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* [Bug c/111210] New: Wrong code at -Os on x86_64-linux-gnu since r12-4849-gf19791565d7
@ 2023-08-28 7:13 shaohua.li at inf dot ethz.ch
2023-08-28 7:29 ` [Bug c/111210] " amonakov at gcc dot gnu.org
` (4 more replies)
0 siblings, 5 replies; 6+ messages in thread
From: shaohua.li at inf dot ethz.ch @ 2023-08-28 7:13 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111210
Bug ID: 111210
Summary: Wrong code at -Os on x86_64-linux-gnu since
r12-4849-gf19791565d7
Product: gcc
Version: 14.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: shaohua.li at inf dot ethz.ch
Target Milestone: ---
gcc at -Os produces the wrong code.
Bisected to r12-4849-gf19791565d7
Compiler explorer: https://godbolt.org/z/fjcEvKdYT
$ cat a.c
int printf(const char *, ...);
int a;
int *b = &a;
int c(long *d) {
if (*d)
return *(int *)0;
return *(int *)(d + 1);
}
int main() {
long e[] = {0, 100};
int f = c(e);
*b = f;
printf("%d\n", a);
}
$
$ gcc -O3 a.c && ./a.out
100
$ gcc -Os a.c && ./a.out
0
$
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug c/111210] Wrong code at -Os on x86_64-linux-gnu since r12-4849-gf19791565d7
2023-08-28 7:13 [Bug c/111210] New: Wrong code at -Os on x86_64-linux-gnu since r12-4849-gf19791565d7 shaohua.li at inf dot ethz.ch
@ 2023-08-28 7:29 ` amonakov at gcc dot gnu.org
2023-08-28 12:46 ` shaohua.li at inf dot ethz.ch
` (3 subsequent siblings)
4 siblings, 0 replies; 6+ messages in thread
From: amonakov at gcc dot gnu.org @ 2023-08-28 7:29 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111210
Alexander Monakov <amonakov at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Resolution|--- |INVALID
Status|UNCONFIRMED |RESOLVED
CC| |amonakov at gcc dot gnu.org
--- Comment #1 from Alexander Monakov <amonakov at gcc dot gnu.org> ---
'c' is called with 'd' pointing to 'long e[2]', so
return *(int *)(d + 1);
is an aliasing violation (dereferencing a pointer to an incompatible type).
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug c/111210] Wrong code at -Os on x86_64-linux-gnu since r12-4849-gf19791565d7
2023-08-28 7:13 [Bug c/111210] New: Wrong code at -Os on x86_64-linux-gnu since r12-4849-gf19791565d7 shaohua.li at inf dot ethz.ch
2023-08-28 7:29 ` [Bug c/111210] " amonakov at gcc dot gnu.org
@ 2023-08-28 12:46 ` shaohua.li at inf dot ethz.ch
2023-08-28 12:50 ` xry111 at gcc dot gnu.org
` (2 subsequent siblings)
4 siblings, 0 replies; 6+ messages in thread
From: shaohua.li at inf dot ethz.ch @ 2023-08-28 12:46 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111210
--- Comment #2 from Shaohua Li <shaohua.li at inf dot ethz.ch> ---
(In reply to Alexander Monakov from comment #1)
> 'c' is called with 'd' pointing to 'long e[2]', so
>
> return *(int *)(d + 1);
>
> is an aliasing violation (dereferencing a pointer to an incompatible type).
Thanks for the quick diagnosis. I tried to enable -Wall -Wextra -pedantic but
got no warning about the test case. Could you share how you diagnose this
issue?
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug c/111210] Wrong code at -Os on x86_64-linux-gnu since r12-4849-gf19791565d7
2023-08-28 7:13 [Bug c/111210] New: Wrong code at -Os on x86_64-linux-gnu since r12-4849-gf19791565d7 shaohua.li at inf dot ethz.ch
2023-08-28 7:29 ` [Bug c/111210] " amonakov at gcc dot gnu.org
2023-08-28 12:46 ` shaohua.li at inf dot ethz.ch
@ 2023-08-28 12:50 ` xry111 at gcc dot gnu.org
2023-08-28 12:55 ` amonakov at gcc dot gnu.org
2023-08-28 12:58 ` shaohua.li at inf dot ethz.ch
4 siblings, 0 replies; 6+ messages in thread
From: xry111 at gcc dot gnu.org @ 2023-08-28 12:50 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111210
Xi Ruoyao <xry111 at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |xry111 at gcc dot gnu.org
--- Comment #3 from Xi Ruoyao <xry111 at gcc dot gnu.org> ---
(In reply to Shaohua Li from comment #2)
> (In reply to Alexander Monakov from comment #1)
> > 'c' is called with 'd' pointing to 'long e[2]', so
> >
> > return *(int *)(d + 1);
> >
> > is an aliasing violation (dereferencing a pointer to an incompatible type).
>
> Thanks for the quick diagnosis. I tried to enable -Wall -Wextra -pedantic
> but got no warning about the test case. Could you share how you diagnose
> this issue?
The red banner in the bug creation page says clearly:
"Similarly, if compiling with -fno-strict-aliasing -fwrapv makes a difference,
your code probably is not correct."
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug c/111210] Wrong code at -Os on x86_64-linux-gnu since r12-4849-gf19791565d7
2023-08-28 7:13 [Bug c/111210] New: Wrong code at -Os on x86_64-linux-gnu since r12-4849-gf19791565d7 shaohua.li at inf dot ethz.ch
` (2 preceding siblings ...)
2023-08-28 12:50 ` xry111 at gcc dot gnu.org
@ 2023-08-28 12:55 ` amonakov at gcc dot gnu.org
2023-08-28 12:58 ` shaohua.li at inf dot ethz.ch
4 siblings, 0 replies; 6+ messages in thread
From: amonakov at gcc dot gnu.org @ 2023-08-28 12:55 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111210
--- Comment #4 from Alexander Monakov <amonakov at gcc dot gnu.org> ---
The testcase is small enough to notice the issue by inspection.
Note that you get the "expected" answer with -fno-strict-aliasing, and as
explained in https://gcc.gnu.org/bugs/ it is one of the things you should check
when submitting a bugreport:
Before reporting that GCC compiles your code incorrectly, compile it with gcc
-Wall -Wextra and see whether this shows anything wrong with your code.
Similarly, if compiling with -fno-strict-aliasing -fwrapv
-fno-aggressive-loop-optimizations makes a difference, or if compiling with
-fsanitize=undefined produces any run-time errors, then your code is probably
not correct.
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug c/111210] Wrong code at -Os on x86_64-linux-gnu since r12-4849-gf19791565d7
2023-08-28 7:13 [Bug c/111210] New: Wrong code at -Os on x86_64-linux-gnu since r12-4849-gf19791565d7 shaohua.li at inf dot ethz.ch
` (3 preceding siblings ...)
2023-08-28 12:55 ` amonakov at gcc dot gnu.org
@ 2023-08-28 12:58 ` shaohua.li at inf dot ethz.ch
4 siblings, 0 replies; 6+ messages in thread
From: shaohua.li at inf dot ethz.ch @ 2023-08-28 12:58 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111210
--- Comment #5 from Shaohua Li <shaohua.li at inf dot ethz.ch> ---
Thanks for all your comments!
^ permalink raw reply [flat|nested] 6+ messages in thread
end of thread, other threads:[~2023-08-28 12:58 UTC | newest]
Thread overview: 6+ messages (download: mbox.gz / follow: Atom feed)
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2023-08-28 7:13 [Bug c/111210] New: Wrong code at -Os on x86_64-linux-gnu since r12-4849-gf19791565d7 shaohua.li at inf dot ethz.ch
2023-08-28 7:29 ` [Bug c/111210] " amonakov at gcc dot gnu.org
2023-08-28 12:46 ` shaohua.li at inf dot ethz.ch
2023-08-28 12:50 ` xry111 at gcc dot gnu.org
2023-08-28 12:55 ` amonakov at gcc dot gnu.org
2023-08-28 12:58 ` shaohua.li at inf dot ethz.ch
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