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* [Bug regression/16456] New: PowerPC - redundant subtract involving pointer types
@ 2004-07-09 18:05 gcc-bugzilla at gcc dot gnu dot org
2004-07-09 19:15 ` [Bug rtl-optimization/16456] " pinskia at gcc dot gnu dot org
2004-11-03 11:20 ` nathan at gcc dot gnu dot org
0 siblings, 2 replies; 5+ messages in thread
From: gcc-bugzilla at gcc dot gnu dot org @ 2004-07-09 18:05 UTC (permalink / raw)
To: gcc-bugs
Description:
A non-optimal code sequence is illustrated. Two subtract operations are
done where only one is necessary. Duplicate using gcc 3.5 and command
line:
gcc -O3 -m64 -c test.c
Testcase:
char * ptr1;
char * ptr2;
int x;
long int y;
void foo ()
{
x = ptr1 - ptr2;
y = ptr1 - ptr2;
}
Assembly:
ld 9,.LC0@toc(2)
ld 11,.LC1@toc(2)
ld 0,0(9)
ld 10,0(11)
ld 8,.LC3@toc(2)
subf 11,10,0
ld 9,.LC2@toc(2)
subf 0,10,0 -- no need to do this subtract, just use the previous
result.
std 11,0(8)
stw 0,0(9)
blr
--
Summary: PowerPC - redundant subtract involving pointer types
Product: gcc
Version: 3.5.0
Status: UNCONFIRMED
Severity: enhancement
Priority: P1
Component: regression
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: steinmtz at us dot ibm dot com
CC: gcc-bugs at gcc dot gnu dot org,steinmtz at us dot ibm
dot com
GCC build triplet: powerpc64-linux
GCC host triplet: powerpc64-linux
GCC target triplet: powerpc64-linux
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=16456
^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug rtl-optimization/16456] PowerPC - redundant subtract involving pointer types
2004-07-09 18:05 [Bug regression/16456] New: " gcc-bugzilla at gcc dot gnu dot org
@ 2004-07-09 19:15 ` pinskia at gcc dot gnu dot org
2004-11-03 11:20 ` nathan at gcc dot gnu dot org
1 sibling, 0 replies; 5+ messages in thread
From: pinskia at gcc dot gnu dot org @ 2004-07-09 19:15 UTC (permalink / raw)
To: gcc-bugs
------- Additional Comments From pinskia at gcc dot gnu dot org 2004-07-09 19:15 -------
Confirmed, the problem is they are different modes.
(set (reg:SI 124)
(minus:SI (subreg:SI (reg:DI 119 [ ptr1.1 ]) 4)
(subreg:SI (reg:DI 120 [ ptr2.4 ]) 4)))
(set (reg:DI 126)
(minus:DI (reg:DI 119 [ ptr1.1 ])
(reg:DI 120 [ ptr2.4 ])))
--
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |NEW
Component|other |rtl-optimization
Ever Confirmed| |1
Last reconfirmed|0000-00-00 00:00:00 |2004-07-09 19:15:24
date| |
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=16456
^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug rtl-optimization/16456] PowerPC - redundant subtract involving pointer types
2004-07-09 18:05 [Bug regression/16456] New: " gcc-bugzilla at gcc dot gnu dot org
2004-07-09 19:15 ` [Bug rtl-optimization/16456] " pinskia at gcc dot gnu dot org
@ 2004-11-03 11:20 ` nathan at gcc dot gnu dot org
1 sibling, 0 replies; 5+ messages in thread
From: nathan at gcc dot gnu dot org @ 2004-11-03 11:20 UTC (permalink / raw)
To: gcc-bugs
------- Additional Comments From nathan at gcc dot gnu dot org 2004-11-03 11:20 -------
What is happening is that convert_to_integer, which is used to convert the
subtraction result to the type of 'x', reassociates the cast from
(int)((long)ptr1 - (long)ptr2)
to
(int)ptr1 - (int)ptr2
As it thinks that's a win. Unfortunately this stops CSE noticing that
the later subtraction is actually the same.
This is another example of premature tree optimization, and it should be addressed
in the reworking of fold itself. I don't think this should be attempted at stage 3
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=16456
^ permalink raw reply [flat|nested] 5+ messages in thread
end of thread, other threads:[~2009-05-14 3:00 UTC | newest]
Thread overview: 5+ messages (download: mbox.gz / follow: Atom feed)
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2005-11-02 17:16 ` [Bug rtl-optimization/16456] PowerPC - redundant subtract involving pointer types pinskia at gcc dot gnu dot org
2006-10-22 23:13 ` pinskia at gcc dot gnu dot org
2009-05-14 3:00 ` bje at gcc dot gnu dot org
2004-07-09 18:05 [Bug regression/16456] New: " gcc-bugzilla at gcc dot gnu dot org
2004-07-09 19:15 ` [Bug rtl-optimization/16456] " pinskia at gcc dot gnu dot org
2004-11-03 11:20 ` nathan at gcc dot gnu dot org
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