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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 --- - constant vector value splatted at runtime with broadcast instruction instead of loaded (only core-avx2 target?)"
href="http://llvm.org/bugs/show_bug.cgi?id=20054">20054</a>
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<th>Summary</th>
<td>constant vector value splatted at runtime with broadcast instruction instead of loaded (only core-avx2 target?)
</td>
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<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>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
</td>
</tr>
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<th>Reporter</th>
<td>spatel+llvm@rotateright.com
</td>
</tr>
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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>$ ./clang -v
clang version 3.5.0 (210980)
Target: x86_64-apple-darwin13.2.0
Thread model: posix
$ cat splat.c
#include <xmmintrin.h>
__m128 splat(__m128 x) {
return _mm_add_ps(x, _mm_set1_ps(1.0f));
}
---------------------------------------------------------
That "1.0f" constant vector is being loaded as a scalar value and then splatted
across the xmm register for the core-avx2 target:
$ ./clang -O1 -fomit-frame-pointer splat.c -march=core-avx2 -S -o -
.section __TEXT,__text,regular,pure_instructions
.macosx_version_min 10, 9
.section __TEXT,__literal4,4byte_literals
.align 2
LCPI0_0:
.long 1065353216 ## float 1
.section __TEXT,__text,regular,pure_instructions
.globl _splat
.align 4, 0x90
_splat: ## @splat
.cfi_startproc
## BB#0: ## %entry
vbroadcastss LCPI0_0(%rip), %xmm1
vaddps %xmm1, %xmm0, %xmm0
retq
.cfi_endproc
.subsections_via_symbols
--------------------------------------------------------
But with any (?) other x86 targets, we make the size-speed tradeoff and just
load a full 128-bit vector:
$ ./clang -O1 -fomit-frame-pointer splat.c -march=corei7-avx -S -o -
.section __TEXT,__text,regular,pure_instructions
.macosx_version_min 10, 9
.section __TEXT,__literal16,16byte_literals
.align 4
LCPI0_0:
.long 1065353216 ## float 1.000000e+00
.long 1065353216 ## float 1.000000e+00
.long 1065353216 ## float 1.000000e+00
.long 1065353216 ## float 1.000000e+00
.section __TEXT,__text,regular,pure_instructions
.globl _splat
.align 4, 0x90
_splat: ## @splat
.cfi_startproc
## BB#0: ## %entry
vaddps LCPI0_0(%rip), %xmm0, %xmm0
retq
.cfi_endproc</pre>
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