linux-arm.hpp

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#ifndef BOOST_DETAIL_ATOMIC_LINUX_ARM_HPP
#define BOOST_DETAIL_ATOMIC_LINUX_ARM_HPP

//  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
//  Linux-specific code by Phil Endecott

// 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 garaunteed 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.

#include <boost/memory_order.hpp>
#include <boost/atomic/detail/base.hpp>

#define BOOST_ATOMIC_CHAR_LOCK_FREE 2
#define BOOST_ATOMIC_CHAR16_T_LOCK_FREE 2
#define BOOST_ATOMIC_CHAR32_T_LOCK_FREE 2
#define BOOST_ATOMIC_WCHAR_T_LOCK_FREE 2
#define BOOST_ATOMIC_SHORT_LOCK_FREE 2
#define BOOST_ATOMIC_INT_LOCK_FREE 2
#define BOOST_ATOMIC_LONG_LOCK_FREE 2
#define BOOST_ATOMIC_LLONG_LOCK_FREE 0
#define BOOST_ATOMIC_ADDRESS_LOCK_FREE 2
#define BOOST_ATOMIC_BOOL_LOCK_FREE 2

namespace boost {
namespace detail {
namespace atomic {

static inline void
arm_barrier(void)
{
        void (*kernel_dmb)(void) = (void (*)(void)) 0xffff0fa0;
        kernel_dmb();
}

static inline void
platform_fence_before(memory_order order)
{
        switch(order) {
                case memory_order_release:
                case memory_order_acq_rel:
                case memory_order_seq_cst:
                        arm_barrier();
                case memory_order_consume:
                default:;
        }
}

static inline void
platform_fence_after(memory_order order)
{
        switch(order) {
                case memory_order_acquire:
                case memory_order_acq_rel:
                case memory_order_seq_cst:
                        arm_barrier();
                default:;
        }
}

static inline void
platform_fence_before_store(memory_order order)
{
        platform_fence_before(order);
}

static inline void
platform_fence_after_store(memory_order order)
{
        if (order == memory_order_seq_cst)
                arm_barrier();
}

static inline void
platform_fence_after_load(memory_order order)
{
        platform_fence_after(order);
}

template<typename T>
bool
platform_cmpxchg32(T & expected, T desired, volatile T * ptr)
{
        typedef T (*kernel_cmpxchg32_t)(T oldval, T newval, volatile T * ptr);
        
        if (((kernel_cmpxchg32_t) 0xffff0fc0)(expected, desired, ptr) == 0) {
                return true;
        } else {
                expected = *ptr;
                return false;
        }
}

}
}

#define BOOST_ATOMIC_THREAD_FENCE 2
static inline void
atomic_thread_fence(memory_order order)
{
        switch(order) {
                case memory_order_acquire:
                case memory_order_release:
                case memory_order_acq_rel:
                case memory_order_seq_cst:
                        detail::atomic::arm_barrier();
                default:;
        }
}

#define BOOST_ATOMIC_SIGNAL_FENCE 2
static inline void
atomic_signal_fence(memory_order)
{
        __asm__ __volatile__ ("" ::: "memory");
}

}

#include <boost/atomic/detail/cas32weak.hpp>

#endif