Using the GNU Compiler Collection (GCC): x86 transactional memory intrinsics

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#### 6.60.33 x86 Transactional Memory Intrinsics

These hardware transactional memory intrinsics for x86 allow you to use memory transactions with RTM (Restricted Transactional Memory). This support is enabled with the `-mrtm` option. For using HLE (Hardware Lock Elision) see x86 specific memory model extensions for transactional memory instead.

A memory transaction commits all changes to memory in an atomic way, as visible to other threads. If the transaction fails it is rolled back and all side effects discarded.

Generally there is no guarantee that a memory transaction ever succeeds and suitable fallback code always needs to be supplied.

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`.

Here is an example showing handling for `_XABORT_RETRY` and a fallback path for other failures:

    #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...
      }

Note that, in most cases, the transactional and non-transactional code must synchronize together to ensure consistency.

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