LOOP_HEADER tree code?

[Richard Sandiford]

Zdenek Dvorak <rakdver@atrey.karlin.mff.cuni.cz> writes:
> 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.
>
> 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?

I haven't seen anyone else say this, but this sounds worryingly
like the old RTL loop notes to me.  I'm sure the loop notes seemed
like a good idea when they were first introduced (probably for
similar reasons) but they became such a bug-bear in the end.

Not a very constructive comment, I'll be the first to admit.

Richard