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<base href="https://llvm.org/bugs/" />
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - [x86, SSE] failed to simplify vector select with zero constant to 'and'"
href="https://llvm.org/bugs/show_bug.cgi?id=28895">28895</a>
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<th>Summary</th>
<td>[x86, SSE] failed to simplify vector select with zero constant to 'and'
</td>
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<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>
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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>normal
</td>
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<th>Priority</th>
<td>P
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<th>Component</th>
<td>Backend: X86
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
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<th>Reporter</th>
<td>spatel+llvm@rotateright.com
</td>
</tr>
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org
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<th>Classification</th>
<td>Unclassified
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</tr></table>
<p>
<div>
<pre>While looking at the select fold in <a href="https://reviews.llvm.org/D22975">https://reviews.llvm.org/D22975</a>, I noticed
this problem:
define <4 x float> @fselvec(<4 x float> %x, <4 x float> %y) {
%cmp = fcmp oeq <4 x float> %x, zeroinitializer
%sel = select <4 x i1> %cmp, <4 x float> %y, <4 x float> zeroinitializer
ret <4 x float> %sel
}
This should be an 'and' bitwise logic op, but:
$ ./llc fsel.ll -o - -mattr=avx
vxorps %xmm2, %xmm2, %xmm2
vcmpeqps %xmm2, %xmm0, %xmm0
vblendvps %xmm0, %xmm1, %xmm2, %xmm0
retq
The optimization exists for AArch64:
$ ./llc fsel.ll -o - -mtriple=aarch64
fcmeq v0.4s, v0.4s, #0.0
and v0.16b, v1.16b, v0.16b
ret
The optimization also exists for x86 FP scalar and x86 integer vector, so some
refactoring may be needed:
define float @fsel(float %x, float %y) {
%cmp = fcmp oeq float %x, 0.0
%sel = select i1 %cmp, float %y, float 0.0
ret float %sel
}
define <4 x i32> @iselvec(<4 x i32> %x, <4 x i32> %y) {
%cmp = icmp eq <4 x i32> %x, zeroinitializer
%sel = select <4 x i1> %cmp, <4 x i32> %y, <4 x i32> zeroinitializer
ret <4 x i32> %sel
}
_fsel: ## @fsel
vxorps %xmm2, %xmm2, %xmm2
vcmpeqss %xmm2, %xmm0, %xmm0
vandps %xmm1, %xmm0, %xmm0
retq
_iselvec: ## @iselvec
vpxor %xmm2, %xmm2, %xmm2
vpcmpeqd %xmm2, %xmm0, %xmm0
vpand %xmm1, %xmm0, %xmm0
retq</pre>
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