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89 lines
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89 lines
3.4 KiB
HTML
4 years ago
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<title>Loop-Specific Pragmas - Using the GNU Compiler Collection (GCC)</title>
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<link rel="up" href="Pragmas.html#Pragmas" title="Pragmas">
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<link rel="prev" href="Function-Specific-Option-Pragmas.html#Function-Specific-Option-Pragmas" title="Function Specific Option Pragmas">
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A GNU Manual
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</head>
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<body>
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<div class="node">
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<a name="Loop-Specific-Pragmas"></a>
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<a name="Loop_002dSpecific-Pragmas"></a>
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<p>
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Previous: <a rel="previous" accesskey="p" href="Function-Specific-Option-Pragmas.html#Function-Specific-Option-Pragmas">Function Specific Option Pragmas</a>,
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Up: <a rel="up" accesskey="u" href="Pragmas.html#Pragmas">Pragmas</a>
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<hr>
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</div>
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<h4 class="subsection">6.60.14 Loop-Specific Pragmas</h4>
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<dl>
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<dt><code>#pragma GCC ivdep</code><dd><a name="index-pragma-GCC-ivdep-4188"></a></dl>
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<p>With this pragma, the programmer asserts that there are no loop-carried
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dependencies which would prevent consecutive iterations of
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the following loop from executing concurrently with SIMD
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(single instruction multiple data) instructions.
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<p>For example, the compiler can only unconditionally vectorize the following
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loop with the pragma:
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<pre class="smallexample"> void foo (int n, int *a, int *b, int *c)
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{
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int i, j;
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#pragma GCC ivdep
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for (i = 0; i < n; ++i)
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a[i] = b[i] + c[i];
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}
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</pre>
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<p class="noindent">In this example, using the <code>restrict</code> qualifier had the same
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effect. In the following example, that would not be possible. Assume
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k < -m or k >= m. Only with the pragma, the compiler knows
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that it can unconditionally vectorize the following loop:
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<pre class="smallexample"> void ignore_vec_dep (int *a, int k, int c, int m)
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{
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#pragma GCC ivdep
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for (int i = 0; i < m; i++)
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a[i] = a[i + k] * c;
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}
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</pre>
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</body></html>
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