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<title>Using the GNU Compiler Collection (GCC): x86 transactional memory intrinsics</title>
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<a name="x86-transactional-memory-intrinsics"></a>
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<p>
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Previous: <a href="x86-Built_002din-Functions.html#x86-Built_002din-Functions" accesskey="p" rel="prev">x86 Built-in Functions</a>, Up: <a href="Target-Builtins.html#Target-Builtins" accesskey="u" rel="up">Target Builtins</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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<hr>
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<a name="x86-Transactional-Memory-Intrinsics"></a>
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<h4 class="subsection">6.59.32 x86 Transactional Memory Intrinsics</h4>
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<p>These hardware transactional memory intrinsics for x86 allow you to use
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memory transactions with RTM (Restricted Transactional Memory).
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This support is enabled with the <samp>-mrtm</samp> option.
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For using HLE (Hardware Lock Elision) see
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<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.
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</p>
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<p>A memory transaction commits all changes to memory in an atomic way,
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as visible to other threads. If the transaction fails it is rolled back
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and all side effects discarded.
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</p>
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<p>Generally there is no guarantee that a memory transaction ever succeeds
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and suitable fallback code always needs to be supplied.
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</p>
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<dl>
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<dt><a name="index-_005fxbegin"></a>RTM Function: <em>unsigned</em> <strong>_xbegin</strong> <em>()</em></dt>
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<dd><p>Start a RTM (Restricted Transactional Memory) transaction.
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Returns <code>_XBEGIN_STARTED</code> when the transaction
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started successfully (note this is not 0, so the constant has to be
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explicitly tested).
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</p>
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<p>If the transaction aborts, all side-effects
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are undone and an abort code encoded as a bit mask is returned.
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The following macros are defined:
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</p>
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<dl compact="compact">
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<dt><code>_XABORT_EXPLICIT</code></dt>
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<dd><p>Transaction was explicitly aborted with <code>_xabort</code>. The parameter passed
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to <code>_xabort</code> is available with <code>_XABORT_CODE(status)</code>.
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</p></dd>
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<dt><code>_XABORT_RETRY</code></dt>
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<dd><p>Transaction retry is possible.
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</p></dd>
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<dt><code>_XABORT_CONFLICT</code></dt>
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<dd><p>Transaction abort due to a memory conflict with another thread.
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</p></dd>
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<dt><code>_XABORT_CAPACITY</code></dt>
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<dd><p>Transaction abort due to the transaction using too much memory.
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</p></dd>
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<dt><code>_XABORT_DEBUG</code></dt>
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<dd><p>Transaction abort due to a debug trap.
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</p></dd>
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<dt><code>_XABORT_NESTED</code></dt>
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<dd><p>Transaction abort in an inner nested transaction.
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</p></dd>
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</dl>
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<p>There is no guarantee
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any transaction ever succeeds, so there always needs to be a valid
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fallback path.
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</p></dd></dl>
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<dl>
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<dt><a name="index-_005fxend"></a>RTM Function: <em>void</em> <strong>_xend</strong> <em>()</em></dt>
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<dd><p>Commit the current transaction. When no transaction is active this faults.
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All memory side-effects of the transaction become visible
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to other threads in an atomic manner.
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</p></dd></dl>
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<dl>
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<dt><a name="index-_005fxtest"></a>RTM Function: <em>int</em> <strong>_xtest</strong> <em>()</em></dt>
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<dd><p>Return a nonzero value if a transaction is currently active, otherwise 0.
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</p></dd></dl>
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<dl>
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<dt><a name="index-_005fxabort"></a>RTM Function: <em>void</em> <strong>_xabort</strong> <em>(status)</em></dt>
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<dd><p>Abort the current transaction. When no transaction is active this is a no-op.
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The <var>status</var> is an 8-bit constant; its value is encoded in the return
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value from <code>_xbegin</code>.
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</p></dd></dl>
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<p>Here is an example showing handling for <code>_XABORT_RETRY</code>
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and a fallback path for other failures:
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</p>
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<div class="smallexample">
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<pre class="smallexample">#include <immintrin.h>
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int n_tries, max_tries;
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unsigned status = _XABORT_EXPLICIT;
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...
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for (n_tries = 0; n_tries < max_tries; n_tries++)
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{
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status = _xbegin ();
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if (status == _XBEGIN_STARTED || !(status & _XABORT_RETRY))
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break;
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}
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if (status == _XBEGIN_STARTED)
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{
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... transaction code...
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_xend ();
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}
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else
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{
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... non-transactional fallback path...
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}
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</pre></div>
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<p>Note that, in most cases, the transactional and non-transactional code
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must synchronize together to ensure consistency.
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</p>
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<hr>
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<div class="header">
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<p>
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Previous: <a href="x86-Built_002din-Functions.html#x86-Built_002din-Functions" accesskey="p" rel="prev">x86 Built-in Functions</a>, Up: <a href="Target-Builtins.html#Target-Builtins" accesskey="u" rel="up">Target Builtins</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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