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

[sebastian dot pop at cri dot ensmp dot fr]

------- Additional Comments From sebastian dot pop at cri dot ensmp dot fr  2005-01-25 12:44 -------
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?

> x_1 + 1
> 

IMO this would be the answer, although I didn't checked.



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