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* [Bug target/62055] New: missed optimization: recognize fnabs (FP negative absolute value) (x86-64)
@ 2014-08-07 19:17 spatel at rotateright dot com
2014-08-08 8:04 ` [Bug target/62055] " rguenth at gcc dot gnu.org
0 siblings, 1 reply; 2+ messages in thread
From: spatel at rotateright dot com @ 2014-08-07 19:17 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=62055
Bug ID: 62055
Summary: missed optimization: recognize fnabs (FP negative
absolute value) (x86-64)
Product: gcc
Version: 4.9.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: spatel at rotateright dot com
$ cat fnabs.c
#include <math.h>
float foo(float a) {
return -fabsf(a);
}
$ gcc49 -O1 fnabs.c -S -o -
.text
.globl _foo
_foo:
LFB19:
movss LC0(%rip), %xmm1
andps %xmm1, %xmm0
movss LC1(%rip), %xmm1
xorps %xmm1, %xmm0
ret
LFE19:
.literal16
.align 4
LC0:
.long 2147483647
.long 0
.long 0
.long 0
.align 4
LC1:
.long 2147483648
.long 0
.long 0
.long 0
---------------------------------------------------
That's a lot of constant pool data and instructions to turn on a single bit.
I think there are 2 steps to improving this. First, recognize that -(fabs(a))
can be transformed into an 'or' op:
movss LC0(%rip), %xmm1
orps %xmm1, %xmm0
LC0:
.long 2147483648
Second, I don't think we need the extra 0 longs here; movss only loads 4 bytes.
This may require understanding that the upper vector elements for the 'orps'
are don't cares?
^ permalink raw reply [flat|nested] 2+ messages in thread
* [Bug target/62055] missed optimization: recognize fnabs (FP negative absolute value) (x86-64)
2014-08-07 19:17 [Bug target/62055] New: missed optimization: recognize fnabs (FP negative absolute value) (x86-64) spatel at rotateright dot com
@ 2014-08-08 8:04 ` rguenth at gcc dot gnu.org
0 siblings, 0 replies; 2+ messages in thread
From: rguenth at gcc dot gnu.org @ 2014-08-08 8:04 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=62055
Richard Biener <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Keywords| |missed-optimization
Target| |x86_64-*-*, i?86-*-*
Status|UNCONFIRMED |NEW
Last reconfirmed| |2014-08-08
Ever confirmed|0 |1
--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
Confirmed also with -ffast-math.
^ permalink raw reply [flat|nested] 2+ messages in thread
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2014-08-07 19:17 [Bug target/62055] New: missed optimization: recognize fnabs (FP negative absolute value) (x86-64) spatel at rotateright dot com
2014-08-08 8:04 ` [Bug target/62055] " rguenth at gcc dot gnu.org
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