LOOP_HEADER tree code?

[Devang Patel]

> 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.
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