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<base href="https://bugs.llvm.org/">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - -std=gnu11 behaves different than gcc wrt FMA"
href="https://bugs.llvm.org/show_bug.cgi?id=36846">36846</a>
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<th>Summary</th>
<td>-std=gnu11 behaves different than gcc wrt FMA
</td>
</tr>
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<th>Product</th>
<td>new-bugs
</td>
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<th>Version</th>
<td>5.0
</td>
</tr>
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<th>Hardware</th>
<td>PC
</td>
</tr>
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<th>OS</th>
<td>All
</td>
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<th>Status</th>
<td>NEW
</td>
</tr>
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<th>Severity</th>
<td>enhancement
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>new bugs
</td>
</tr>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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</tr>
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<th>Reporter</th>
<td>arjanvandeven@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>gcc allows the compiler to use FMA with -std=gnu11 (and its friends) while LLVM
seems to be much more conservative and not emit FMA.
double foo2(double A, double B, double C)
{
double D;
D = A * B + C;
return D;
}
generates with "clang -O3 -std=gnu11 -march=native -c -S foo.c"
vmulsd %xmm1, %xmm0, %xmm0
vaddsd %xmm2, %xmm0, %xmm0
retq
while gcc generates with "gcc -O3 -std=gnu11 -march=native -c -S foo.c"
vfmadd132sd %xmm1, %xmm2, %xmm0
ret</pre>
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</p>
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