Re: Some thoughts and quetsions about the data flow infrastracture

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

[trimming down the Cc list]

> > 1) what to do with (clobber (const_int 0)).  This should be not so much 
> > of a problem thanks to validate_change, but I'd be weary of having such 
> > CLOBBER rtx-en in REG_EQUAL notes!
>
> Just return NULL.  The philosophy of simplify_rtx is different from combine.
> In the former, you're being asked "can you simplify this?" while in the
> latter, there's a lot of intertwining between simplification and
> substitution.  So you can just return "no, I can't simplify it" and combine
> would then do what it wants do with that result.

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.

To some extent, simplify-rtx.c could *know* about CLOBBER.  It would 
just not return it unless in combine.

Paolo