[Bug c/69612] Optimizer does not consider overflow

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69612

Jakub Jelinek  changed:

   What|Removed |Added

 Status|UNCONFIRMED |RESOLVED
 CC||jakub at gcc dot gnu.org
 Resolution|--- |INVALID

--- Comment #1 from Jakub Jelinek  ---
This program triggers undefined behavior.
You can e.g. use -fsanitize=undefined to see it diagnosed at runtime.
Once undefined behavior is reached, anything can happen.

[Bug c/69612] Optimizer does not consider overflow

[roarl at pvv dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69612

--- Comment #2 from Roar Lauritzsen  ---
Thanks a lot for the quick analysis. Now that I know what it is I can fix my
program, and the -fsanitize=undefined will come in handy for localizing problem
areas. For future googlers, I am planning to fix my program along these lines:

unsigned test2(unsigned a) {
if ((int)a < 0)
return 1;
a++; // 0x7FFF + 1 is not undefined anymore
if ((int)a < 0)
return 2;
return a;
}

This code executes the way I want also with -O2, and there are no errors from
-fsanitize=undefined. Personally, I find it somewhat incongruent that integer
overflow is undefined, while integer casting is not, and I also find it very
disturbing that optimized and non-optimized code may behave differently.

[Bug c/69612] Optimizer does not consider overflow

[manu at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69612

Manuel López-Ibáñez  changed:

   What|Removed |Added

 CC||manu at gcc dot gnu.org

--- Comment #3 from Manuel López-Ibáñez  ---
(In reply to Roar Lauritzsen from comment #2)
> Thanks a lot for the quick analysis. Now that I know what it is I can fix my
> program, and the -fsanitize=undefined will come in handy for localizing
> problem areas. For future googlers, I am planning to fix my program along
> these lines:

Better use https://gcc.gnu.org/onlinedocs/gcc/Integer-Overflow-Builtins.html