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| <a name="x86-Transactional-Memory-Intrinsics"></a> |
| <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>-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 <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></div> |
| |
| <p>Note that, in most cases, the transactional and non-transactional code |
| must synchronize together to ensure consistency. |
| </p> |
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