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| <a name="X86-transaction-memory-intrinsics"></a> |
| <h4 class="subsection">6.57.13 X86 transaction memory intrinsics</h4> |
| |
| <p>Hardware transactional memory intrinsics for i386. These allow to use |
| memory transactions with RTM (Restricted Transactional Memory). |
| 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. |
| This support is enabled with the <samp>-mrtm</samp> option. |
| </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 _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. |
| </p></dd></dl> |
| |
| <div class="smallexample"> |
| <pre class="smallexample">#include <immintrin.h> |
| |
| if ((status = _xbegin ()) == _XBEGIN_STARTED) { |
| ... transaction code... |
| _xend (); |
| } else { |
| ... non transactional fallback path... |
| } |
| </pre></div> |
| |
| <p>Valid abort status bits (when the value is not <code>_XBEGIN_STARTED</code>) are: |
| </p> |
| <dl compact="compact"> |
| <dt><code>_XABORT_EXPLICIT</code></dt> |
| <dd><p>Transaction explicitely 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 a inner nested transaction |
| </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 will |
| fault. All memory side effects of the transactions will become visible |
| to other threads in an atomic matter. |
| </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 value not zero when 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. |
| status must be a 8bit constant, that is included in the status code returned |
| by <code>_xbegin</code> |
| </p></dd></dl> |
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