> -----Ursprüngliche Nachricht-----
> Von: LIU Hao <lh_mouse@xxxxxxx>
> Gesendet: Mittwoch, 10. April 2024 11:51
> An: Xi Ruoyao <xry111@xxxxxxxxxxx>; Alexander Monakov
> <amonakov@xxxxxxxxx>; stefan@xxxxxxxxx
> Cc: gcc-help@xxxxxxxxxxx
> Betreff: Re: optimizer discards sign information
>
> 在 2024-04-10 17:44, Xi Ruoyao 写道:
> > You only get a "different result" when an undefined behavior happens,
> > thus it **is** a valid point to say there is no wrong-code issue.
> >
> >> It's a real bug. There are many PRs on bugzilla.
> >
> > You may argue it's a missed-optimization, but we were discussing about
> > wrong-code or not.
>
> Nobody in this thread has been thinking it's wrong code.
>
> When there is no overflow, it's missed optimization. When there is an
> overflow, I don't care.
>
> Do you agree?
>
>
> --
> Best regards,
> LIU Hao
Yes, there is an overflow when the value gets assigned to x
u32 x = *a * *b;
And after that line of code, x is a valid unsigned int, no matter what value was assigned. And the compiler must not throw away that unsignedness.
Also an add can overflow:
u64 faa(int a, int b) {
u32 x = a + b;
u64 r = x;
And in this case the optimizer doesn't discard the variable x
With the multiplication the optimizer kills it:
int _5;
_5 = _2 * _4;
x_9 = (u32) _5;
r_10 = (u64) x_9;
becomes:
int _5;
_5 = _2 * _4;
r_10 = (u64) _5;
The cast to (u32) is an important information, which IMHO must not be discarded.
Regards
Stefan
