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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - AVX512: We don't always need EVEX for VEX-encodable instructions."
href="https://llvm.org/bugs/show_bug.cgi?id=23376">23376</a>
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<th>Summary</th>
<td>AVX512: We don't always need EVEX for VEX-encodable instructions.
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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
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<th>Status</th>
<td>NEW
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<th>Severity</th>
<td>normal
</td>
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<th>Priority</th>
<td>P
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<th>Component</th>
<td>Backend: X86
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>ahmed.bougacha@gmail.com
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<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
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<th>Classification</th>
<td>Unclassified
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<p>
<div>
<pre>This:
define float @t(float %a, float %b) {
%res = fadd float %a, %b
ret float %res
}
Generates the EVEX-prefixed:
62f17e0858c1 vaddss %xmm1, %xmm0, %xmm0
instead of using VEX:
c5fa58c1 vaddss %xmm1, %xmm0, %xmm0
AFAIK, there is no transition penalty between AVX512 and AVX, so I believe the
only reason to use EVEX in this case is to access more registers (xmm0-31).
That's not always useful, so we should recognize that, and relax EVEX->VEX when
possible.</pre>
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