* [Bug tree-optimization/56125] -O2 -ffast-math generates bad code when dividing a double by the square of another double.
2013-01-28 2:58 [Bug c/56125] New: -O2 -ffast-math generates bad code when dividing a double by the square of another double mednafen at gmail dot com
@ 2013-01-28 7:23 ` glisse at gcc dot gnu.org
2013-01-28 7:30 ` [Bug tree-optimization/56125] [4.7/4.8 Regression] " glisse at gcc dot gnu.org
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From: glisse at gcc dot gnu.org @ 2013-01-28 7:23 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56125
Marc Glisse <glisse at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Keywords| |wrong-code
Status|UNCONFIRMED |NEW
Last reconfirmed| |2013-01-28
Component|c |tree-optimization
Ever Confirmed|0 |1
--- Comment #1 from Marc Glisse <glisse at gcc dot gnu.org> 2013-01-28 07:23:24 UTC ---
It is turned to x*pow(n,-2.) and the expansion of pow notices that 2*2 is an
integer and uses the code meant for 2.5...
^ permalink raw reply [flat|nested] 9+ messages in thread* [Bug tree-optimization/56125] [4.7/4.8 Regression] -O2 -ffast-math generates bad code when dividing a double by the square of another double.
2013-01-28 2:58 [Bug c/56125] New: -O2 -ffast-math generates bad code when dividing a double by the square of another double mednafen at gmail dot com
2013-01-28 7:23 ` [Bug tree-optimization/56125] " glisse at gcc dot gnu.org
@ 2013-01-28 7:30 ` glisse at gcc dot gnu.org
2013-01-28 9:15 ` jakub at gcc dot gnu.org
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From: glisse at gcc dot gnu.org @ 2013-01-28 7:30 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56125
Marc Glisse <glisse at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Summary|-O2 -ffast-math generates |[4.7/4.8 Regression] -O2
|bad code when dividing a |-ffast-math generates bad
|double by the square of |code when dividing a double
|another double. |by the square of another
| |double.
--- Comment #2 from Marc Glisse <glisse at gcc dot gnu.org> 2013-01-28 07:30:05 UTC ---
4.6 generated a very suboptimal call to the libm pow function, but it didn't
have the completely wrong result.
^ permalink raw reply [flat|nested] 9+ messages in thread* [Bug tree-optimization/56125] [4.7/4.8 Regression] -O2 -ffast-math generates bad code when dividing a double by the square of another double.
2013-01-28 2:58 [Bug c/56125] New: -O2 -ffast-math generates bad code when dividing a double by the square of another double mednafen at gmail dot com
2013-01-28 7:23 ` [Bug tree-optimization/56125] " glisse at gcc dot gnu.org
2013-01-28 7:30 ` [Bug tree-optimization/56125] [4.7/4.8 Regression] " glisse at gcc dot gnu.org
@ 2013-01-28 9:15 ` jakub at gcc dot gnu.org
2013-01-28 10:27 ` jakub at gcc dot gnu.org
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From: jakub at gcc dot gnu.org @ 2013-01-28 9:15 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56125
Jakub Jelinek <jakub at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |jakub at gcc dot gnu.org
Target Milestone|--- |4.7.3
--- Comment #3 from Jakub Jelinek <jakub at gcc dot gnu.org> 2013-01-28 09:15:09 UTC ---
Regressed with http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=174446
Looking into it.
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2013-01-28 2:58 [Bug c/56125] New: -O2 -ffast-math generates bad code when dividing a double by the square of another double mednafen at gmail dot com
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2013-01-28 9:15 ` jakub at gcc dot gnu.org
@ 2013-01-28 10:27 ` jakub at gcc dot gnu.org
2013-01-28 14:43 ` jakub at gcc dot gnu.org
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From: jakub at gcc dot gnu.org @ 2013-01-28 10:27 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56125
Jakub Jelinek <jakub at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|NEW |ASSIGNED
AssignedTo|unassigned at gcc dot |jakub at gcc dot gnu.org
|gnu.org |
--- Comment #4 from Jakub Jelinek <jakub at gcc dot gnu.org> 2013-01-28 10:27:19 UTC ---
Created attachment 29292
--> http://gcc.gnu.org/bugzilla/attachment.cgi?id=29292
gcc48-pr56125.patch
The bug is that the last two optimizations of pow (where 2c resp. 3c is a
non-zero integer) silently assume that the earlier optimizations already
handled the cases where already c (or 2c for the last optimization) is an
integer.
But that doesn't have to be the case, as shown by the testcase, the integer
optimization is guarded by c in [-1,2] or optimization for speed.
The && optimize_function_for_speed_p () (or should that be
optimize_insn_for_speed_p?, the pow folding is inconsistent in that, and I
don't see e.g. rtl_profile_for_bb being called during this pass to make it
accurate)
is up for discussions, say on x86_64
__attribute__((cold)) double
foo (double x, double n)
{
double u = __builtin_pow (x, -1.5);
return u;
}
with it we get smaller code:
movsd .LC0(%rip), %xmm1
jmp pow
compared to:
sqrtsd %xmm0, %xmm1
mulsd %xmm0, %xmm1
movsd .LC0(%rip), %xmm0
divsd %xmm1, %xmm0
without it, 7 bytes shorter.
^ permalink raw reply [flat|nested] 9+ messages in thread* [Bug tree-optimization/56125] [4.7/4.8 Regression] -O2 -ffast-math generates bad code when dividing a double by the square of another double.
2013-01-28 2:58 [Bug c/56125] New: -O2 -ffast-math generates bad code when dividing a double by the square of another double mednafen at gmail dot com
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2013-01-28 10:27 ` jakub at gcc dot gnu.org
@ 2013-01-28 14:43 ` jakub at gcc dot gnu.org
2013-01-28 15:07 ` [Bug tree-optimization/56125] [4.7 " jakub at gcc dot gnu.org
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From: jakub at gcc dot gnu.org @ 2013-01-28 14:43 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56125
--- Comment #5 from Jakub Jelinek <jakub at gcc dot gnu.org> 2013-01-28 14:43:07 UTC ---
Author: jakub
Date: Mon Jan 28 14:43:03 2013
New Revision: 195507
URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=195507
Log:
PR tree-optimization/56125
* tree-ssa-math-opts.c (gimple_expand_builtin_pow): Don't optimize
pow(x,c) into sqrt(x) * powi(x, n/2) or
1.0 / (sqrt(x) * powi(x, abs(n/2))) if c is an integer or when
optimizing for size.
Don't optimize pow(x,c) into powi(x, n/3) * powi(cbrt(x), n%3) or
1.0 / (powi(x, abs(n)/3) * powi(cbrt(x), abs(n)%3)) if 2c is an
integer.
* gcc.dg/pr56125.c: New test.
Added:
trunk/gcc/testsuite/gcc.dg/pr56125.c
Modified:
trunk/gcc/ChangeLog
trunk/gcc/testsuite/ChangeLog
trunk/gcc/tree-ssa-math-opts.c
^ permalink raw reply [flat|nested] 9+ messages in thread* [Bug tree-optimization/56125] [4.7 Regression] -O2 -ffast-math generates bad code when dividing a double by the square of another double.
2013-01-28 2:58 [Bug c/56125] New: -O2 -ffast-math generates bad code when dividing a double by the square of another double mednafen at gmail dot com
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2013-01-28 14:43 ` jakub at gcc dot gnu.org
@ 2013-01-28 15:07 ` jakub at gcc dot gnu.org
2013-02-01 14:18 ` jakub at gcc dot gnu.org
2013-02-01 14:39 ` jakub at gcc dot gnu.org
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From: jakub at gcc dot gnu.org @ 2013-01-28 15:07 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56125
Jakub Jelinek <jakub at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Summary|[4.7/4.8 Regression] -O2 |[4.7 Regression] -O2
|-ffast-math generates bad |-ffast-math generates bad
|code when dividing a double |code when dividing a double
|by the square of another |by the square of another
|double. |double.
--- Comment #6 from Jakub Jelinek <jakub at gcc dot gnu.org> 2013-01-28 15:07:24 UTC ---
Fixed on the trunk so far.
^ permalink raw reply [flat|nested] 9+ messages in thread* [Bug tree-optimization/56125] [4.7 Regression] -O2 -ffast-math generates bad code when dividing a double by the square of another double.
2013-01-28 2:58 [Bug c/56125] New: -O2 -ffast-math generates bad code when dividing a double by the square of another double mednafen at gmail dot com
` (5 preceding siblings ...)
2013-01-28 15:07 ` [Bug tree-optimization/56125] [4.7 " jakub at gcc dot gnu.org
@ 2013-02-01 14:18 ` jakub at gcc dot gnu.org
2013-02-01 14:39 ` jakub at gcc dot gnu.org
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From: jakub at gcc dot gnu.org @ 2013-02-01 14:18 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56125
--- Comment #7 from Jakub Jelinek <jakub at gcc dot gnu.org> 2013-02-01 14:17:58 UTC ---
Author: jakub
Date: Fri Feb 1 14:17:50 2013
New Revision: 195664
URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=195664
Log:
Backported from mainline
2013-01-28 Jakub Jelinek <jakub@redhat.com>
PR tree-optimization/56125
* tree-ssa-math-opts.c (gimple_expand_builtin_pow): Don't optimize
pow(x,c) into sqrt(x) * powi(x, n/2) or
1.0 / (sqrt(x) * powi(x, abs(n/2))) if c is an integer or when
optimizing for size.
Don't optimize pow(x,c) into powi(x, n/3) * powi(cbrt(x), n%3) or
1.0 / (powi(x, abs(n)/3) * powi(cbrt(x), abs(n)%3)) if 2c is an
integer.
* gcc.dg/pr56125.c: New test.
Added:
branches/gcc-4_7-branch/gcc/testsuite/gcc.dg/pr56125.c
Modified:
branches/gcc-4_7-branch/gcc/ChangeLog
branches/gcc-4_7-branch/gcc/testsuite/ChangeLog
branches/gcc-4_7-branch/gcc/tree-ssa-math-opts.c
^ permalink raw reply [flat|nested] 9+ messages in thread* [Bug tree-optimization/56125] [4.7 Regression] -O2 -ffast-math generates bad code when dividing a double by the square of another double.
2013-01-28 2:58 [Bug c/56125] New: -O2 -ffast-math generates bad code when dividing a double by the square of another double mednafen at gmail dot com
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2013-02-01 14:18 ` jakub at gcc dot gnu.org
@ 2013-02-01 14:39 ` jakub at gcc dot gnu.org
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From: jakub at gcc dot gnu.org @ 2013-02-01 14:39 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56125
Jakub Jelinek <jakub at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|ASSIGNED |RESOLVED
Resolution| |FIXED
--- Comment #8 from Jakub Jelinek <jakub at gcc dot gnu.org> 2013-02-01 14:38:51 UTC ---
Fixed.
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