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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - will it blend? apparently not [SSE, AVX, X86]"
href="http://llvm.org/bugs/show_bug.cgi?id=22483">22483</a>
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<th>Summary</th>
<td>will it blend? apparently not [SSE, AVX, X86]
</td>
</tr>
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<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
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<th>Hardware</th>
<td>PC
</td>
</tr>
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<th>OS</th>
<td>All
</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>spatel+llvm@rotateright.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
</td>
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>define float @blendv(float %x, float %y) {
%cmp = fcmp oge float %x, %y
%sel = select i1 %cmp, float %x, float %y
ret float %sel
}
Or in C:
float blendv(float x, float y) {
if (x >= y) return x;
return y;
}
There are no scalar FP select instructions for xmm registers (at least through
AVX2 from what I can tell)...just like there are no scalar FP logical ops (and,
xor, or, andn). Consistent unorthogonality?
Currently (r228316), we generate:
$ llc -mattr=avx blend.ll -o -
...
vcmpless %xmm0, %xmm1, %xmm2
vandps %xmm0, %xmm2, %xmm0
vandnps %xmm1, %xmm2, %xmm1
vorps %xmm0, %xmm1, %xmm0
retq
I think that we'd be better off using 'vblendvps'; this was added with SSE4.1:
vcmpless %xmm0, %xmm1, %xmm2
vblendvps %xmm2, %xmm0, %xmm1, %xmm0
retq
I'm not sure what's in bits 32:127 of the output reg in either case, but we're
not worse off using blendv?
FWIW, icc 15 just does a compare and branch:
vcomiss %xmm1, %xmm0
jae L_L3
vmovaps %xmm1, %xmm0
L_L3:
ret</pre>
</div>
</p>
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