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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 - [X86][SSE] Failure to convert (x&1) bittest to a shift to drive a blendvps"
href="https://bugs.llvm.org/show_bug.cgi?id=46531">46531</a>
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<tr>
<th>Summary</th>
<td>[X86][SSE] Failure to convert (x&1) bittest to a shift to drive a blendvps
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
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<th>OS</th>
<td>Windows NT
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>Backend: X86
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>llvm-dev@redking.me.uk
</td>
</tr>
<tr>
<th>CC</th>
<td>craig.topper@gmail.com, llvm-bugs@lists.llvm.org, llvm-dev@redking.me.uk, spatel+llvm@rotateright.com
</td>
</tr></table>
<p>
<div>
<pre><a href="https://c.godbolt.org/z/cvDB4B">https://c.godbolt.org/z/cvDB4B</a>
#include <x86intrin.h>
inline unsigned opt(unsigned x, unsigned y) {
unsigned val = x | y;
if (y & 1) { val = x ^ y; }
return val;
}
void opt( unsigned * __restrict dst, const unsigned *x, const unsigned *y )
{
*dst++ = opt(*x++, *y++);
*dst++ = opt(*x++, *y++);
*dst++ = opt(*x++, *y++);
*dst++ = opt(*x++, *y++);
}
.LCPI0_0:
.long 1 # 0x1
opt:
vmovdqu (%rdx), %xmm1
vpbroadcastd .LCPI0_0(%rip), %xmm3 # xmm3 = [1,1,1,1]
vmovdqu (%rsi), %xmm0
vpxor %xmm4, %xmm4, %xmm4
vpand %xmm3, %xmm1, %xmm3
vpor %xmm0, %xmm1, %xmm2
vpxor %xmm0, %xmm1, %xmm0
vpcmpeqd %xmm4, %xmm3, %xmm3
vblendvps %xmm3, %xmm2, %xmm0, %xmm0
vmovups %xmm0, (%rdi)
retq
but we should be able to replace the vpcmpeqd(vpand(y, 1), 0) with vpslli(y,31)
to drive the vblendvps (with inverted selections):
opt:
vmovdqu (%rdx), %xmm1
vmovdqu (%rsi), %xmm0
vpslld $31, %xmm1, %xmm3
vpor %xmm0, %xmm1, %xmm2
vpxor %xmm0, %xmm1, %xmm0
vblendvps %xmm3, %xmm0, %xmm2, %xmm0
vmovups %xmm0, (%rdi)
retq</pre>
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