Using the GNU Compiler Collection (GCC): x86 specific memory model extensions for transactional memory

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### 6.55 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);