6.53 x86 specific memory model extensions for transactional memory

The i386 architecture supports additional memory ordering flags to mark lock critical sections for hardware lock elision. These must be specified in addition to an existing memory model to atomic intrinsics.

__ATOMIC_HLE_ACQUIRE

Start lock elision on a lock variable. Memory model must be __ATOMIC_ACQUIRE or stronger.

__ATOMIC_HLE_RELEASE

End lock elision on a lock variable. Memory model must be __ATOMIC_RELEASE or stronger.

When a lock acquire fails it is required for good performance to abort the transaction quickly. This can be done with a _mm_pause

#include <immintrin.h> // For _mm_pause

int lockvar;

/* Acquire lock with lock elision */
while (__atomic_exchange_n(&lockvar, 1, __ATOMIC_ACQUIRE|__ATOMIC_HLE_ACQUIRE))
    _mm_pause(); /* Abort failed transaction */
...
/* Free lock with lock elision */
__atomic_store_n(&lockvar, 0, __ATOMIC_RELEASE|__ATOMIC_HLE_RELEASE);

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