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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - AVX512: fmsub intrinsic generates fmadd instruction"
href="https://llvm.org/bugs/show_bug.cgi?id=28892">28892</a>
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<th>Summary</th>
<td>AVX512: fmsub intrinsic generates fmadd instruction
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
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<th>Version</th>
<td>trunk
</td>
</tr>
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<th>Hardware</th>
<td>PC
</td>
</tr>
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<th>OS</th>
<td>All
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
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<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>Backend: X86
</td>
</tr>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>wenzel.jakob@epfl.ch
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr>
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>Consider the following simple snippet which uses the fused multiply subtract
instruction on AVX512 and AVX:
=============
#include <immintrin.h>
__m512 test(__m512 a, __m512 b, __m512 c) {
return _mm512_fmsub_ps(a, b, c);
}
__m256 test(__m256 a, __m256 b, __m256 c) {
return _mm256_fmsub_ps(a, b, c);
}
=============
In the AVX512 case, LLVM trunk generates the following weird code which does a
sign flip first followed by a FMADD instruction. The AVX backend, in
comparison, generates the expected (single FMADD instruction).
__Z4testDv16_fS_S_:
vpxord LCPI0_0(%rip), %zmm2, %zmm2
vfmadd213ps %zmm2, %zmm1, %zmm0
retq
__Z4testDv8_fS_S_:
vfmsub213ps %ymm2, %ymm1, %ymm0
retq</pre>
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</p>
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