Re: LOOP_HEADER tree code?

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

Hello,

> 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

I am not sure what you mean by that?  IMHO this is what I propose the
LOOP_HEADER tree node for.

> 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...

for data dependences, you need to transfer just dependence
vectors/distances, i.e., you do not need to worry about symbols.
So I do not see a reason for using some node in IL for data dependences.

Zdenek