Using the GNU Compiler Collection (GCC): X86 transactional memory intrinsics

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#### 6.57.13 X86 transaction memory intrinsics

Hardware transactional memory intrinsics for i386. These allow to use memory transactions with RTM (Restricted Transactional Memory). For using HLE (Hardware Lock Elision) see x86 specific memory model extensions for transactional memory instead. This support is enabled with the `-mrtm` option.

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 explicitely tested). When the transaction aborts all side effects are undone and an abort code is returned. There is no guarantee any transaction ever succeeds, so there always needs to be a valid tested fallback path.

    #include <immintrin.h>

    if ((status = _xbegin ()) == _XBEGIN_STARTED) {
        ... transaction code...
        _xend ();
    } else {
        ... non transactional fallback path...
    }

Valid abort status bits (when the value is not `_XBEGIN_STARTED`) are:

`_XABORT_EXPLICIT`

  Transaction explicitely 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 a inner nested transaction

RTM Function: void _xend ()

  Commit the current transaction. When no transaction is active this will fault. All memory side effects of the transactions will become visible to other threads in an atomic matter.

RTM Function: int _xtest ()

  Return a value not zero when 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. status must be a 8bit constant, that is included in the status code returned by `_xbegin`

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