Re: [CFG] Unswitching cleanup

[Zdenek Dvorak <rakdver at atrey dot karlin dot mff dot cuni dot cz>]

Hello.

> > This patch cleans up the code concerning loop unswitching a lot, removing most
> > of the dark magic.  It also allows us to really remove the loop when we peel it
> > completely (till now we were just leaving it in place, hoping that some
> > following optimization pass will get rid of it).
> 
> Thanks.  I will take a look at it shortly. One thing I was thinking about
> is that we may make the unswitching independent on the midrtl just
> by using canonicalize_comparison on the condition we get from condtiional
> and re-emiting the jump again based on the comparison.  I will take a look
> how dificult it is to integrate into current design.
> 
> > *************** void verify_loop_structure (loops, flags
> > *** 1140,1145 ****
> > --- 1201,1209 ----
> >     for (i = 1; i < loops->num; i++)
> >       {
> >         loop = loops->parray[i];
> > +       if (!loop)
> > + 	continue;
> > + 
> 
> This looks fine. I am afraid there is a lot more places we will have
> to care NULL pointers tought.

Not on relevant places (i.e. where we could have holes in this array); I'm
almost sure I've found all the places in loop optimizer where we use it,
and nowhere else is the loop structure manipulated.

> > ! 
> > !   /* Find border hexes.  */
> What do you mean by the border hexes?

Hexes on border of the removed area (i.e. such that they have at least one
removed entry edge).

> > *************** count_loop_iterations (desc, niter, rnit
> > *** 418,427 ****
> >     unsigned HOST_WIDE_INT abs_diff = 0;
> >     enum rtx_code cond = desc->cond;
> >   
> >     if (desc->grow)
> >       {
> >         /* Bypass nonsential tests.  */
> > !       if (cond == GE || cond == GT || cond == GEU || cond == GTU)
> >   	return false;
> >         if (rniter)
> >   	{
> > --- 432,449 ----
> >     unsigned HOST_WIDE_INT abs_diff = 0;
> >     enum rtx_code cond = desc->cond;
> >   
> > +   /* Ensure that we always handle the condition to stay inside loop.  */
> > +   if (desc->neg)
> > +     {
> > +       cond = reverse_condition (cond);
> > +       if (cond == UNKNOWN)
> > + 	return false;
> > +     }
> > + 
> 
> Hmm, how can the UNKNOWNS get here?  They can happen only for floats
> and we hsould rule them out earlier.  I guess I will just add the
> test into simple_loop to verify that induction variable is integral.

They cannot; I didn't look inside reverse_condition, I just read its
description :-)

Zdenek