator++(int) { return Base::Intrinsics::inc(Base::mValue); }
T operator--(int) { return Base::Intrinsics::dec(Base::mValue); }
T operator++() { return Base::Intrinsics::inc(Base::mValue) + 1; }
T operator--() { return Base::Intrinsics::dec(Base::mValue) - 1; }
private:
template<MemoryOrdering AnyOrder>
AtomicBaseIncDec(const AtomicBaseIncDec<T, AnyOrder>& aCopy) MOZ_DELETE;
};
} // namespace detail
/**
* A wrapper for a type that enforces that all memory accesses are atomic.
*
* In general, where a variable |T foo| exists, |Atomic<T> foo| can be used in
* its place. Implementations for integral and pointer types are provided
* below.
*
* Atomic accesses are sequentially consistent by default. You should
* use the default unless you are tall enough to ride the
* memory-ordering roller coaster (if you're not sure, you aren't) and
* you have a compelling reason to do otherwise.
*
* There is one exception to the case of atomic memory accesses: providing an
* initial value of the atomic value is not guaranteed to be atomic. This is a
* deliberate design choice that enables static atomic variables to be declared
* without introducing extra static constructors.
*/
template<typename T,
MemoryOrdering Order = SequentiallyConsistent,
typename Enable = void>
class Atomic;
/**
* Atomic<T> implementation for integral types.
*
* In addition to atomic store and load operations, compound assignment and
* increment/decrement operators are implemented which perform the
* corresponding read-modify-write operation atomically. Finally, an atomic
* swap method is provided.
*/
template<typename T, MemoryOrdering Order>
class Atomic<T, Order, typename EnableIf<IsIntegral<T>::value>::Type>
: public detail::AtomicBaseIncDec<T, Order>
{
typedef typename detail::AtomicBaseIncDec<T, Order> Base;
public:
MOZ_CONSTEXPR Atomic() : Base() {}
MOZ_CONSTEXPR Atomic(T aInit) : Base(aInit) {}
using Base::operator=;
T operator+=(T delta) { return Base::Intrinsics::add(Base::mValue, delta) + delta; }
T operator-=(T delta) { return Base::Intrinsics::sub(Base::mValue, delta) - delta; }
T operator|=(T val) { return Base::Intrinsics::or_(Base::mValue, val) | val; }
T operator^=(T val) { return Base::Intrinsics::xor_(Base::mValue, val) ^ val; }
T operator&=(T val) { return Base::Intrinsics::and_(Base::mValue, val) & val; }
private:
Atomic(Atomic<T, Order>& aOther) MOZ_DELETE;
};
/**
* Atomic<T> implementation for pointer types.
*
* An atomic compare-and-swap primitive for pointer variables is provided, as
* are atomic increment and decement operators. Also provided are the compound
* assignment operators for addition and subtraction. Atomic swap (via
* exchange()) is included as well.
*/
template<typename T, MemoryOrdering Order>
class Atomic<T*, Order> : public detail::AtomicBaseIncDec<T*, Order>
{
typedef typename detail::AtomicBaseIncDec<T*, Order> Base;
public:
MOZ_CONSTEXPR Atomic() : Base() {}
MOZ_CONSTEXPR Atomic(T* aInit) : Base(aInit) {}
using Base::operator=;
T* operator+=(ptrdiff_t delta) {
return Base::Intrinsics::add(Base::mValue, delta) + delta;
}
T* operator-=(ptrdiff_t delta) {
return Base::Intrinsics::sub(Base::mValue, delta) - delta;
}
private:
Atomic(Atomic<T*, Order>& aOther) MOZ_DELETE;
};
/**
* Atomic<T> implementation for enum types.
*
* The atomic store and load operations and the atomic swap method is provided.
*/
template<typename T, MemoryOrdering Order>
class Atomic<T, Order, typename EnableIf<IsEnum<T>::value>::Type>
: public detail::AtomicBase<T, Order>
{
typedef typename detail::AtomicBase<T, Order> Base;
public:
MOZ_CONSTEXPR Atomic() : Base() {}
MOZ_CONSTEXPR Atomic(T aInit) : Base(aInit) {}
using Base::operator=;
private:
Atomic(Atomic<T, Order>& aOther) MOZ_DELETE;
};
} // namespace mozilla
#endif /* mozilla_Atomics_h */
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