DXR is a code search and navigation tool aimed at making sense of large projects. It supports full-text and regex searches as well as structural queries.

Mercurial (8e969cc9aff4)

VCS Links

Atomic

Atomic

Atomic

Atomic

AtomicBase

AtomicBaseIncDec

AtomicIntrinsics

AtomicIntrinsics

AtomicOrderConstraints

AtomicOrderConstraints

AtomicOrderConstraints

IntrinsicAddSub

IntrinsicAddSub

IntrinsicBase

IntrinsicIncDec

IntrinsicMemoryOps

MemoryOrdering

ToStorageTypeArgument

Macros

Line Code
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552 553 554 555 556 557 558 559 560 561 562 563 564 565 566 567 568 569 570 571 572 573 574 575 576 577 578 579 580 581 582 583 584 585 586 587 588 589 590 591 592 593 594 595 596 597 598 599 600 601 602 603 604 605 606 607 608 609 610 611 612 613 614 615 616 617 618 619 620 621 622 623 624 625 626 627 628 629 630 631 632 633 634 635 636 637 638 639 640 641 642 643 644 645 646 647 648 649 650 651 652 653 654 655 656 657 658 659 660 661 662 663 664 665 666 667 668 669 670 671 672 673 674 675 676 677 678 679 680 681 682 683 684 685 686 687 688 689 690 691 692 693 694 695 696 697 698 699 700 701 702 703 704 705 706 707 708 709 710 711 712 713 714 715 716 717 718 719 720 721 722 723 724 725 726 727 728 729 730 731 732 733 734 735 736 737 738 739 740 741 742 743 744 745 746 747 748 749 750 751 752 753 754 755 756 757 758 759 760 761 762 763 764 765 766 767 768 769 770 771 772 773 774 775 776 777 778 779 780 781 782 783 784 785 786 787 788 789 790 791 792 793 794 795 796 797 798 799 800
/* -*- 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(_MSC_VER)
#  define MOZ_HAVE_CXX11_ATOMICS
#elif 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++.
    *
    * libc++ <atomic> is only functional with clang.
    */
#  if MOZ_USING_LIBSTDCXX && MOZ_LIBSTDCXX_VERSION_AT_LEAST(4, 7, 0)
#    define MOZ_HAVE_CXX11_ATOMICS
#  elif MOZ_USING_LIBCXX && defined(__clang__)
#    define MOZ_HAVE_CXX11_ATOMICS
#  endif
#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& 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; }
};

} // 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, bool TIsEnum = IsEnum<T>::value>
struct AtomicStorageType
{
  // For non-enums, just use the type directly.
  typedef T Type;
};

template<typename T>
struct AtomicStorageType<T, true>
  : Conditional<sizeof(T) == 4, uint32_t, uint64_t>
{
  static_assert(sizeof(T) == 4 || sizeof(T) == 8,
                "wrong type computed in conditional above");
};

template<typename T, MemoryOrdering Order>
struct IntrinsicMemoryOps
{
  typedef typename AtomicStorageType<T>::Type ValueType;

  static T load(const ValueType& aPtr)
  {
    Barrier<Order>::beforeLoad();
    T val = T(aPtr);
    Barrier<Order>::afterLoad();
    return val;
  }

  static void store(ValueType& aPtr, T aVal)
  {
    Barrier<Order>::beforeStore();
    aPtr = ValueType(aVal);
    Barrier<Order>::afterStore();
  }

  static T exchange(ValueType& aPtr, T aVal)
  {
    // __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 T(__sync_lock_test_and_set(&aPtr, ValueType(aVal)));
  }

  static bool compareExchange(ValueType& aPtr, T aOldVal, T aNewVal)
  {
    return __sync_bool_compare_and_swap(&aPtr, ValueType(aOldVal), ValueType(aNewVal));
  }
};

template<typename T, MemoryOrdering Order>
struct IntrinsicAddSub
  : public IntrinsicMemoryOps<T, Order>
{
  typedef IntrinsicMemoryOps<T, Order> Base;
  typedef typename Base::ValueType ValueType;

  static T add(ValueType& aPtr, T aVal)
  {
    return T(__sync_fetch_and_add(&aPtr, ValueType(aVal)));
  }

  static T sub(ValueType& aPtr, T aVal)
  {
    return T(__sync_fetch_and_sub(&aPtr, ValueType(aVal)));
  }
};

template<typename T, MemoryOrdering Order>
struct IntrinsicAddSub<T*, Order>
  : public IntrinsicMemoryOps<T*, Order>
{
  typedef IntrinsicMemoryOps<T*, Order> Base;
  typedef typename Base::ValueType 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& aPtr, ptrdiff_t aVal)
  {
    ValueType amount = reinterpret_cast<ValueType>(aVal * sizeof(T));
    return __sync_fetch_and_add(&aPtr, amount);
  }

  static ValueType sub(ValueType& aPtr, ptrdiff_t aVal)
  {
    ValueType amount = reinterpret_cast<ValueType>(aVal * sizeof(T));
    return __sync_fetch_and_sub(&aPtr, amount);
  }
};

template<typename T, MemoryOrdering Order>
struct IntrinsicIncDec : public IntrinsicAddSub<T, Order>
{
  typedef IntrinsicAddSub<T, Order> Base;
  typedef typename Base::ValueType ValueType;

  static T inc(ValueType& aPtr) { return Base::add(aPtr, 1); }
  static T dec(ValueType& aPtr) { return Base::sub(aPtr, 1); }
};

template<typename T, MemoryOrdering Order>
struct AtomicIntrinsics : public IntrinsicIncDec<T, Order>
{
  static T or_( T& aPtr, T aVal) { return __sync_fetch_and_or(&aPtr, aVal); }
  static T xor_(T& aPtr, T aVal) { return __sync_fetch_and_xor(&aPtr, aVal); }
  static T and_(T& aPtr, T aVal) { return __sync_fetch_and_and(&aPtr, aVal); }
};

template<typename T, MemoryOrdering Order>
struct AtomicIntrinsics<T*, Order> : public IntrinsicIncDec<T*, Order>
{
};

template<typename T, bool TIsEnum = IsEnum<T>::value>
struct ToStorageTypeArgument
{
  typedef typename AtomicStorageType<T>::Type ResultType;

  static constexpr ResultType convert (T aT) { return ResultType(aT); }
};

template<typename T>
struct ToStorageTypeArgument<T, false>
{
  static constexpr T convert (T aT) { return aT; }
};

} // 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
{
  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:
  template<MemoryOrdering AnyOrder>
  AtomicBase(const AtomicBase<T, AnyOrder>& 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:
  template<MemoryOrdering AnyOrder>
  AtomicBaseIncDec(const AtomicBaseIncDec<T, AnyOrder>& 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, typename EnableIf<IsIntegral<T>::value &&
                       !IsSame<T, bool>::value>::Type>
  : 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<T, Order>& 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<T*, Order>& 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, typename EnableIf<IsEnum<T>::value>::Type>
  : 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<T, Order>& 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<bool, Order>& aOther) = delete;
};

} // namespace mozilla

#endif /* mozilla_Atomics_h */