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* [Bug middle-end/109578] New: fail to remove dead code due to division by zero
@ 2023-04-20 21:17 vincent-gcc at vinc17 dot net
2023-04-20 21:18 ` [Bug middle-end/109578] " pinskia at gcc dot gnu.org
` (4 more replies)
0 siblings, 5 replies; 6+ messages in thread
From: vincent-gcc at vinc17 dot net @ 2023-04-20 21:17 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109578
Bug ID: 109578
Summary: fail to remove dead code due to division by zero
Product: gcc
Version: 12.2.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: middle-end
Assignee: unassigned at gcc dot gnu.org
Reporter: vincent-gcc at vinc17 dot net
Target Milestone: ---
This is about the opposite of the invalid bug 29968:
#include <stdio.h>
int f (int i, int k)
{
if (k == 0)
printf ("k = 0\n");
return i/k;
}
int main (void)
{
return f (1, 0);
}
With gcc-12 (Debian 12.2.0-14) 12.2.0 and -O3 optimization, I get:
k = 0
zsh: illegal hardware instruction (core dumped) ./tst
But since the case k == 0 corresponds to an undefined behavior (which is the
justification behind that GCC is correct in bug 29968), the code
if (k == 0)
printf ("k = 0\n");
should have been removed as an optimization.
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug middle-end/109578] fail to remove dead code due to division by zero
2023-04-20 21:17 [Bug middle-end/109578] New: fail to remove dead code due to division by zero vincent-gcc at vinc17 dot net
@ 2023-04-20 21:18 ` pinskia at gcc dot gnu.org
2023-04-20 21:26 ` vincent-gcc at vinc17 dot net
` (3 subsequent siblings)
4 siblings, 0 replies; 6+ messages in thread
From: pinskia at gcc dot gnu.org @ 2023-04-20 21:18 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109578
Andrew Pinski <pinskia at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Resolution|--- |WONTFIX
Status|UNCONFIRMED |RESOLVED
--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
We don't removing code before undefined behavior ...
That is GCC does not know that printf does not have side effects.
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug middle-end/109578] fail to remove dead code due to division by zero
2023-04-20 21:17 [Bug middle-end/109578] New: fail to remove dead code due to division by zero vincent-gcc at vinc17 dot net
2023-04-20 21:18 ` [Bug middle-end/109578] " pinskia at gcc dot gnu.org
@ 2023-04-20 21:26 ` vincent-gcc at vinc17 dot net
2023-04-20 21:37 ` pinskia at gcc dot gnu.org
` (2 subsequent siblings)
4 siblings, 0 replies; 6+ messages in thread
From: vincent-gcc at vinc17 dot net @ 2023-04-20 21:26 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109578
--- Comment #2 from Vincent Lefèvre <vincent-gcc at vinc17 dot net> ---
(In reply to Andrew Pinski from comment #1)
> We don't removing code before undefined behavior ...
> That is GCC does not know that printf does not have side effects.
Then GCC is incorrect in bug 29968, because it does the division *before* the
printf.
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug middle-end/109578] fail to remove dead code due to division by zero
2023-04-20 21:17 [Bug middle-end/109578] New: fail to remove dead code due to division by zero vincent-gcc at vinc17 dot net
2023-04-20 21:18 ` [Bug middle-end/109578] " pinskia at gcc dot gnu.org
2023-04-20 21:26 ` vincent-gcc at vinc17 dot net
@ 2023-04-20 21:37 ` pinskia at gcc dot gnu.org
2023-04-20 21:49 ` vincent-gcc at vinc17 dot net
2023-04-20 21:50 ` pinskia at gcc dot gnu.org
4 siblings, 0 replies; 6+ messages in thread
From: pinskia at gcc dot gnu.org @ 2023-04-20 21:37 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109578
--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Vincent Lefèvre from comment #2)
> (In reply to Andrew Pinski from comment #1)
> > We don't removing code before undefined behavior ...
> > That is GCC does not know that printf does not have side effects.
>
> Then GCC is incorrect in bug 29968, because it does the division *before*
> the printf.
Or was a bug which has since been fixed.
For sparc GCC 12.2.0 we get:
sethi %hi(.LC0), %o0
call puts, 0
or %o0, %lo(.LC0), %o0
ta 5
Anyways maybe the issue with PR 29968 was a scheduling issue which was fixed
later on that GCC didn't realize divide could trap.
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug middle-end/109578] fail to remove dead code due to division by zero
2023-04-20 21:17 [Bug middle-end/109578] New: fail to remove dead code due to division by zero vincent-gcc at vinc17 dot net
` (2 preceding siblings ...)
2023-04-20 21:37 ` pinskia at gcc dot gnu.org
@ 2023-04-20 21:49 ` vincent-gcc at vinc17 dot net
2023-04-20 21:50 ` pinskia at gcc dot gnu.org
4 siblings, 0 replies; 6+ messages in thread
From: vincent-gcc at vinc17 dot net @ 2023-04-20 21:49 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109578
--- Comment #4 from Vincent Lefèvre <vincent-gcc at vinc17 dot net> ---
(In reply to Andrew Pinski from comment #3)
> Anyways maybe the issue with PR 29968 was a scheduling issue which was fixed
> later on that GCC didn't realize divide could trap.
OK, thanks, I can see your update marking PR 29968 as a duplicate of PR 41239
(fixed).
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug middle-end/109578] fail to remove dead code due to division by zero
2023-04-20 21:17 [Bug middle-end/109578] New: fail to remove dead code due to division by zero vincent-gcc at vinc17 dot net
` (3 preceding siblings ...)
2023-04-20 21:49 ` vincent-gcc at vinc17 dot net
@ 2023-04-20 21:50 ` pinskia at gcc dot gnu.org
4 siblings, 0 replies; 6+ messages in thread
From: pinskia at gcc dot gnu.org @ 2023-04-20 21:50 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109578
--- Comment #5 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Andrew Pinski from comment #3)
> Anyways maybe the issue with PR 29968 was a scheduling issue which was fixed
> later on that GCC didn't realize divide could trap.
I was right on that, it was a dup of bug 41239 which was fixed in GCC 4.5.0.
The problem was exactly what I thought it was, a trapping instruction being
scheduled before the call. And yes it has been fixed for a long time, 13 years
in fact.
^ permalink raw reply [flat|nested] 6+ messages in thread
end of thread, other threads:[~2023-04-20 21:50 UTC | newest]
Thread overview: 6+ messages (download: mbox.gz / follow: Atom feed)
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2023-04-20 21:17 [Bug middle-end/109578] New: fail to remove dead code due to division by zero vincent-gcc at vinc17 dot net
2023-04-20 21:18 ` [Bug middle-end/109578] " pinskia at gcc dot gnu.org
2023-04-20 21:26 ` vincent-gcc at vinc17 dot net
2023-04-20 21:37 ` pinskia at gcc dot gnu.org
2023-04-20 21:49 ` vincent-gcc at vinc17 dot net
2023-04-20 21:50 ` pinskia at gcc dot gnu.org
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