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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 - Failure to produce vxorps to avoid latency on AVX operations"
href="https://bugs.llvm.org/show_bug.cgi?id=46075">46075</a>
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<th>Summary</th>
<td>Failure to produce vxorps to avoid latency on AVX operations
</td>
</tr>
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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>Linux
</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>Backend: X86
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>gabravier@gmail.com
</td>
</tr>
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<th>CC</th>
<td>craig.topper@gmail.com, llvm-bugs@lists.llvm.org, llvm-dev@redking.me.uk, spatel+llvm@rotateright.com
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<p>
<div>
<pre>double f(double x, int i)
{
return x * i;
}
With `-O3 -mavx2`, Clang produces this :
f(double, int): # @f(double, int)
vcvtsi2sd xmm1, xmm1, edi
vmulsd xmm0, xmm1, xmm0
ret
GCC produces this :
f(double, int):
vmovsd xmm1, xmm0, xmm0
vxorps xmm0, xmm0, xmm0
vcvtsi2sd xmm0, xmm0, edi
vmulsd xmm0, xmm0, xmm1
ret
According to GCC developers on here :
<a href="https://gcc.gnu.org/bugzilla/show_bug.cgi?id=57954">https://gcc.gnu.org/bugzilla/show_bug.cgi?id=57954</a>, using `vxorps` avoids a
latency which is caused by non-zeroing an AVX register before using it. This
seems to affect literally all X86 processors (as GCC uses `vxorps` for them
all) and Clang uses `vxorps` for none.</pre>
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