LOOP_HEADER tree code?

[Sebastian Pop]

Hi,

On 10/23/06, Zdenek Dvorak <rakdver@atrey.karlin.mff.cuni.cz> wrote:
> 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.
>
> 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?
>

Why not keeping this information outside the IL, and making sure that code
transformers are updating that information if they modify the symbols involved
in that representation?

The same problems will also occur when transmitting to the backend
other informations attached to loop structures.  I'm not sure that
we want to create a new node for data dependences as well...

Sebastian