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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] Perform unsigned compare as signed when signbits are zero"
href="https://bugs.llvm.org/show_bug.cgi?id=33276">33276</a>
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<th>Summary</th>
<td>[X86][SSE] Perform unsigned compare as signed when signbits are zero
</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>
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<th>OS</th>
<td>Windows NT
</td>
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<th>Status</th>
<td>NEW
</td>
</tr>
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<th>Severity</th>
<td>enhancement
</td>
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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>llvm-dev@redking.me.uk
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org, spatel+llvm@rotateright.com
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<p>
<div>
<pre>SSE doesn't have an unsigned integer compare, so we have to flip the sign bits
and use the signed compare instructions. But if we know that the sign bits of
both values are zero then we should be able to avoid the flip:
llc -mtriple=x86_64-unknown -mcpu=btver2
define <4 x i32> @cmp_ugt_4i32(<4 x i32> %a0, <4 x i32> %a1) {
%1 = lshr <4 x i32> %a0, <i32 1, i32 1, i32 1, i32 1>
%2 = lshr <4 x i32> %a1, <i32 1, i32 1, i32 1, i32 1>
%3 = icmp ugt <4 x i32> %1, %2
%4 = sext <4 x i1> %3 to <4 x i32>
ret <4 x i32> %4
}
cmp_ugt:
vmovdqa .LCPI0_0(%rip), %xmm2 # xmm2 =
[2147483648,2147483648,2147483648,2147483648]
vpsrld $1, %xmm0, %xmm0
vpsrld $1, %xmm1, %xmm1
vpxor %xmm2, %xmm1, %xmm1
vpxor %xmm2, %xmm0, %xmm0
vpcmpgtd %xmm1, %xmm0, %xmm0
retq
Ideally:
cmp_ugt:
vpsrld $1, %xmm0, %xmm0
vpsrld $1, %xmm1, %xmm1
vpcmpgtd %xmm1, %xmm0, %xmm0
retq</pre>
</div>
</p>
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