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<h4 class="subsection">6.60.14 Loop-Specific Pragmas</h4>
<dl>
<dt><code>#pragma GCC ivdep</code><dd><a name="index-pragma-GCC-ivdep-4188"></a></dl>
<p>With this pragma, the programmer asserts that there are no loop-carried
dependencies which would prevent consecutive iterations of
the following loop from executing concurrently with SIMD
(single instruction multiple data) instructions.
<p>For example, the compiler can only unconditionally vectorize the following
loop with the pragma:
<pre class="smallexample"> void foo (int n, int *a, int *b, int *c)
{
int i, j;
#pragma GCC ivdep
for (i = 0; i < n; ++i)
a[i] = b[i] + c[i];
}
</pre>
<p class="noindent">In this example, using the <code>restrict</code> qualifier had the same
effect. In the following example, that would not be possible. Assume
k < -m or k >= m. Only with the pragma, the compiler knows
that it can unconditionally vectorize the following loop:
<pre class="smallexample"> void ignore_vec_dep (int *a, int k, int c, int m)
{
#pragma GCC ivdep
for (int i = 0; i < m; i++)
a[i] = a[i + k] * c;
}
</pre>
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