6.58.31 x86 Transactional Memory Intrinsics

— RTM Function: unsigned _xbegin ()

Start a RTM (Restricted Transactional Memory) transaction. Returns _XBEGIN_STARTED when the transaction started successfully (note this is not 0, so the constant has to be explicitly tested).

If the transaction aborts, all side-effects are undone and an abort code encoded as a bit mask is returned. The following macros are defined:

_XABORT_EXPLICIT

Transaction was explicitly aborted with _xabort. The parameter passed to _xabort is available with _XABORT_CODE(status).

_XABORT_RETRY

Transaction retry is possible.

_XABORT_CONFLICT

Transaction abort due to a memory conflict with another thread.

_XABORT_CAPACITY

Transaction abort due to the transaction using too much memory.

_XABORT_DEBUG

Transaction abort due to a debug trap.

_XABORT_NESTED

Transaction abort in an inner nested transaction.

There is no guarantee any transaction ever succeeds, so there always needs to be a valid fallback path.

— RTM Function: void _xend ()

Commit the current transaction. When no transaction is active this faults. All memory side-effects of the transaction become visible to other threads in an atomic manner.

— RTM Function: int _xtest ()

Return a nonzero value if a transaction is currently active, otherwise 0.

— RTM Function: void _xabort (status)

Abort the current transaction. When no transaction is active this is a no-op. The status is an 8-bit constant; its value is encoded in the return value from _xbegin.

#include <immintrin.h>

int n_tries, max_tries;
unsigned status = _XABORT_EXPLICIT;
...

for (n_tries = 0; n_tries < max_tries; n_tries++)
  {
    status = _xbegin ();
    if (status == _XBEGIN_STARTED || !(status & _XABORT_RETRY))
      break;
  }
if (status == _XBEGIN_STARTED)
  {
    ... transaction code...
    _xend ();
  }
else
  {
    ... non-transactional fallback path...
  }