Re: GCC turns &~ into | due to undefined bit-shift without warning

[Alexander Monakov <amonakov at ispras dot ru>]

On Thu, 21 Mar 2019, Richard Biener wrote:
> > Maybe an example would help.
> >
> > Consider this code:
> >
> > for (int i = start; i < limit; i++) {
> >   foo(i * 5);
> > }
> >
> > Should GCC be entitled to turn it into
> >
> > int limit_tmp = i * 5;
> > for (int i = start * 5; i < limit_tmp; i += 5) {
> >   foo(i);
> > }
> >
> > If you answered "Yes, GCC should be allowed to do this", would you
> > want a warning? And how many such warnings might there be in a typical
> > program?
> 
> I assume i is signed int.  Even then GCC may not do this unless it knows
> the loop is entered (start < limit).

Additionally, the compiler needs to prove that 'foo' always returns normally 
(i.e. cannot invoke exit/longjmp or such).

Alexander