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<a name="x86-transactional-memory-intrinsics"></a>
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<a name="x86-Transactional-Memory-Intrinsics"></a>
<h4 class="subsection">6.60.33 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>-mrtm</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>
<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>
<p>Generally there is no guarantee that a memory transaction ever succeeds
and suitable fallback code always needs to be supplied.
</p>
<dl>
<dt><a name="index-_005fxbegin"></a>RTM Function: <em>unsigned</em> <strong>_xbegin</strong> <em>()</em></dt>
<dd><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>
<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:
</p>
<dl compact="compact">
<dt><code>_XABORT_EXPLICIT</code></dt>
<dd><p>Transaction was explicitly aborted with <code>_xabort</code>.  The parameter passed
to <code>_xabort</code> is available with <code>_XABORT_CODE(status)</code>.
</p></dd>
<dt><code>_XABORT_RETRY</code></dt>
<dd><p>Transaction retry is possible.
</p></dd>
<dt><code>_XABORT_CONFLICT</code></dt>
<dd><p>Transaction abort due to a memory conflict with another thread.
</p></dd>
<dt><code>_XABORT_CAPACITY</code></dt>
<dd><p>Transaction abort due to the transaction using too much memory.
</p></dd>
<dt><code>_XABORT_DEBUG</code></dt>
<dd><p>Transaction abort due to a debug trap.
</p></dd>
<dt><code>_XABORT_NESTED</code></dt>
<dd><p>Transaction abort in an inner nested transaction.
</p></dd>
</dl>

<p>There is no guarantee
any transaction ever succeeds, so there always needs to be a valid
fallback path.
</p></dd></dl>

<dl>
<dt><a name="index-_005fxend"></a>RTM Function: <em>void</em> <strong>_xend</strong> <em>()</em></dt>
<dd><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></dd></dl>

<dl>
<dt><a name="index-_005fxtest"></a>RTM Function: <em>int</em> <strong>_xtest</strong> <em>()</em></dt>
<dd><p>Return a nonzero value if a transaction is currently active, otherwise 0.
</p></dd></dl>

<dl>
<dt><a name="index-_005fxabort"></a>RTM Function: <em>void</em> <strong>_xabort</strong> <em>(status)</em></dt>
<dd><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></dd></dl>

<p>Here is an example showing handling for <code>_XABORT_RETRY</code>
and a fallback path for other failures:
</p>
<div class="smallexample">
<pre class="smallexample">#include &lt;immintrin.h&gt;

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

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

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