https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83133
--- Comment #6 from Maxim Egorushkin <maxim.yegorushkin at gmail dot com> ---
(In reply to Uroš Bizjak from comment #3)
> (In reply to Maxim Egorushkin from comment #2)
>
> > Could you provide an example where that "dangerous optimization" would break
> > well-formed code please?
>
> --cut here--
> #include <stdio.h>
>
> void positive (int a) { printf ("positive: %i\n", a); }
> void nonpositive (int a) { printf ("nonpositive: %i\n", a); }
>
> void
> __attribute__((noinline))
> g (int a, int b)
> {
> int diff = a - b;
>
> if (diff > 0)
> return positive (diff);
> else
> return nonpositive (diff);
> }
>
> int
> main ()
> {
> int a = -0x80000000;
> int b = 0x01;
>
> g (a, b);
>
> return 0;
> }
> --cut here--
>
> $ gcc -O2 ttt.c
> $ ./a.out
> positive: 2147483647
>
> $ gcc -O0 ttt.c
> $ ./a.out
> positive: 2147483647
>
> $ clang -O2 ttt.c
> $ ./a.out
> nonpositive: 2147483647 <--------- HERE!
>
> $ clang -O0 ttt.c
> $ ./a.out
> positive: 2147483647
This code underflows a signed integer, which is undefined behaviour, if I am
not mistaken. So, this would not be a valid example, would it?
