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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 --- - missed opportunity to use absolute value vector instruction (vpabsd)"
href="http://llvm.org/bugs/show_bug.cgi?id=17172">17172</a>
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<th>Summary</th>
<td>missed opportunity to use absolute value vector instruction (vpabsd)
</td>
</tr>
<tr>
<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>
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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>kkhoo@perfwizard.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>The following code is performing an absolute value operation on a 4 integer
array. LLVM is generating the straightforward unrolled loop code, but not
recognizing that these operations could be mapped to a single 'VPABSD'
instruction.
$ ./clang -v
clang version 3.4 (trunk 189776)
Target: x86_64-apple-darwin11.4.2
Thread model: posix
$ cat vpabsd.c
void foo(int *x) {
int i;
for (i=0; i<4; i++) {
x[i] = x[i] < 0 ? -x[i] : x[i];
}
}
$ ./clang -S -O3 -fomit-frame-pointer -march=corei7-avx -o /dev/stdout vpabsd.c
.section __TEXT,__text,regular,pure_instructions
.globl _foo
.align 4, 0x90
_foo: ## @foo
.cfi_startproc
## BB#0: ## %entry
movl (%rdi), %eax
movl %eax, %ecx
negl %ecx
cmovll %eax, %ecx
movl %ecx, (%rdi)
movl 4(%rdi), %eax
movl %eax, %ecx
negl %ecx
cmovll %eax, %ecx
movl %ecx, 4(%rdi)
movl 8(%rdi), %eax
movl %eax, %ecx
negl %ecx
cmovll %eax, %ecx
movl %ecx, 8(%rdi)
movl 12(%rdi), %eax
movl %eax, %ecx
negl %ecx
cmovll %eax, %ecx
movl %ecx, 12(%rdi)
ret</pre>
</div>
</p>
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