The Tor Browser: mfbt/Atomics.h@97036ab72558 (annotated)

mfbt/Atomics.h@97036ab72558 (annotated)

mfbt/Atomics.h

Tue, 06 Jan 2015 21:39:09 +0100

author: Michael Schloh von Bennewitz <michael@schloh.com>
date: Tue, 06 Jan 2015 21:39:09 +0100
branch: TOR_BUG_9701
changeset 8: 97036ab72558
permissions: -rw-r--r--

Conditionally force memory storage according to privacy.thirdparty.isolate;
This solves Tor bug #9701, complying with disk avoidance documented in
https://www.torproject.org/projects/torbrowser/design/#disk-avoidance.

 /* -*- 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
 #  if defined(DEBUG)
      /*
       * Provide our own failure code since we're having trouble linking to
       * std::_Debug_message (bug 982310).
       */
 #    define _INVALID_MEMORY_ORDER MOZ_CRASH("Invalid memory order")
 #  endif
 #  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>
 {
     typedef typename IntrinsicBase<T>::ValueType ValueType;
     typedef typename IntrinsicBase<T>::Primitives Primitives;
     typedef typename IntrinsicBase<T>::PrimType PrimType;
     typedef typename IntrinsicBase<T>::Cast Cast;
     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 typename IntrinsicBase<T>::ValueType ValueType;
     typedef typename IntrinsicBase<T>::PrimType PrimType;
     typedef typename IntrinsicBase<T>::Cast Cast;
     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>
 {
     typedef typename IntrinsicApplyHelper<T>::ValueType ValueType;
     typedef typename IntrinsicBase<T>::Primitives Primitives;
     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*>
 {
     typedef typename IntrinsicApplyHelper<T*>::ValueType ValueType;
     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>
 {
     typedef typename IntrinsicAddSub<T>::ValueType ValueType;
     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>
 {
     typedef typename IntrinsicIncDec<T>::ValueType ValueType;
     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*>
 {
     typedef typename IntrinsicMemoryOps<T*, Order>::ValueType ValueType;
 };
 } // 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) {}
     // 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 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=;
     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:
     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 && !IsSame<T, bool>::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) {}
     operator T() const { return Base::Intrinsics::load(Base::mValue); }
     using Base::operator=;
   private:
     Atomic(Atomic<T, Order>& aOther) MOZ_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:
     MOZ_CONSTEXPR Atomic() : Base() {}
     MOZ_CONSTEXPR Atomic(bool aInit) : Base(aInit) {}
     // We provide boolean wrappers for the underlying AtomicBase methods.
     operator bool() const { return Base::Intrinsics::load(Base::mValue); }
     bool operator=(bool aValue) { return Base::operator=(aValue); }
     bool exchange(bool aValue) { return Base::exchange(aValue); }
     bool compareExchange(bool aOldValue, bool aNewValue) {
       return Base::compareExchange(aOldValue, aNewValue);
     }
   private:
     Atomic(Atomic<bool, Order>& aOther) MOZ_DELETE;
 };
 } // namespace mozilla
 #endif /* mozilla_Atomics_h */

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mfbt/Atomics.h@97036ab72558 (annotated)

mfbt/Atomics.h