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* [Bug middle-end/23669] New: fold does convert (-a)/10 into a/-10.
@ 2005-09-01 2:04 pinskia at gcc dot gnu dot org
2005-09-01 2:38 ` [Bug middle-end/23669] " pinskia at gcc dot gnu dot org
` (3 more replies)
0 siblings, 4 replies; 5+ messages in thread
From: pinskia at gcc dot gnu dot org @ 2005-09-01 2:04 UTC (permalink / raw)
To: gcc-bugs
Another idea from LLVM.
The following two functions should be the same asm:
int f(int a)
{
return (-a)/10;
}
int f1(int a)
{
return a/-10;
}
----
We currently get on PPC:
_f:
lis r0,0x6666
neg r3,r3
ori r0,r0,26215
mulhw r0,r3,r0
srawi r3,r3,31
srawi r0,r0,2
subf r3,r3,r0
blr
_f1:
lis r0,0x6666
ori r0,r0,26215
mulhw r0,r3,r0
srawi r3,r3,31
srawi r0,r0,2
subf r3,r0,r3
blr
As you can see f1 has one less instruction which should be better.
--
Summary: fold does convert (-a)/10 into a/-10.
Product: gcc
Version: 4.1.0
Status: UNCONFIRMED
Keywords: missed-optimization
Severity: normal
Priority: P2
Component: middle-end
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: pinskia at gcc dot gnu dot org
CC: gcc-bugs at gcc dot gnu dot org
OtherBugsDependingO 19987
nThis:
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=23669
^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug middle-end/23669] fold does convert (-a)/10 into a/-10.
2005-09-01 2:04 [Bug middle-end/23669] New: fold does convert (-a)/10 into a/-10 pinskia at gcc dot gnu dot org
@ 2005-09-01 2:38 ` pinskia at gcc dot gnu dot org
2005-09-17 2:12 ` pinskia at gcc dot gnu dot org
` (2 subsequent siblings)
3 siblings, 0 replies; 5+ messages in thread
From: pinskia at gcc dot gnu dot org @ 2005-09-01 2:38 UTC (permalink / raw)
To: gcc-bugs
--
What |Removed |Added
----------------------------------------------------------------------------
Severity|normal |enhancement
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=23669
^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug middle-end/23669] fold does convert (-a)/10 into a/-10.
2005-09-01 2:04 [Bug middle-end/23669] New: fold does convert (-a)/10 into a/-10 pinskia at gcc dot gnu dot org
2005-09-01 2:38 ` [Bug middle-end/23669] " pinskia at gcc dot gnu dot org
@ 2005-09-17 2:12 ` pinskia at gcc dot gnu dot org
2005-10-01 17:25 ` mrnobo1024 at yahoo dot com
2005-10-01 17:34 ` [Bug middle-end/23669] fold does convert (-a)/10 into a/-10 with -fno-wrapv pinskia at gcc dot gnu dot org
3 siblings, 0 replies; 5+ messages in thread
From: pinskia at gcc dot gnu dot org @ 2005-09-17 2:12 UTC (permalink / raw)
To: gcc-bugs
------- Additional Comments From pinskia at gcc dot gnu dot org 2005-09-17 02:12 -------
Confirmed.
--
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |NEW
Ever Confirmed| |1
Last reconfirmed|0000-00-00 00:00:00 |2005-09-17 02:12:00
date| |
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=23669
^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug middle-end/23669] fold does convert (-a)/10 into a/-10.
2005-09-01 2:04 [Bug middle-end/23669] New: fold does convert (-a)/10 into a/-10 pinskia at gcc dot gnu dot org
2005-09-01 2:38 ` [Bug middle-end/23669] " pinskia at gcc dot gnu dot org
2005-09-17 2:12 ` pinskia at gcc dot gnu dot org
@ 2005-10-01 17:25 ` mrnobo1024 at yahoo dot com
2005-10-01 17:34 ` [Bug middle-end/23669] fold does convert (-a)/10 into a/-10 with -fno-wrapv pinskia at gcc dot gnu dot org
3 siblings, 0 replies; 5+ messages in thread
From: mrnobo1024 at yahoo dot com @ 2005-10-01 17:25 UTC (permalink / raw)
To: gcc-bugs
------- Additional Comments From mrnobo1024 at yahoo dot com 2005-10-01 17:25 -------
Those expressions are only the same for floating point types.
f(-2147483648) == -214748364
f1(-2147483648) == 214748364
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=23669
^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug middle-end/23669] fold does convert (-a)/10 into a/-10 with -fno-wrapv
2005-09-01 2:04 [Bug middle-end/23669] New: fold does convert (-a)/10 into a/-10 pinskia at gcc dot gnu dot org
` (2 preceding siblings ...)
2005-10-01 17:25 ` mrnobo1024 at yahoo dot com
@ 2005-10-01 17:34 ` pinskia at gcc dot gnu dot org
3 siblings, 0 replies; 5+ messages in thread
From: pinskia at gcc dot gnu dot org @ 2005-10-01 17:34 UTC (permalink / raw)
To: gcc-bugs
------- Additional Comments From pinskia at gcc dot gnu dot org 2005-10-01 17:34 -------
(In reply to comment #2)
> Those expressions are only the same for floating point types.
But -a for INT_MIN is undefined for signed type since it is an overflow. So the conversion is only valid
for -fno-wrapv which is default for C, C++, Ada, and fortran.
--
What |Removed |Added
----------------------------------------------------------------------------
Summary|fold does convert (-a)/10 |fold does convert (-a)/10
|into a/-10. |into a/-10 with -fno-wrapv
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=23669
^ permalink raw reply [flat|nested] 5+ messages in thread
end of thread, other threads:[~2005-10-01 17:34 UTC | newest]
Thread overview: 5+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2005-09-01 2:04 [Bug middle-end/23669] New: fold does convert (-a)/10 into a/-10 pinskia at gcc dot gnu dot org
2005-09-01 2:38 ` [Bug middle-end/23669] " pinskia at gcc dot gnu dot org
2005-09-17 2:12 ` pinskia at gcc dot gnu dot org
2005-10-01 17:25 ` mrnobo1024 at yahoo dot com
2005-10-01 17:34 ` [Bug middle-end/23669] fold does convert (-a)/10 into a/-10 with -fno-wrapv pinskia at gcc dot gnu dot org
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