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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] LLVM produces sub-optimal assembly for zext from 8xi8 to 8xi32"
href="https://llvm.org/bugs/show_bug.cgi?id=25718">25718</a>
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<th>Summary</th>
<td>[X86][SSE] LLVM produces sub-optimal assembly for zext from 8xi8 to 8xi32
</td>
</tr>
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<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>Linux
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
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<th>Severity</th>
<td>normal
</td>
</tr>
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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
</td>
</tr>
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<th>Reporter</th>
<td>congh@google.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>For the following IR:
define void @zext_v8i8_to_v8i32(<8 x i8>* %a) {
%1 = load <8 x i8>, <8 x i8>* %a
%2 = zext <8 x i8> %1 to <8 x i32>
store <8 x i32> %2, <8 x i32>* undef, align 4
ret void
}
LLVM emits the following assembly:
movq (%rdi), %xmm0 # xmm0 = mem[0],zero
punpcklbw %xmm0, %xmm0 # xmm0 = xmm0[0,0,1,1,2,2,3,3,4,4,5,5,6,6,7,7]
movdqa %xmm0, %xmm1
punpcklwd %xmm0, %xmm1 # xmm1 =
xmm1[0],xmm0[0],xmm1[1],xmm0[1],xmm1[2],xmm0[2],xmm1[3],xmm0[3]
movdqa .LCPI0_0(%rip), %xmm2 # xmm2 =
[255,0,0,0,255,0,0,0,255,0,0,0,255,0,0,0]
pand %xmm2, %xmm1
punpckhwd %xmm0, %xmm0 # xmm0 = xmm0[4,4,5,5,6,6,7,7]
pand %xmm2, %xmm0
movdqu %xmm0, (%rax)
movdqu %xmm1, (%rax)
retq
This is sub-optimal as we can eliminate two pand and one movdaq by using zeros
as one operand on all punpck*** (so we have one more xor).</pre>
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