------- Additional Comments From rakdver at atrey dot karlin dot mff dot cuni
dot cz 2005-01-25 15:54 -------
Subject: Re: [4.0 Regression] IV-OPTS is O(N^3)
> rakdver at atrey dot karlin dot mff dot cuni dot cz wrote:
> > More seriously -- which of the possibilities? If I have loops like
> >
> > while (...)
> > {
> > while (...)
> > {
> > x_1 = something ();
> > }
> > x_2 = phi (x_1);
> > x_3 = x_2 + 1;
> > }
> >
> > What will analyze_scalar_evolutions return for x_3? There are (at least)
> > three possible valid values:
> >
> > x_3
>
> This would be the answer if analyze_scalar_evolutions would be the
> identity function. If you want, you could change analyze_scalar_evolutions
> such that it behaves like that, and decide that the instantiation do
> the rest of the work (I mean moving the code that is currently in
> analyze_scalar_evolutions to the instantiation phase).
>
> > x_2 + 1
>
> If you decide to reconstruct the tree expression, there is no reason
> to stop on a phi node that has a single argument. Why would you like
> to get this answer as the reconstructed tree?
because this answer preserves loop closed ssa form -- the answer x_1 + 1
copy propagates the value outsied of the loop. In some applications
this choice could make sense.
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=18595
