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* [Bug c/26867] New: wrong optimization (-O2), wrong instruction reordering.
@ 2006-03-25 11:46 renzo at cs dot unibo dot it
2006-03-25 16:14 ` [Bug c/26867] " rguenth at gcc dot gnu dot org
0 siblings, 1 reply; 2+ messages in thread
From: renzo at cs dot unibo dot it @ 2006-03-25 11:46 UTC (permalink / raw)
To: gcc-bugs
The program here enclosed has this behavior (tested on i386 and ppc, gcc
4.0.3):
$ gcc -O1 test.c
$ ./a.out 42
42
$ gcc -O2 test.c
$ ./a.out 42
0
on ppc (from debian sid) this chunk of the code:
25: tmp=((struct a *)bp)->a2;
26: bp->b1=0;
27: bp->b2=0;
28: bp->b3=0;
29: bp->b5 = tmp;
gets translated in this (wrong) way (gcc -S -g -O2):
.loc 1 22 0
lha 29,0(4)
.loc 1 26 0
stw 0,0(3) <-- b1 is zeroed line 26
.loc 1 27 0
stw 0,4(3)
.loc 1 17 0
mr 31,4
.loc 1 25 0
lha 11,2(3) <-- line 25, reads a2 which is part of b1!
.LVL5:
.loc 1 22 0
lha 9,2(4)
.loc 1 28 0
stb 0,8(3)
.LVL6:
.loc 1 29 0
sth 11,10(3) <-- store R11 into b5
I have tried to read the open bugs and it seems to me that this bug has not
been
submitted yet. If it has been already filed, I apologize in advance.
renzo
----------
$ gcc -v
Using built-in specs.
Target: powerpc-linux-gnu
Configured with: ../src/configure -v
--enable-languages=c,c++,java,f95,objc,ada,treelang --prefix=/usr
--enable-shared --with-system-zlib --libexecdir=/usr/lib
--without-included-gettext --enable-threads=posix --enable-nls
--program-suffix=-4.0 --enable-__cxa_atexit --enable-clocale=gnu
--enable-libstdcxx-debug --enable-java-awt=gtk-default --enable-gtk-cairo
--with-java-home=/usr/lib/jvm/java-1.4.2-gcj-4.0-1.4.2.0/jre --enable-mpfr
--disable-softfloat --enable-targets=powerpc-linux,powerpc64-linux
--with-cpu=default32 --disable-werror --enable-checking=release
powerpc-linux-gnu
Thread model: posix
gcc version 4.0.3 (Debian 4.0.3-1)
The C test source is:
-------------------------
#include <stdio.h>
struct a {
short a1;
short a2;
};
struct b {
int b1;
int b2;
char b3;
char b4;
short b5;
};
void f(struct b *bp, void *m)
{
int tmp;
struct a *ap;
struct a savea;
ap=(struct a *)m;
savea = *ap;
savea.a2 += 4;
tmp=((struct a *)bp)->a2;
bp->b1=0;
bp->b2=0;
bp->b3=0;
bp->b5 = tmp;
if (fc()) {
*ap = savea;
}
}
int fc()
{
return (time() % 2);
}
main(int argc,char *argv[])
{
struct b myb;
struct a mya;
mya.a2=0;
((struct a *)(&myb))->a2 = atoi(argv[1]);
f(&myb,&mya);
printf("%d\n",myb.b5);
}
--
Summary: wrong optimization (-O2), wrong instruction reordering.
Product: gcc
Version: 4.0.3
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: renzo at cs dot unibo dot it
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26867
^ permalink raw reply [flat|nested] 2+ messages in thread
* [Bug c/26867] wrong optimization (-O2), wrong instruction reordering.
2006-03-25 11:46 [Bug c/26867] New: wrong optimization (-O2), wrong instruction reordering renzo at cs dot unibo dot it
@ 2006-03-25 16:14 ` rguenth at gcc dot gnu dot org
0 siblings, 0 replies; 2+ messages in thread
From: rguenth at gcc dot gnu dot org @ 2006-03-25 16:14 UTC (permalink / raw)
To: gcc-bugs
------- Comment #1 from rguenth at gcc dot gnu dot org 2006-03-25 16:14 -------
It's the most reported bug. And it's invalid, you are violating C aliasing
rules.
Use -fno-strict-aliasing.
*** This bug has been marked as a duplicate of 21920 ***
--
rguenth at gcc dot gnu dot org changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |RESOLVED
Resolution| |DUPLICATE
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26867
^ permalink raw reply [flat|nested] 2+ messages in thread
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2006-03-25 11:46 [Bug c/26867] New: wrong optimization (-O2), wrong instruction reordering renzo at cs dot unibo dot it
2006-03-25 16:14 ` [Bug c/26867] " rguenth at gcc dot gnu dot org
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