[Bug middle-end/38729] long time needed in tree canonical iv

[rguenther at suse dot de]

------- Comment #6 from rguenther at suse dot de  2009-06-20 17:12 -------
Subject: Re:  long time needed in tree canonical iv

On Sat, 20 Jun 2009, rakdver at gcc dot gnu dot org wrote:

> ------- Comment #5 from rakdver at gcc dot gnu dot org  2009-06-20 17:08 -------
> (In reply to comment #4)
> > With MAX_DOMINATORS_TO_WALK zero and find_loop_niter_by_eval completely
> > disabled
> > (checking enabled compiler, built with -O0):
> > 
> >  tree iv optimization  :  11.12 ( 6%) usr   0.07 ( 5%) sys  12.02 ( 6%) wall  
> > 59123 kB (20%) ggc
> > 
> > With just find_loop_niter_by_eval disabled:
> > 
> >  tree iv optimization  :  10.81 ( 6%) usr   0.05 ( 3%) sys  11.44 ( 6%) wall  
> > 59123 kB (20%) ggc
> > 
> > so that's it.
> 
> I assume you were really looking at tree canonical iv counter, not on the tree
> iv optimization one, otherwise I don't see how you got to this conclusion.

Ah, yes.  tree canonical iv is at 0% in all cases ...

> > Zdenek - if for one exit we cannot determine the number of executions,
> > shouldn't we return chrec_dont_know here instead of just examining the
> > other exits?
> 
> Why should we?  The fact that we cannot determine number of iterations before a
> particular exit is taken does not tell you anything about the rest of the
> exits.

wouldn't that possibly overestimate the number of iterations?  Ah, I
see - the function searches for the exit with the least known number
of iterations.

Richard.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38729