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<base href="https://llvm.org/bugs/" />
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<body><table border="1" cellspacing="0" cellpadding="8">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - [X86] Unnecessary register moves"
href="https://llvm.org/bugs/show_bug.cgi?id=31024">31024</a>
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</tr>
<tr>
<th>Summary</th>
<td>[X86] Unnecessary register moves
</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>
<tr>
<th>OS</th>
<td>Windows NT
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>normal
</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>llvm-bugs@lists.llvm.org
</td>
</tr>
<tr>
<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>define <8 x i64> @sext_16i8_to_8i64(<16 x i8> %A) nounwind uwtable readnone ssp
{
entry:
%B = shufflevector <16 x i8> %A, <16 x i8> undef, <8 x i32> <i32 0, i32 1,
i32 2, i32 3, i32 4, i32 5, i32 6, i32 7>
%C = sext <8 x i8> %B to <8 x i64>
ret <8 x i64> %C
}
llc -mtriple=x86_64-unknown-unknown -mattr=+avx2
sext_16i8_to_8i64:
vpmovsxbq %xmm0, %ymm2
vpshufd {{.*#+}} xmm0 = xmm0[1,1,2,3]
vpmovsxbq %xmm0, %ymm1
vmovdqa %ymm2, %ymm0
retq
This could avoid the register move:
sext_16i8_to_8i64:
vpshufd {{.*#+}} xmm1 = xmm0[1,1,2,3]
vpmovsxbq %xmm0, %ymm0
vpmovsxbq %xmm1, %ymm1
retq
Reg-reg moves are almost free but its still better to avoid them if we can.</pre>
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