[Bug tree-optimization/18595] [4.0 Regression] IV-OPTS is O(N^3)

["rakdver at atrey dot karlin dot mff dot cuni dot cz" <gcc-bugzilla at gcc dot gnu dot org>]

------- 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.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=18595