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.

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.

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.

int _xtest()

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

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.