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/* -*- 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"
#ifdef __wasi__
# include "mozilla/WasiAtomic.h"
#else
# include <atomic>
#endif // __wasi__
#include <stdint.h>
#include <type_traits>
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* (reads and writes both) to "memory" may only resolve to out-of-date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.
*
* 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 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& aPtr) {
return aPtr.load(Base::OrderedOp::LoadOrder);
}
static void store(typename Base::ValueType& aPtr, T aVal) {
aPtr.store(aVal, Base::OrderedOp::StoreOrder);
}
static T exchange(typename Base::ValueType& aPtr, T aVal) {
return aPtr.exchange(aVal, Base::OrderedOp::AtomicRMWOrder);
}
static bool compareExchange(typename Base::ValueType& aPtr, T aOldVal,
T aNewVal) {
return aPtr.compare_exchange_strong(
aOldVal, aNewVal, 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& aPtr, T aVal) {
return aPtr.fetch_add(aVal, Base::OrderedOp::AtomicRMWOrder);
}
static T sub(typename Base::ValueType& aPtr, T aVal) {
return aPtr.fetch_sub(aVal, 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& aPtr, ptrdiff_t aVal) {
return aPtr.fetch_add(aVal, Base::OrderedOp::AtomicRMWOrder);
}
static T* sub(typename Base::ValueType& aPtr, ptrdiff_t aVal) {
return aPtr.fetch_sub(aVal, Base::OrderedOp::AtomicRMWOrder);
}
};
template <typename T, MemoryOrdering Order>
struct IntrinsicIncDec : public IntrinsicAddSub<T, Order> {
typedef IntrinsicBase<T, Order> Base;
static T inc(typename Base::ValueType& aPtr) {
return IntrinsicAddSub<T, Order>::add(aPtr, 1);
}
static T dec(typename Base::ValueType& aPtr) {
return IntrinsicAddSub<T, Order>::sub(aPtr, 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& aPtr, T aVal) {
return aPtr.fetch_or(aVal, Base::OrderedOp::AtomicRMWOrder);
}
static T xor_(typename Base::ValueType& aPtr, T aVal) {
return aPtr.fetch_xor(aVal, Base::OrderedOp::AtomicRMWOrder);
}
static T and_(typename Base::ValueType& aPtr, T aVal) {
return aPtr.fetch_and(aVal, Base::OrderedOp::AtomicRMWOrder);
}
};
template <typename T, MemoryOrdering Order>
struct AtomicIntrinsics<T*, Order> : public IntrinsicMemoryOps<T*, Order>,
public IntrinsicIncDec<T*, Order> {};
template <typename T>
struct ToStorageTypeArgument {
static constexpr T convert(T aT) { return aT; }
};
template <typename T, MemoryOrdering Order>
class AtomicBase {
static_assert(sizeof(T) == 4 || sizeof(T) == 8,
"mozilla/Atomics.h only supports 32-bit and 64-bit types");
protected:
typedef typename detail::AtomicIntrinsics<T, Order> Intrinsics;
typedef typename Intrinsics::ValueType ValueType;
ValueType mValue;
public:
constexpr AtomicBase() : mValue() {}
explicit constexpr AtomicBase(T aInit)
: mValue(ToStorageTypeArgument<T>::convert(aInit)) {}
// Note: we can't provide operator T() here because Atomic<bool> inherits
// from AtomcBase with T=uint32_t and not T=bool. If we implemented
// operator T() here, it would cause errors when comparing Atomic<bool> with
// a regular bool.
T operator=(T aVal) {
Intrinsics::store(mValue, aVal);
return aVal;
}
/**
* Performs an atomic swap operation. aVal is stored and the previous
* value of this variable is returned.
*/
T exchange(T aVal) { return Intrinsics::exchange(mValue, aVal); }
/**
* 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:
AtomicBase(const AtomicBase& aCopy) = delete;
};
template <typename T, MemoryOrdering Order>
class AtomicBaseIncDec : public AtomicBase<T, Order> {
typedef typename detail::AtomicBase<T, Order> Base;
public:
constexpr AtomicBaseIncDec() : Base() {}
explicit constexpr AtomicBaseIncDec(T aInit) : Base(aInit) {}
using Base::operator=;
operator T() const { return Base::Intrinsics::load(Base::mValue); }
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:
AtomicBaseIncDec(const AtomicBaseIncDec& aCopy) = 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,
std::enable_if_t<std::is_integral_v<T> && !std::is_same_v<T, bool>>>
: public detail::AtomicBaseIncDec<T, Order> {
typedef typename detail::AtomicBaseIncDec<T, Order> Base;
public:
constexpr Atomic() : Base() {}
explicit constexpr Atomic(T aInit) : Base(aInit) {}
using Base::operator=;
T operator+=(T aDelta) {
return Base::Intrinsics::add(Base::mValue, aDelta) + aDelta;
}
T operator-=(T aDelta) {
return Base::Intrinsics::sub(Base::mValue, aDelta) - aDelta;
}
T operator|=(T aVal) {
return Base::Intrinsics::or_(Base::mValue, aVal) | aVal;
}
T operator^=(T aVal) {
return Base::Intrinsics::xor_(Base::mValue, aVal) ^ aVal;
}
T operator&=(T aVal) {
return Base::Intrinsics::and_(Base::mValue, aVal) & aVal;
}
private:
Atomic(Atomic& aOther) = 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:
constexpr Atomic() : Base() {}
explicit constexpr Atomic(T* aInit) : Base(aInit) {}
using Base::operator=;
T* operator+=(ptrdiff_t aDelta) {
return Base::Intrinsics::add(Base::mValue, aDelta) + aDelta;
}
T* operator-=(ptrdiff_t aDelta) {
return Base::Intrinsics::sub(Base::mValue, aDelta) - aDelta;
}
private:
Atomic(Atomic& aOther) = 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, std::enable_if_t<std::is_enum_v<T>>>
: public detail::AtomicBase<T, Order> {
typedef typename detail::AtomicBase<T, Order> Base;
public:
constexpr Atomic() : Base() {}
explicit constexpr Atomic(T aInit) : Base(aInit) {}
operator T() const { return T(Base::Intrinsics::load(Base::mValue)); }
using Base::operator=;
private:
Atomic(Atomic& aOther) = delete;
};
/**
* Atomic<T> implementation for boolean types.
*
* The atomic store and load operations and the atomic swap method is provided.
*
* Note:
*
* - sizeof(Atomic<bool>) != sizeof(bool) for some implementations of
* bool and/or some implementations of std::atomic. This is allowed in
* [atomic.types.generic]p9.
*
* - It's not obvious whether the 8-bit atomic functions on Windows are always
* inlined or not. If they are not inlined, the corresponding functions in the
* runtime library are not available on Windows XP. This is why we implement
* Atomic<bool> with an underlying type of uint32_t.
*/
template <MemoryOrdering Order>
class Atomic<bool, Order> : protected detail::AtomicBase<uint32_t, Order> {
typedef typename detail::AtomicBase<uint32_t, Order> Base;
public:
constexpr Atomic() : Base() {}
explicit constexpr Atomic(bool aInit) : Base(aInit) {}
// We provide boolean wrappers for the underlying AtomicBase methods.
MOZ_IMPLICIT operator bool() const {
return Base::Intrinsics::load(Base::mValue);
}
bool operator=(bool aVal) { return Base::operator=(aVal); }
bool exchange(bool aVal) { return Base::exchange(aVal); }
bool compareExchange(bool aOldValue, bool aNewValue) {
return Base::compareExchange(aOldValue, aNewValue);
}
private:
Atomic(Atomic& aOther) = delete;
};
} // namespace mozilla
namespace std {
// If you want to atomically swap two atomic values, use exchange().
template <typename T, mozilla::MemoryOrdering Order>
void swap(mozilla::Atomic<T, Order>&, mozilla::Atomic<T, Order>&) = delete;
} // namespace std
#endif /* mozilla_Atomics_h */