Re: [sa]: Revert partial def stuff, use fake variables instead

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

Hello,

> > 2) To make things even more funny, we require that the ssa names with
> >    the same pt_vars also have the same tag.
> 
> This is also true, because it's necessary if we group.
> This is because the grouping algorithms may cause us to use the NMT
> name, instead of it's aliases, as the alias.
> 
> If you have different names for the same set of pointed-to pointers, you
> either 
> 1. are screwed
> 2. have to make sure to mark anything with the same set of pointed-to
> pointers as being in the same sets anyway, which causes the same
> quadratic behavior.

see group_aliases.  We already have to look for pointers whose may-alias
set intersects with the group that is merged together.

> > So far I did not notice any reason for having NMT's at all -- it seems
> > to me that everything would work just fine (and much more efficiently)
> > if tree-ssa-operands just looked at pt_vars instead.  What do I miss?
> 
> That if you have too many pt_vars, the number of virtual variables
> explodes if you do that.  In order to avoid having an infinite number of
> virtual variables that we are renaming to represent aliasing, we group
> if the number of virtual ops is going to be more than 500 in a single
> functions.
> 
> If you don't group, you could certainly just use pt_vars (there may be
> some caveats to this right now that i'm sure diego know sabout).

As far as I can tell, the following approach should work as well,
without need for NMT's:

1) Find a pointer with largest pt_vars set S.
2) Create a new tag T to represent the set of variables S.
3) For each pointer P, if P->pt_vars has nonempty intersection with S,
   set P->pt_vars = P->pt_vars \setminus S \cup {T}.

Zdenek