text: /*
   * Copyright (c) 1999, 2019, Oracle and/or its affiliates. All rights reserved.
   * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   *
   * This code is free software; you can redistribute it and/or modify it
   * under the terms of the GNU General Public License version 2 only, as
   * published by the Free Software Foundation.
   *
   * This code is distributed in the hope that it will be useful, but WITHOUT
   * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
   * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
   * version 2 for more details (a copy is included in the LICENSE file that
   * accompanied this code).
   *
   * You should have received a copy of the GNU General Public License version
   * 2 along with this work; if not, write to the Free Software Foundation,
   * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
   *
   * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
   * or visit www.oracle.com if you need additional information or have any
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  #ifndef SHARE_RUNTIME_ATOMIC_HPP
  #define SHARE_RUNTIME_ATOMIC_HPP
  
  #include "memory/allocation.hpp"
  #include "metaprogramming/conditional.hpp"
  #include "metaprogramming/enableIf.hpp"
  #include "metaprogramming/isIntegral.hpp"
  #include "metaprogramming/isPointer.hpp"
  #include "metaprogramming/isSame.hpp"
  #include "metaprogramming/primitiveConversions.hpp"
  #include "metaprogramming/removeCV.hpp"
  #include "metaprogramming/removePointer.hpp"
  #include "runtime/orderAccess.hpp"
  #include "utilities/align.hpp"
  #include "utilities/bytes.hpp"
  #include "utilities/macros.hpp"
  #include <type_traits>
  
  enum atomic_memory_order {
    // The modes that align with C++11 are intended to
    // follow the same semantics.
    memory_order_relaxed = 0,
    memory_order_acquire = 2,
    memory_order_release = 3,
    memory_order_acq_rel = 4,
    // Strong two-way memory barrier.
    memory_order_conservative = 8
  };
  
  enum ScopedFenceType {
      X_ACQUIRE
    , RELEASE_X
    , RELEASE_X_FENCE
  };
  
  class Atomic : AllStatic {
  public:
    // Atomic operations on int64 types are not available on all 32-bit
    // platforms. If atomic ops on int64 are defined here they must only
    // be used from code that verifies they are available at runtime and
    // can provide an alternative action if not - see supports_cx8() for
    // a means to test availability.
  
    // The memory operations that are mentioned with each of the atomic
    // function families come from src/share/vm/runtime/orderAccess.hpp,
    // e.g., <fence> is described in that file and is implemented by the
    // OrderAccess::fence() function. See that file for the gory details
    // on the Memory Access Ordering Model.
  
    // All of the atomic operations that imply a read-modify-write action
    // guarantee a two-way memory barrier across that operation. Historically
    // these semantics reflect the strength of atomic operations that are
    // provided on SPARC/X86. We assume that strength is necessary unless
    // we can prove that a weaker form is sufficiently safe.
  
    // Atomically store to a location
    // The type T must be either a pointer type convertible to or equal
    // to D, an integral/enum type equal to D, or a type equal to D that
    // is primitive convertible using PrimitiveConversions.
    template<typename D, typename T>
    inline static void store(volatile D* dest, T store_value);
  
    template <typename D, typename T>
    inline static void release_store(volatile D* dest, T store_value);
  
    template <typename D, typename T>
    inline static void release_store_fence(volatile D* dest, T store_value);
  
    // Atomically load from a location
    // The type T must be either a pointer type, an integral/enum type,
    // or a type that is primitive convertible using PrimitiveConversions.
    template<typename T>
    inline static T load(const volatile T* dest);
  
    template <typename T>
    inline static T load_acquire(const volatile T* dest);
  
    // Atomically add to a location. *add*() provide:
    // <fence> add-value-to-dest <membar StoreLoad|StoreStore>
  
    // Returns updated value.
    template<typename D, typename I>
    inline static D add(D volatile* dest, I add_value,
                        atomic_memory_order order = memory_order_conservative);
  
    // Returns previous value.
    template<typename D, typename I>
    inline static D fetch_and_add(D volatile* dest, I add_value,
                                  atomic_memory_order order = memory_order_conservative);
  
    template<typename D, typename I>
    inline static D sub(D volatile* dest, I sub_value,
                        atomic_memory_order order = memory_order_conservative);
  
    // Atomically increment location. inc() provide:
    // <fence> increment-dest <membar StoreLoad|StoreStore>
    // The type D may be either a pointer type, or an integral
    // type. If it is a pointer type, then the increment is
    // scaled to the size of the type pointed to by the pointer.
    template<typename D>
    inline static void inc(D volatile* dest,
                           atomic_memory_order order = memory_order_conservative);
  
    // Atomically decrement a location. dec() provide:
    // <fence> decrement-dest <membar StoreLoad|StoreStore>
    // The type D may be either a pointer type, or an integral
    // type. If it is a pointer type, then the decrement is
    // scaled to the size of the type pointed to by the pointer.
    template<typename D>
    inline static void dec(D volatile* dest,
                           atomic_memory_order order = memory_order_conservative);
  
    // Performs atomic exchange of *dest with exchange_value. Returns old
    // prior value of *dest. xchg*() provide:
    // <fence> exchange-value-with-dest <membar StoreLoad|StoreStore>
    // The type T must be either a pointer type convertible to or equal
    // to D, an integral/enum type equal to D, or a type equal to D that
    // is primitive convertible using PrimitiveConversions.
    template<typename D, typename T>
    inline static D xchg(volatile D* dest, T exchange_value,
                         atomic_memory_order order = memory_order_conservative);
  
    // Performs atomic compare of *dest and compare_value, and exchanges
    // *dest with exchange_value if the comparison succeeded. Returns prior
    // value of *dest. cmpxchg*() provide:
    // <fence> compare-and-exchange <membar StoreLoad|StoreStore>
  
    template<typename D, typename U, typename T>
    inline static D cmpxchg(D volatile* dest,
                            U compare_value,
                            T exchange_value,
                            atomic_memory_order order = memory_order_conservative);
  
    // Performs atomic compare of *dest and NULL, and replaces *dest
    // with exchange_value if the comparison succeeded.  Returns true if
    // the comparison succeeded and the exchange occurred.  This is
    // often used as part of lazy initialization, as a lock-free
    // alternative to the Double-Checked Locking Pattern.
    template<typename D, typename T>
    inline static bool replace_if_null(D* volatile* dest, T* value,
                                       atomic_memory_order order = memory_order_conservative);
  
  private:
  WINDOWS_ONLY(public:) // VS2017 warns (C2027) use of undefined type if IsPointerConvertible is declared private
    // Test whether From is implicitly convertible to To.
    // From and To must be pointer types.
    // Note: Provides the limited subset of C++11 std::is_convertible
    // that is needed here.
    template<typename From, typename To> struct IsPointerConvertible;
  
  protected:
    // Dispatch handler for store.  Provides type-based validity
    // checking and limited conversions around calls to the platform-
    // specific implementation layer provided by PlatformOp.
    template<typename D, typename T, typename PlatformOp, typename Enable = void>
    struct StoreImpl;
  
    // Platform-specific implementation of store.  Support for sizes
    // of 1, 2, 4, and (if different) pointer size bytes are required.
    // The class is a function object that must be default constructable,
    // with these requirements:
    //
    // either:
    // - dest is of type D*, an integral, enum or pointer type.
    // - new_value are of type T, an integral, enum or pointer type D or
    //   pointer type convertible to D.
    // or:
    // - T and D are the same and are primitive convertible using PrimitiveConversions
    // and either way:
    // - platform_store is an object of type PlatformStore<sizeof(T)>.
    //
    // Then
    //   platform_store(new_value, dest)
    // must be a valid expression.
    //
    // The default implementation is a volatile store. If a platform
    // requires more for e.g. 64 bit stores, a specialization is required
    template<size_t byte_size> struct PlatformStore;
  
    // Dispatch handler for load.  Provides type-based validity
    // checking and limited conversions around calls to the platform-
    // specific implementation layer provided by PlatformOp.
    template<typename T, typename PlatformOp, typename Enable = void>
    struct LoadImpl;
  
    // Platform-specific implementation of load. Support for sizes of
    // 1, 2, 4 bytes and (if different) pointer size bytes are required.
    // The class is a function object that must be default
    // constructable, with these requirements:
    //
    // - dest is of type T*, an integral, enum or pointer type, or
    //   T is convertible to a primitive type using PrimitiveConversions
    // - platform_load is an object of type PlatformLoad<sizeof(T)>.
    //
    // Then
    //   platform_load(src)
    // must be a valid expression, returning a result convertible to T.
    //
    // The default implementation is a volatile load. If a platform
    // requires more for e.g. 64 bit loads, a specialization is required
    template<size_t byte_size> struct PlatformLoad;
  
    // Give platforms a variation point to specialize.
    template<size_t byte_size, ScopedFenceType type> struct PlatformOrderedStore;
    template<size_t byte_size, ScopedFenceType type> struct PlatformOrderedLoad;
  
  private:
    // Dispatch handler for add.  Provides type-based validity checking
    // and limited conversions around calls to the platform-specific
    // implementation layer provided by PlatformAdd.
    template<typename D, typename I, typename Enable = void>
    struct AddImpl;
  
    // Platform-specific implementation of add.  Support for sizes of 4
    // bytes and (if different) pointer size bytes are required.  The
    // class must be default constructable, with these requirements:
    //
    // - dest is of type D*, an integral or pointer type.
    // - add_value is of type I, an integral type.
    // - sizeof(I) == sizeof(D).
    // - if D is an integral type, I == D.
    // - order is of type atomic_memory_order.
    // - platform_add is an object of type PlatformAdd<sizeof(D)>.
    //
    // Then both
    //   platform_add.add_and_fetch(dest, add_value, order)
    //   platform_add.fetch_and_add(dest, add_value, order)
    // must be valid expressions returning a result convertible to D.
    //
    // add_and_fetch atomically adds add_value to the value of dest,
    // returning the new value.
    //
    // fetch_and_add atomically adds add_value to the value of dest,
    // returning the old value.
    //
    // When D is a pointer type P*, both add_and_fetch and fetch_and_add
    // treat it as if it were an uintptr_t; they do not perform any
    // scaling of add_value, as that has already been done by the caller.
    //
    // No definition is provided; all platforms must explicitly define
    // this class and any needed specializations.
    template<size_t byte_size> struct PlatformAdd;
  
    // Support for platforms that implement some variants of add using a
    // (typically out of line) non-template helper function.  The
    // generic arguments passed to PlatformAdd need to be translated to
    // the appropriate type for the helper function, the helper function
    // invoked on the translated arguments, and the result translated
    // back.  Type is the parameter / return type of the helper
    // function.  No scaling of add_value is performed when D is a pointer
    // type, so this function can be used to implement the support function
    // required by AddAndFetch.
    template<typename Type, typename Fn, typename D, typename I>
    static D add_using_helper(Fn fn, D volatile* dest, I add_value);
  
    // Dispatch handler for cmpxchg.  Provides type-based validity
    // checking and limited conversions around calls to the
    // platform-specific implementation layer provided by
    // PlatformCmpxchg.
    template<typename D, typename U, typename T, typename Enable = void>
    struct CmpxchgImpl;
  
    // Platform-specific implementation of cmpxchg.  Support for sizes
    // of 1, 4, and 8 are required.  The class is a function object that
    // must be default constructable, with these requirements:
    //
    // - dest is of type T*.
    // - exchange_value and compare_value are of type T.
    // - order is of type atomic_memory_order.
    // - platform_cmpxchg is an object of type PlatformCmpxchg<sizeof(T)>.
    //
    // Then
    //   platform_cmpxchg(dest, compare_value, exchange_value, order)
    // must be a valid expression, returning a result convertible to T.
    //
    // A default definition is provided, which declares a function template
    //   T operator()(T volatile*, T, T, atomic_memory_order) const
    //
    // For each required size, a platform must either provide an
    // appropriate definition of that function, or must entirely
    // specialize the class template for that size.
    template<size_t byte_size> struct PlatformCmpxchg;
  
    // Support for platforms that implement some variants of cmpxchg
    // using a (typically out of line) non-template helper function.
    // The generic arguments passed to PlatformCmpxchg need to be
    // translated to the appropriate type for the helper function, the
    // helper invoked on the translated arguments, and the result
    // translated back.  Type is the parameter / return type of the
    // helper function.
    template<typename Type, typename Fn, typename T>
    static T cmpxchg_using_helper(Fn fn,
                                  T volatile* dest,
                                  T compare_value,
                                  T exchange_value);
  
    // Support platforms that do not provide Read-Modify-Write
    // byte-level atomic access. To use, derive PlatformCmpxchg<1> from
    // this class.
  public: // Temporary, can't be private: C++03 11.4/2. Fixed by C++11.
    struct CmpxchgByteUsingInt;
  private:
  
    // Dispatch handler for xchg.  Provides type-based validity
    // checking and limited conversions around calls to the
    // platform-specific implementation layer provided by
    // PlatformXchg.
    template<typename D, typename T, typename Enable = void>
    struct XchgImpl;
  
    // Platform-specific implementation of xchg.  Support for sizes
    // of 4, and sizeof(intptr_t) are required.  The class is a function
    // object that must be default constructable, with these requirements:
    //
    // - dest is of type T*.
    // - exchange_value is of type T.
    // - platform_xchg is an object of type PlatformXchg<sizeof(T)>.
    //
    // Then
    //   platform_xchg(dest, exchange_value)
    // must be a valid expression, returning a result convertible to T.
    //
    // A default definition is provided, which declares a function template
    //   T operator()(T volatile*, T, atomic_memory_order) const
    //
    // For each required size, a platform must either provide an
    // appropriate definition of that function, or must entirely
    // specialize the class template for that size.
    template<size_t byte_size> struct PlatformXchg;
  
    // Support for platforms that implement some variants of xchg
    // using a (typically out of line) non-template helper function.
    // The generic arguments passed to PlatformXchg need to be
    // translated to the appropriate type for the helper function, the
    // helper invoked on the translated arguments, and the result
    // translated back.  Type is the parameter / return type of the
    // helper function.
    template<typename Type, typename Fn, typename T>
    static T xchg_using_helper(Fn fn,
                               T volatile* dest,
                               T exchange_value);
  };
  
  template<typename From, typename To>
  struct Atomic::IsPointerConvertible<From*, To*> : AllStatic {
    // Determine whether From* is implicitly convertible to To*, using
    // the "sizeof trick".
    typedef char yes;
    typedef char (&no)[2];
  
    static yes test(To*);
    static no test(...);
    static From* test_value;
  
    static const bool value = (sizeof(yes) == sizeof(test(test_value)));
  };
  
  // Handle load for pointer, integral and enum types.
  template<typename T, typename PlatformOp>
  struct Atomic::LoadImpl<
    T,
    PlatformOp,
    typename EnableIf<IsIntegral<T>::value || std::is_enum<T>::value || IsPointer<T>::value>::type>
  {
    T operator()(T const volatile* dest) const {
      // Forward to the platform handler for the size of T.
      return PlatformOp()(dest);
    }
  };
  
  // Handle load for types that have a translator.
  //
  // All the involved types must be identical.
  //
  // This translates the original call into a call on the decayed
  // arguments, and returns the recovered result of that translated
  // call.
  template<typename T, typename PlatformOp>
  struct Atomic::LoadImpl<
    T,
    PlatformOp,
    typename EnableIf<PrimitiveConversions::Translate<T>::value>::type>
  {
    T operator()(T const volatile* dest) const {
      typedef PrimitiveConversions::Translate<T> Translator;
      typedef typename Translator::Decayed Decayed;
      STATIC_ASSERT(sizeof(T) == sizeof(Decayed));
      Decayed result = PlatformOp()(reinterpret_cast<Decayed const volatile*>(dest));
      return Translator::recover(result);
    }
  };
  
  // Default implementation of atomic load if a specific platform
  // does not provide a specialization for a certain size class.
  // For increased safety, the default implementation only allows
  // load types that are pointer sized or smaller. If a platform still
  // supports wide atomics, then it has to use specialization
  // of Atomic::PlatformLoad for that wider size class.
  template<size_t byte_size>
  struct Atomic::PlatformLoad {
    template<typename T>
    T operator()(T const volatile* dest) const {
      STATIC_ASSERT(sizeof(T) <= sizeof(void*)); // wide atomics need specialization
      return *dest;
    }
  };
  
  // Handle store for integral and enum types.
  //
  // All the involved types must be identical.
  template<typename T, typename PlatformOp>
  struct Atomic::StoreImpl<
    T, T,
    PlatformOp,
    typename EnableIf<IsIntegral<T>::value || std::is_enum<T>::value>::type>
  {
    void operator()(T volatile* dest, T new_value) const {
      // Forward to the platform handler for the size of T.
      PlatformOp()(dest, new_value);
    }
  };
  
  // Handle store for pointer types.
  //
  // The new_value must be implicitly convertible to the
  // destination's type; it must be type-correct to store the
  // new_value in the destination.
  template<typename D, typename T, typename PlatformOp>
  struct Atomic::StoreImpl<
    D*, T*,
    PlatformOp,
    typename EnableIf<Atomic::IsPointerConvertible<T*, D*>::value>::type>
  {
    void operator()(D* volatile* dest, T* new_value) const {
      // Allow derived to base conversion, and adding cv-qualifiers.
      D* value = new_value;
      PlatformOp()(dest, value);
    }
  };
  
  // Handle store for types that have a translator.
  //
  // All the involved types must be identical.
  //
  // This translates the original call into a call on the decayed
  // arguments.
  template<typename T, typename PlatformOp>
  struct Atomic::StoreImpl<
    T, T,
    PlatformOp,
    typename EnableIf<PrimitiveConversions::Translate<T>::value>::type>
  {
    void operator()(T volatile* dest, T new_value) const {
      typedef PrimitiveConversions::Translate<T> Translator;
      typedef typename Translator::Decayed Decayed;
      STATIC_ASSERT(sizeof(T) == sizeof(Decayed));
      PlatformOp()(reinterpret_cast<Decayed volatile*>(dest),
                   Translator::decay(new_value));
    }
  };
  
  // Default implementation of atomic store if a specific platform
  // does not provide a specialization for a certain size class.
  // For increased safety, the default implementation only allows
  // storing types that are pointer sized or smaller. If a platform still
  // supports wide atomics, then it has to use specialization
  // of Atomic::PlatformStore for that wider size class.
  template<size_t byte_size>
  struct Atomic::PlatformStore {
    template<typename T>
    void operator()(T volatile* dest,
                    T new_value) const {
      STATIC_ASSERT(sizeof(T) <= sizeof(void*)); // wide atomics need specialization
      (void)const_cast<T&>(*dest = new_value);
    }
  };
  
  template<typename D>
  inline void Atomic::inc(D volatile* dest, atomic_memory_order order) {
    STATIC_ASSERT(IsPointer<D>::value || IsIntegral<D>::value);
    typedef typename Conditional<IsPointer<D>::value, ptrdiff_t, D>::type I;
    Atomic::add(dest, I(1), order);
  }
  
  template<typename D>
  inline void Atomic::dec(D volatile* dest, atomic_memory_order order) {
    STATIC_ASSERT(IsPointer<D>::value || IsIntegral<D>::value);
    typedef typename Conditional<IsPointer<D>::value, ptrdiff_t, D>::type I;
    // Assumes two's complement integer representation.
    #pragma warning(suppress: 4146)
    Atomic::add(dest, I(-1), order);
  }
  
  template<typename D, typename I>
  inline D Atomic::sub(D volatile* dest, I sub_value, atomic_memory_order order) {
    STATIC_ASSERT(IsPointer<D>::value || IsIntegral<D>::value);
    STATIC_ASSERT(IsIntegral<I>::value);
    // If D is a pointer type, use [u]intptr_t as the addend type,
    // matching signedness of I.  Otherwise, use D as the addend type.
    typedef typename Conditional<IsSigned<I>::value, intptr_t, uintptr_t>::type PI;
    typedef typename Conditional<IsPointer<D>::value, PI, D>::type AddendType;
    // Only allow conversions that can't change the value.
    STATIC_ASSERT(IsSigned<I>::value == IsSigned<AddendType>::value);
    STATIC_ASSERT(sizeof(I) <= sizeof(AddendType));
    AddendType addend = sub_value;
    // Assumes two's complement integer representation.
    #pragma warning(suppress: 4146) // In case AddendType is not signed.
    return Atomic::add(dest, -addend, order);
  }
  
  // Define the class before including platform file, which may specialize
  // the operator definition.  No generic definition of specializations
  // of the operator template are provided, nor are there any generic
  // specializations of the class.  The platform file is responsible for
  // providing those.
  template<size_t byte_size>
  struct Atomic::PlatformCmpxchg {
    template<typename T>
    T operator()(T volatile* dest,
                 T compare_value,
                 T exchange_value,
                 atomic_memory_order order) const;
  };
  
  // Define the class before including platform file, which may use this
  // as a base class, requiring it be complete.  The definition is later
  // in this file, near the other definitions related to cmpxchg.
  struct Atomic::CmpxchgByteUsingInt {
    static uint8_t get_byte_in_int(uint32_t n, uint32_t idx);
    static uint32_t set_byte_in_int(uint32_t n, uint8_t b, uint32_t idx);
    template<typename T>
    T operator()(T volatile* dest,
                 T compare_value,
                 T exchange_value,
                 atomic_memory_order order) const;
  };
  
  // Define the class before including platform file, which may specialize
  // the operator definition.  No generic definition of specializations
  // of the operator template are provided, nor are there any generic
  // specializations of the class.  The platform file is responsible for
  // providing those.
  template<size_t byte_size>
  struct Atomic::PlatformXchg {
    template<typename T>
    T operator()(T volatile* dest,
                 T exchange_value,
                 atomic_memory_order order) const;
  };
  
  template <ScopedFenceType T>
  class ScopedFenceGeneral: public StackObj {
   public:
    void prefix() {}
    void postfix() {}
  };
  
  // The following methods can be specialized using simple template specialization
  // in the platform specific files for optimization purposes. Otherwise the
  // generalized variant is used.
  
  template<> inline void ScopedFenceGeneral<X_ACQUIRE>::postfix()       { OrderAccess::acquire(); }
  template<> inline void ScopedFenceGeneral<RELEASE_X>::prefix()        { OrderAccess::release(); }
  template<> inline void ScopedFenceGeneral<RELEASE_X_FENCE>::prefix()  { OrderAccess::release(); }
  template<> inline void ScopedFenceGeneral<RELEASE_X_FENCE>::postfix() { OrderAccess::fence();   }
  
  template <ScopedFenceType T>
  class ScopedFence : public ScopedFenceGeneral<T> {
    void *const _field;
   public:
    ScopedFence(void *const field) : _field(field) { prefix(); }
    ~ScopedFence() { postfix(); }
    void prefix() { ScopedFenceGeneral<T>::prefix(); }
    void postfix() { ScopedFenceGeneral<T>::postfix(); }
  };
  
  // platform specific in-line definitions - must come before shared definitions
  
  #include OS_CPU_HEADER(atomic)
  
  // shared in-line definitions
  
  // size_t casts...
  #if (SIZE_MAX != UINTPTR_MAX)
  #error size_t is not WORD_SIZE, interesting platform, but missing implementation here
  #endif
  
  template<typename T>
  inline T Atomic::load(const volatile T* dest) {
    return LoadImpl<T, PlatformLoad<sizeof(T)> >()(dest);
  }
  
  template<size_t byte_size, ScopedFenceType type>
  struct Atomic::PlatformOrderedLoad {
    template <typename T>
    T operator()(const volatile T* p) const {
      ScopedFence<type> f((void*)p);
      return Atomic::load(p);
    }
  };
  
  template <typename T>
  inline T Atomic::load_acquire(const volatile T* p) {
    return LoadImpl<T, PlatformOrderedLoad<sizeof(T), X_ACQUIRE> >()(p);
  }
  
  template<typename D, typename T>
  inline void Atomic::store(volatile D* dest, T store_value) {
    StoreImpl<D, T, PlatformStore<sizeof(D)> >()(dest, store_value);
  }
  
  template<size_t byte_size, ScopedFenceType type>
  struct Atomic::PlatformOrderedStore {
    template <typename T>
    void operator()(volatile T* p, T v) const {
      ScopedFence<type> f((void*)p);
      Atomic::store(p, v);
    }
  };
  
  template <typename D, typename T>
  inline void Atomic::release_store(volatile D* p, T v) {
    StoreImpl<D, T, PlatformOrderedStore<sizeof(D), RELEASE_X> >()(p, v);
  }
  
  template <typename D, typename T>
  inline void Atomic::release_store_fence(volatile D* p, T v) {
    StoreImpl<D, T, PlatformOrderedStore<sizeof(D), RELEASE_X_FENCE> >()(p, v);
  }
  
  template<typename D, typename I>
  inline D Atomic::add(D volatile* dest, I add_value,
                       atomic_memory_order order) {
    return AddImpl<D, I>::add_and_fetch(dest, add_value, order);
  }
  
  template<typename D, typename I>
  inline D Atomic::fetch_and_add(D volatile* dest, I add_value,
                                 atomic_memory_order order) {
    return AddImpl<D, I>::fetch_and_add(dest, add_value, order);
  }
  
  template<typename D, typename I>
  struct Atomic::AddImpl<
    D, I,
    typename EnableIf<IsIntegral<I>::value &&
                      IsIntegral<D>::value &&
                      (sizeof(I) <= sizeof(D)) &&
                      (IsSigned<I>::value == IsSigned<D>::value)>::type>
  {
    static D add_and_fetch(D volatile* dest, I add_value, atomic_memory_order order) {
      D addend = add_value;
      return PlatformAdd<sizeof(D)>().add_and_fetch(dest, addend, order);
    }
    static D fetch_and_add(D volatile* dest, I add_value, atomic_memory_order order) {
      D addend = add_value;
      return PlatformAdd<sizeof(D)>().fetch_and_add(dest, addend, order);
    }
  };
  
  template<typename P, typename I>
  struct Atomic::AddImpl<
    P*, I,
    typename EnableIf<IsIntegral<I>::value && (sizeof(I) <= sizeof(P*))>::type>
  {
    STATIC_ASSERT(sizeof(intptr_t) == sizeof(P*));
    STATIC_ASSERT(sizeof(uintptr_t) == sizeof(P*));
    typedef typename Conditional<IsSigned<I>::value,
                                 intptr_t,
                                 uintptr_t>::type CI;
  
    static CI scale_addend(CI add_value) {
      return add_value * sizeof(P);
    }
  
    static P* add_and_fetch(P* volatile* dest, I add_value, atomic_memory_order order) {
      CI addend = add_value;
      return PlatformAdd<sizeof(P*)>().add_and_fetch(dest, scale_addend(addend), order);
    }
    static P* fetch_and_add(P* volatile* dest, I add_value, atomic_memory_order order) {
      CI addend = add_value;
      return PlatformAdd<sizeof(P*)>().fetch_and_add(dest, scale_addend(addend), order);
    }
  };
  
  template<typename Type, typename Fn, typename D, typename I>
  inline D Atomic::add_using_helper(Fn fn, D volatile* dest, I add_value) {
    return PrimitiveConversions::cast<D>(
      fn(PrimitiveConversions::cast<Type>(add_value),
         reinterpret_cast<Type volatile*>(dest)));
  }
  
  template<typename D, typename U, typename T>
  inline D Atomic::cmpxchg(D volatile* dest,
                           U compare_value,
                           T exchange_value,
                           atomic_memory_order order) {
    return CmpxchgImpl<D, U, T>()(dest, compare_value, exchange_value, order);
  }
  
  template<typename D, typename T>
  inline bool Atomic::replace_if_null(D* volatile* dest, T* value,
                                      atomic_memory_order order) {
    // Presently using a trivial implementation in terms of cmpxchg.
    // Consider adding platform support, to permit the use of compiler
    // intrinsics like gcc's __sync_bool_compare_and_swap.
    D* expected_null = NULL;
    return expected_null == cmpxchg(dest, expected_null, value, order);
  }
  
  // Handle cmpxchg for integral and enum types.
  //
  // All the involved types must be identical.
  template<typename T>
  struct Atomic::CmpxchgImpl<
    T, T, T,
    typename EnableIf<IsIntegral<T>::value || std::is_enum<T>::value>::type>
  {
    T operator()(T volatile* dest, T compare_value, T exchange_value,
                 atomic_memory_order order) const {
      // Forward to the platform handler for the size of T.
      return PlatformCmpxchg<sizeof(T)>()(dest,
                                          compare_value,
                                          exchange_value,
                                          order);
    }
  };
  
  // Handle cmpxchg for pointer types.
  //
  // The destination's type and the compare_value type must be the same,
  // ignoring cv-qualifiers; we don't care about the cv-qualifiers of
  // the compare_value.
  //
  // The exchange_value must be implicitly convertible to the
  // destination's type; it must be type-correct to store the
  // exchange_value in the destination.
  template<typename D, typename U, typename T>
  struct Atomic::CmpxchgImpl<
    D*, U*, T*,
    typename EnableIf<Atomic::IsPointerConvertible<T*, D*>::value &&
                      IsSame<typename RemoveCV<D>::type,
                             typename RemoveCV<U>::type>::value>::type>
  {
    D* operator()(D* volatile* dest, U* compare_value, T* exchange_value,
                 atomic_memory_order order) const {
      // Allow derived to base conversion, and adding cv-qualifiers.
      D* new_value = exchange_value;
      // Don't care what the CV qualifiers for compare_value are,
      // but we need to match D* when calling platform support.
      D* old_value = const_cast<D*>(compare_value);
      return PlatformCmpxchg<sizeof(D*)>()(dest, old_value, new_value, order);
    }
  };
  
  // Handle cmpxchg for types that have a translator.
  //
  // All the involved types must be identical.
  //
  // This translates the original call into a call on the decayed
  // arguments, and returns the recovered result of that translated
  // call.
  template<typename T>
  struct Atomic::CmpxchgImpl<
    T, T, T,
    typename EnableIf<PrimitiveConversions::Translate<T>::value>::type>
  {
    T operator()(T volatile* dest, T compare_value, T exchange_value,
                 atomic_memory_order order) const {
      typedef PrimitiveConversions::Translate<T> Translator;
      typedef typename Translator::Decayed Decayed;
      STATIC_ASSERT(sizeof(T) == sizeof(Decayed));
      return Translator::recover(
        cmpxchg(reinterpret_cast<Decayed volatile*>(dest),
                Translator::decay(compare_value),
                Translator::decay(exchange_value),
                order));
    }
  };
  
  template<typename Type, typename Fn, typename T>
  inline T Atomic::cmpxchg_using_helper(Fn fn,
                                        T volatile* dest,
                                        T compare_value,
                                        T exchange_value) {
    STATIC_ASSERT(sizeof(Type) == sizeof(T));
    return PrimitiveConversions::cast<T>(
      fn(PrimitiveConversions::cast<Type>(exchange_value),
         reinterpret_cast<Type volatile*>(dest),
         PrimitiveConversions::cast<Type>(compare_value)));
  }
  
  inline uint32_t Atomic::CmpxchgByteUsingInt::set_byte_in_int(uint32_t n,
                                                               uint8_t b,
                                                               uint32_t idx) {
    int bitsIdx = BitsPerByte * idx;
    return (n & ~(static_cast<uint32_t>(0xff) << bitsIdx))
            | (static_cast<uint32_t>(b) << bitsIdx);
  }
  
  inline uint8_t Atomic::CmpxchgByteUsingInt::get_byte_in_int(uint32_t n,
                                                              uint32_t idx) {
    int bitsIdx = BitsPerByte * idx;
    return (uint8_t)(n >> bitsIdx);
  }
  
  template<typename T>
  inline T Atomic::CmpxchgByteUsingInt::operator()(T volatile* dest,
                                                   T compare_value,
                                                   T exchange_value,
                                                   atomic_memory_order order) const {
    STATIC_ASSERT(sizeof(T) == sizeof(uint8_t));
    uint8_t canon_exchange_value = exchange_value;
    uint8_t canon_compare_value = compare_value;
    volatile uint32_t* aligned_dest
      = reinterpret_cast<volatile uint32_t*>(align_down(dest, sizeof(uint32_t)));
    size_t offset = pointer_delta(dest, aligned_dest, 1);
  
    uint32_t idx = (Endian::NATIVE == Endian::BIG)
                     ? (sizeof(uint32_t) - 1 - offset)
                     : offset;
  
    // current value may not be what we are looking for, so force it
    // to that value so the initial cmpxchg will fail if it is different
    uint32_t cur = set_byte_in_int(Atomic::load(aligned_dest), canon_compare_value, idx);
  
    // always execute a real cmpxchg so that we get the required memory
    // barriers even on initial failure
    do {
      // value to swap in matches current value
      // except for the one byte we want to update
      uint32_t new_value = set_byte_in_int(cur, canon_exchange_value, idx);
  
      uint32_t res = cmpxchg(aligned_dest, cur, new_value, order);
      if (res == cur) break;      // success
  
      // at least one byte in the int changed value, so update
      // our view of the current int
      cur = res;
      // if our byte is still as cur we loop and try again
    } while (get_byte_in_int(cur, idx) == canon_compare_value);
  
    return PrimitiveConversions::cast<T>(get_byte_in_int(cur, idx));
  }
  
  // Handle xchg for integral and enum types.
  //
  // All the involved types must be identical.
  template<typename T>
  struct Atomic::XchgImpl<
    T, T,
    typename EnableIf<IsIntegral<T>::value || std::is_enum<T>::value>::type>
  {
    T operator()(T volatile* dest, T exchange_value, atomic_memory_order order) const {
      // Forward to the platform handler for the size of T.
      return PlatformXchg<sizeof(T)>()(dest, exchange_value, order);
    }
  };
  
  // Handle xchg for pointer types.
  //
  // The exchange_value must be implicitly convertible to the
  // destination's type; it must be type-correct to store the
  // exchange_value in the destination.
  template<typename D, typename T>
  struct Atomic::XchgImpl<
    D*, T*,
    typename EnableIf<Atomic::IsPointerConvertible<T*, D*>::value>::type>
  {
    D* operator()(D* volatile* dest, T* exchange_value, atomic_memory_order order) const {
      // Allow derived to base conversion, and adding cv-qualifiers.
      D* new_value = exchange_value;
      return PlatformXchg<sizeof(D*)>()(dest, new_value, order);
    }
  };
  
  // Handle xchg for types that have a translator.
  //
  // All the involved types must be identical.
  //
  // This translates the original call into a call on the decayed
  // arguments, and returns the recovered result of that translated
  // call.
  template<typename T>
  struct Atomic::XchgImpl<
    T, T,
    typename EnableIf<PrimitiveConversions::Translate<T>::value>::type>
  {
    T operator()(T volatile* dest, T exchange_value, atomic_memory_order order) const {
      typedef PrimitiveConversions::Translate<T> Translator;
      typedef typename Translator::Decayed Decayed;
      STATIC_ASSERT(sizeof(T) == sizeof(Decayed));
      return Translator::recover(
        xchg(reinterpret_cast<Decayed volatile*>(dest),
             Translator::decay(exchange_value),
             order));
    }
  };
  
  template<typename Type, typename Fn, typename T>
  inline T Atomic::xchg_using_helper(Fn fn,
                                     T volatile* dest,
                                     T exchange_value) {
    STATIC_ASSERT(sizeof(Type) == sizeof(T));
    // Notice the swapped order of arguments. Change when/if stubs are rewritten.
    return PrimitiveConversions::cast<T>(
      fn(PrimitiveConversions::cast<Type>(exchange_value),
         reinterpret_cast<Type volatile*>(dest)));
  }
  
  template<typename D, typename T>
  inline D Atomic::xchg(volatile D* dest, T exchange_value, atomic_memory_order order) {
    return XchgImpl<D, T>()(dest, exchange_value, order);
  }
  
  #endif // SHARE_RUNTIME_ATOMIC_HPP
  