birthpoints in rtl.

[Steven Bosscher]

On Tue, Mar 4, 2008 at 7:58 PM, Diego Novillo <dnovillo@google.com> wrote:
>  Both PHIs and birthpoints are merely factoring devices that let you cut
>  down the number of UD links.  They don't need to be part of the IL, much
>  like none of the DF objects are part of the RTL IL.

Maybe they don't need to be, but it may be useful to have them anyway.

>  > What about keeping things up-to-date after applying some
>  > transformations? It is already hard to keep UD/DU chains up-to-date
>  > now (I don't think any pass successfully does so right now). This
>  > should be a lot easier if you fully factorize your UD chains, right?
>
>  In theory, yes.  Code for keeping these things up-to-date already exists
>  in GIMPLE SSA.

That code is IMHO just awfully ugly. And slow too, last I checked.  I
don't know if  this is still true, but update_ssa used to walk a huge
part of the dominator tree even for seemingly trivial updates. We
should not want that on RTL.  I don't think we should allow
transformations on RTL that are too hard to manually update the FUD
chains somehow.

Gr.
Steven