Re: Some thoughts and quetsions about the data flow infrastracture

[Paolo Bonzini <paolo dot bonzini at lu dot unisi dot ch>]

Richard Kenner wrote:
> > Yes, one possibility is to use a RTX hook for this too.  By default you 
> > would return NULL (and this would propagate up); in combine you could 
> > override it to return the CLOBBER.
>
> I really don't see why.  Look at when combine calls the simplify routines now.
> If they return zero, it uses the original value.

Some of the combine simplifications (you obviously know that) work by 
"hoping" that the CLOBBER is simplified away.  I don't think you can 
preserve all their power if you propagate NULL.  In most cases you can 
replace CLOBBER with NULL, but I don't think that's possible everywhere.

Paolo