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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - -ffp-contract=on prevents vectorization of reduction loop"
href="https://bugs.llvm.org/show_bug.cgi?id=52266">52266</a>
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<th>Summary</th>
<td>-ffp-contract=on prevents vectorization of reduction loop
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<th>Product</th>
<td>libraries
</td>
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<th>Version</th>
<td>trunk
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>All
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>enhancement
</td>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>Loop Optimizer
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>craig.topper@gmail.com
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</tr>
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
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<p>
<div>
<pre>Compiling this function with -ffp-contract=on does not vectorize when compiler
for -march=haswell. It works for -ffp-contract=off or -ffp-contract=fast
float foo(float *a, float *b, int n) {
float sum = 0.0f;
for (int i = 0; i != n; ++i) {
#pragma clang fp reassociate(on)
sum += a[i] * b[i];
}
return sum;
}
<a href="https://godbolt.org/z/cdv8dKPj9">https://godbolt.org/z/cdv8dKPj9</a>
I believe the problems is that the -ffp-contract=on causes the multiply and
accumulate in the loop to become a fmuladd intrinsic in the frontend. This
prevents the reduction recurrence from being detected since it is not an FAdd
instruction. With -ffp-contract=off or fast, there are separate fmul and fadd
instructions.</pre>
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</p>
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