gcc/libcilkrts/runtime/config/x86/os-fence.h

/* os.h                  -*-C++-*-
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/* gcc before 4.4 does not implement __sync_synchronize properly */
#if (__ICC >= 1110 && !(__MIC__ || __MIC2__))                      \
    || (!defined __ICC && __GNUC__ * 10 + __GNUC_MINOR__ > 43)
#   define HAVE_SYNC_INTRINSICS 1
#endif


/*
 * void __cilkrts_fence(void)
 *
 * Executes an MFENCE instruction to serialize all load and store instructions
 * that were issued prior the MFENCE instruction. This serializing operation
 * guarantees that every load and store instruction that precedes the MFENCE
 * instruction is globally visible before any load or store instruction that
 * follows the MFENCE instruction. The MFENCE instruction is ordered with
 * respect to all load and store instructions, other MFENCE instructions, any
 * SFENCE and LFENCE instructions, and any serializing instructions (such as
 * the CPUID instruction).
 */
#ifdef HAVE_SYNC_INTRINSICS
#   define __cilkrts_fence() __sync_synchronize()
#elif defined __ICC || defined __GNUC__
    /* mfence is a strict subset of lock add but takes longer on many
     * processors. */
// #   define __cilkrts_fence() __asm__ volatile ("mfence")
    /* On MIC, fence seems to be completely unnecessary.
     * Just for simplicity of 1st implementation, it defaults to x86 */ 
#   define __cilkrts_fence() __asm__ volatile ("lock addl $0,(%rsp)")
// #elif defined _WIN32
// #   pragma intrinsic(_ReadWriteBarrier)
// #   define __cilkrts_fence() _ReadWriteBarrier()
#else
COMMON_SYSDEP void __cilkrts_fence(void); ///< MFENCE instruction
#endif