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<base href="http://llvm.org/bugs/" />
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<body><table border="1" cellspacing="0" cellpadding="8">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - missed optimization: recognize fnabs (FP negative absolute value) (x86-64)"
href="http://llvm.org/bugs/show_bug.cgi?id=20578">20578</a>
</td>
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<th>Summary</th>
<td>missed optimization: recognize fnabs (FP negative absolute value) (x86-64)
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<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>
<tr>
<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>Backend: X86
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>spatel+llvm@rotateright.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
</td>
</tr>
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>Ouch - 32 bytes of constant pool data to turn on a single bit.
Using llc built from r214897:
$ cat fnabs.c
#include <math.h>
float foo(float a) {
return -fabs(a);
}
or in LLVM IR:
$ cat fnabs.ll
define float @foo(float %a) {
%fabsf = tail call float @fabsf(float %a) #1
%fsub = fsub float -0.0, %fabsf
ret float %fsub
}
declare float @fabsf(float)
attributes #1 = { readnone }
$ ./llc -mtriple=x86_64-unknown-unknown fnabs.ll -o -
.text
.file "fnabs.ll"
.section .rodata.cst16,"aM",@progbits,16
.align 16
.LCPI0_0:
.long 2147483647 # float nan
.long 2147483647 # float nan
.long 2147483647 # float nan
.long 2147483647 # float nan
.LCPI0_1:
.long 2147483648 # float -0.000000e+00
.long 2147483648 # float -0.000000e+00
.long 2147483648 # float -0.000000e+00
.long 2147483648 # float -0.000000e+00
.text
.globl foo
.align 16, 0x90
.type foo,@function
foo: # @foo
.cfi_startproc
# BB#0:
andps .LCPI0_0(%rip), %xmm0
xorps .LCPI0_1(%rip), %xmm0
retq</pre>
</div>
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