[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 <roarl at pvv dot org> ---
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++; // 0x7FFFFFFF + 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.