https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106842
Bug ID: 106842
Summary: misleading warning : iteration X invokes undefined
behavior
Product: gcc
Version: 12.2.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: franckbehaghel_gcc at protonmail dot com
Target Milestone: ---
Hello,
I get the following misleading warning with gcc 12.2 with -O2/-O3.
11.3 seems fine.
gcc -O2 main.c
main.c: In function ‘main’:
main.c:16:38: warning: iteration 9 invokes undefined behavior
[-Waggressive-loop-optimizations]
16 | for(int64_t k =0; k<i1 ; k++)
| ~^~
main.c:16:31: note: within this loop
16 | for(int64_t k =0; k<i1 ; k++)
| ~^~~
cat main.c
#include "stdio.h"
#include "stdint.h"
int main(int argc, char** argv)
{
int64_t i1=0;
int64_t i3=0,i2=0; // warning with this declaration order
//int64_t i2=0,i3=0; // but fine (no warning) with this order
for ( i1 = 0; i1<10 ; i1++)
{
for ( ; i2<10 ;i2++ )
printf("L2\n");
for ( ; i3<10 ; i3++ )
for(int64_t k =0; k<i1 ; k++)
printf("L3\n");
}
printf("i1 %lu i2 %lu i3 %lu\n",i1,i2,i3);
return 0;
}
Could someone reproduce ?
The weirdest part of this warning : it depends on the index declaration order.
Even if gcc has a bad(?) way of computing internal range/validity to deduce
such issues, I would expect it in both order.
Regards,
