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<h4 class="subsection">6.58.31 x86 Transactional Memory Intrinsics</h4>
<p>These hardware transactional memory intrinsics for x86 allow you to use
memory transactions with RTM (Restricted Transactional Memory).
This support is enabled with the <samp><span class="option">-mrtm</span></samp> option.
For using HLE (Hardware Lock Elision) see
<a href="x86-specific-memory-model-extensions-for-transactional-memory.html#x86-specific-memory-model-extensions-for-transactional-memory">x86 specific memory model extensions for transactional memory</a> instead.
<p>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.
<p>Generally there is no guarantee that a memory transaction ever succeeds
and suitable fallback code always needs to be supplied.
<div class="defun">
— RTM Function: unsigned <b>_xbegin</b> ()<var><a name="index-g_t_005fxbegin-4151"></a></var><br>
<blockquote><p>Start a RTM (Restricted Transactional Memory) transaction.
Returns <code>_XBEGIN_STARTED</code> when the transaction
started successfully (note this is not 0, so the constant has to be
explicitly tested).
<p>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:
<dl>
<dt><code>_XABORT_EXPLICIT</code><dd>Transaction was explicitly aborted with <code>_xabort</code>. The parameter passed
to <code>_xabort</code> is available with <code>_XABORT_CODE(status)</code>.
<br><dt><code>_XABORT_RETRY</code><dd>Transaction retry is possible.
<br><dt><code>_XABORT_CONFLICT</code><dd>Transaction abort due to a memory conflict with another thread.
<br><dt><code>_XABORT_CAPACITY</code><dd>Transaction abort due to the transaction using too much memory.
<br><dt><code>_XABORT_DEBUG</code><dd>Transaction abort due to a debug trap.
<br><dt><code>_XABORT_NESTED</code><dd>Transaction abort in an inner nested transaction.
</dl>
<p>There is no guarantee
any transaction ever succeeds, so there always needs to be a valid
fallback path.
</p></blockquote></div>
<div class="defun">
— RTM Function: void <b>_xend</b> ()<var><a name="index-g_t_005fxend-4152"></a></var><br>
<blockquote><p>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.
</p></blockquote></div>
<div class="defun">
— RTM Function: int <b>_xtest</b> ()<var><a name="index-g_t_005fxtest-4153"></a></var><br>
<blockquote><p>Return a nonzero value if a transaction is currently active, otherwise 0.
</p></blockquote></div>
<div class="defun">
— RTM Function: void <b>_xabort</b> (<var>status</var>)<var><a name="index-g_t_005fxabort-4154"></a></var><br>
<blockquote><p>Abort the current transaction. When no transaction is active this is a no-op.
The <var>status</var> is an 8-bit constant; its value is encoded in the return
value from <code>_xbegin</code>.
</p></blockquote></div>
<p>Here is an example showing handling for <code>_XABORT_RETRY</code>
and a fallback path for other failures:
<pre class="smallexample"> #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...
}
</pre>
<p class="noindent">Note that, in most cases, the transactional and non-transactional code
must synchronize together to ensure consistency.
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