6.57 x86-Specific Memory Model Extensions for Transactional Memory ¶

The x86 architecture supports additional memory ordering flags to mark critical sections for hardware lock elision. These must be specified in addition to an existing memory order to atomic intrinsics.

__ATOMIC_HLE_ACQUIRE

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

__ATOMIC_HLE_RELEASE

End lock elision on a lock variable. Memory order 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);