On Thu, Mar 21, 2019 at 9:25 AM Alexander Monakov <amona...@ispras.ru> wrote:
>
> 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).
Ah, yes. Andrews example was probably meaning limit_tmp = limit * 5, not i * 5.
Computing start * 5 is fine if the loop is entered.
Richard.
>
> Alexander
