text: /*
   * Distributed under the Boost Software License, Version 1.0.
   * (See accompanying file LICENSE_1_0.txt or copy at
   * http://www.boost.org/LICENSE_1_0.txt)
   *
   * Copyright (c) 2009, 2011 Helge Bahmann
   * Copyright (c) 2009 Phil Endecott
   * Copyright (c) 2013 Tim Blechmann
   * Linux-specific code by Phil Endecott
   * Copyright (c) 2014 Andrey Semashev
   */
  /*!
   * \file   atomic/detail/ops_linux_arm.hpp
   *
   * This header contains implementation of the \c operations template.
   */
  
  #ifndef BOOST_ATOMIC_DETAIL_OPS_LINUX_ARM_HPP_INCLUDED_
  #define BOOST_ATOMIC_DETAIL_OPS_LINUX_ARM_HPP_INCLUDED_
  
  #include <boost/memory_order.hpp>
  #include <boost/atomic/detail/config.hpp>
  #include <boost/atomic/detail/storage_type.hpp>
  #include <boost/atomic/detail/operations_fwd.hpp>
  #include <boost/atomic/capabilities.hpp>
  #include <boost/atomic/detail/ops_cas_based.hpp>
  #include <boost/atomic/detail/ops_extending_cas_based.hpp>
  
  #ifdef BOOST_HAS_PRAGMA_ONCE
  #pragma once
  #endif
  
  namespace mars_boost {} namespace boost = mars_boost; namespace mars_boost {
  namespace atomics {
  namespace detail {
  
  // Different ARM processors have different atomic instructions.  In particular,
  // architecture versions before v6 (which are still in widespread use, e.g. the
  // Intel/Marvell XScale chips like the one in the NSLU2) have only atomic swap.
  // On Linux the kernel provides some support that lets us abstract away from
  // these differences: it provides emulated CAS and barrier functions at special
  // addresses that are guaranteed not to be interrupted by the kernel.  Using
  // this facility is slightly slower than inline assembler would be, but much
  // faster than a system call.
  //
  // While this emulated CAS is "strong" in the sense that it does not fail
  // "spuriously" (i.e.: it never fails to perform the exchange when the value
  // found equals the value expected), it does not return the found value on
  // failure. To satisfy the atomic API, compare_exchange_{weak|strong} must
  // return the found value on failure, and we have to manually load this value
  // after the emulated CAS reports failure. This in turn introduces a race
  // between the CAS failing (due to the "wrong" value being found) and subsequently
  // loading (which might turn up the "right" value). From an application's
  // point of view this looks like "spurious failure", and therefore the
  // emulated CAS is only good enough to provide compare_exchange_weak
  // semantics.
  
  struct linux_arm_cas_base
  {
      static BOOST_FORCEINLINE void fence_before_store(memory_order order) BOOST_NOEXCEPT
      {
          if ((order & memory_order_release) != 0)
              hardware_full_fence();
      }
  
      static BOOST_FORCEINLINE void fence_after_store(memory_order order) BOOST_NOEXCEPT
      {
          if (order == memory_order_seq_cst)
              hardware_full_fence();
      }
  
      static BOOST_FORCEINLINE void fence_after_load(memory_order order) BOOST_NOEXCEPT
      {
          if ((order & (memory_order_consume | memory_order_acquire)) != 0)
              hardware_full_fence();
      }
  
      static BOOST_FORCEINLINE void hardware_full_fence() BOOST_NOEXCEPT
      {
          typedef void (*kernel_dmb_t)(void);
          ((kernel_dmb_t)0xffff0fa0)();
      }
  };
  
  template< bool Signed >
  struct linux_arm_cas :
      public linux_arm_cas_base
  {
      typedef typename make_storage_type< 4u, Signed >::type storage_type;
      typedef typename make_storage_type< 4u, Signed >::aligned aligned_storage_type;
  
      static BOOST_FORCEINLINE void store(storage_type volatile& storage, storage_type v, memory_order order) BOOST_NOEXCEPT
      {
          fence_before_store(order);
          storage = v;
          fence_after_store(order);
      }
  
      static BOOST_FORCEINLINE storage_type load(storage_type const volatile& storage, memory_order order) BOOST_NOEXCEPT
      {
          storage_type v = storage;
          fence_after_load(order);
          return v;
      }
  
      static BOOST_FORCEINLINE bool compare_exchange_strong(
          storage_type volatile& storage, storage_type& expected, storage_type desired, memory_order success_order, memory_order failure_order) BOOST_NOEXCEPT
      {
          while (true)
          {
              storage_type tmp = expected;
              if (compare_exchange_weak(storage, tmp, desired, success_order, failure_order))
                  return true;
              if (tmp != expected)
              {
                  expected = tmp;
                  return false;
              }
          }
      }
  
      static BOOST_FORCEINLINE bool compare_exchange_weak(
          storage_type volatile& storage, storage_type& expected, storage_type desired, memory_order, memory_order) BOOST_NOEXCEPT
      {
          typedef storage_type (*kernel_cmpxchg32_t)(storage_type oldval, storage_type newval, volatile storage_type* ptr);
  
          if (((kernel_cmpxchg32_t)0xffff0fc0)(expected, desired, &storage) == 0)
          {
              return true;
          }
          else
          {
              expected = storage;
              return false;
          }
      }
  
      static BOOST_FORCEINLINE bool is_lock_free(storage_type const volatile&) BOOST_NOEXCEPT
      {
          return true;
      }
  };
  
  template< bool Signed >
  struct operations< 1u, Signed > :
      public extending_cas_based_operations< cas_based_operations< cas_based_exchange< linux_arm_cas< Signed > > >, 1u, Signed >
  {
  };
  
  template< bool Signed >
  struct operations< 2u, Signed > :
      public extending_cas_based_operations< cas_based_operations< cas_based_exchange< linux_arm_cas< Signed > > >, 2u, Signed >
  {
  };
  
  template< bool Signed >
  struct operations< 4u, Signed > :
      public cas_based_operations< cas_based_exchange< linux_arm_cas< Signed > > >
  {
  };
  
  BOOST_FORCEINLINE void thread_fence(memory_order order) BOOST_NOEXCEPT
  {
      if (order != memory_order_relaxed)
          linux_arm_cas_base::hardware_full_fence();
  }
  
  BOOST_FORCEINLINE void signal_fence(memory_order order) BOOST_NOEXCEPT
  {
      if (order != memory_order_relaxed)
          __asm__ __volatile__ ("" ::: "memory");
  }
  
  } // namespace detail
  } // namespace atomics
  } // namespace mars_boost
  
  #endif // BOOST_ATOMIC_DETAIL_OPS_LINUX_ARM_HPP_INCLUDED_
  