Re: LOOP_HEADER tree code?

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

Hello,

> >for project http://gcc.gnu.org/wiki/PreservingLoops, I am considering
> >introducing a tree LOOP_HEADER with single argument N (number of
> >iterations of the loop), that would be present in IL at the beginning of
> >header of each loop.  I have following motivations:
> >
> >1) One of the goals of the project is to avoid recomputing number of
> >  iterations of loops, and to keep this information (analysing # of
> >  iterations is fairly expensive, and we recompute this information
> >  quite a lot at the moment).  To keep the information valid, we need
> >  to prevent optimizations from destroying it (e.g., if the number
> >  is n_1 = n_2 - 1, and this is the last use of n_1, we do not want
> >  DCE to remove it); this is easy to achieve if n_1 would be the
> >  argument of LOOP_HEADER.  Without this tree node in IL, we would need
> >  to handle this specially in DCE, and possibly also other optimizers.
> >2) This statement might make it simpler to ensure that various
> >  optimizers update the loops correctly.
> 
> However, various optimizer needs to know about this special tree node.

not really (not any more than they know about other tree codes that are
not interesting for them).

Zdenek

> Or am I missing something?
> 
> >
> >I am not quite sure whether this is a good idea, though.  Are there some
> >reasons why not to do this, or any other problems with the idea?
> >
> >Zdenek
> 
> 
> -
> Devang