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<base href="http://llvm.org/bugs/" />
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - having a select instruction doesn't mean we need to use it (x86 - blend)"
href="http://llvm.org/bugs/show_bug.cgi?id=20648">20648</a>
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<th>Summary</th>
<td>having a select instruction doesn't mean we need to use it (x86 - blend)
</td>
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<th>Product</th>
<td>libraries
</td>
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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>All
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>normal
</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
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<th>Reporter</th>
<td>spatel+llvm@rotateright.com
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<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>Better hardware earned us more instructions, more data, and worse performance.
Using llc with x86-64 target built from r215293:
$ cat blend.ll
define void @foo(<4 x i32>* %f) {
%select = select <4 x i1> <i1 1, i1 0, i1 0, i1 1>, <4 x i32> <i32 1, i32 1,
i32 1, i32 1>, <4 x i32> <i32 2, i32 2, i32 2, i32 2>
store <4 x i32> %select, <4 x i32>* %f, align 4
ret void
}
$ ./llc blend.ll --mattr="-sse4.1" -o -
...
LCPI0_0:
.long 1 ## 0x1
.long 2 ## 0x2
.long 2 ## 0x2
.long 1 ## 0x1
.section __TEXT,__text,regular,pure_instructions
.globl _foo
.align 4, 0x90
_foo: ## @foo
.cfi_startproc
## BB#0:
movaps LCPI0_0(%rip), %xmm0 <--- precomputed select data loaded
movups %xmm0, (%rdi)
retq
$ ./llc blend.ll --mattr="+sse4.1" -o -
...
LCPI0_0:
.long 2 ## 0x2
.long 2 ## 0x2
.long 2 ## 0x2
.long 2 ## 0x2
LCPI0_1: <--- 16 extra bytes of constant pool
.long 1 ## 0x1
.long 1 ## 0x1
.long 1 ## 0x1
.long 1 ## 0x1
.section __TEXT,__text,regular,pure_instructions
.globl _foo
.align 4, 0x90
_foo: ## @foo
.cfi_startproc
## BB#0:
movaps LCPI0_1(%rip), %xmm0 <--- 2 loads and a select
blendps $6, LCPI0_0(%rip), %xmm0
movups %xmm0, (%rdi)
retq</pre>
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