Re: [tree-ssa] CCP and non-destructive folding problems

[Diego Novillo <dnovillo at redhat dot com>]

On Wed, 2003-02-26 at 18:32, law at redhat dot com wrote:

> First, I'd rather not have the folders call into any of the simple
> routines.  If we really really need to do this, then the code in
> question should probably move into tree-simple or somewhere other
> than fold().  I'd like to keep fold as independent of the simplifier
> as we can as a guiding principle.
> 
OK.  Good point.

> A closely related issue is whether we should consider arithmetic 
> involving constant addresses as a simple operand.  I think this
> is probably on your plate since I think it's something you want
> to do.  I'm not convinced it's a good thing, but I'm not going to
> object if you make the changes to allow constant address arithmetic
> as simple operands.
> 
It's only allowing ADDR_EXPR <VAR_DECL>  (&var) as a simple constant. 
Actually, now that you mention it, this may not be a good idea because
it would change the GIMPLE grammar.  &var is a GIMPLE unary expression. 
I withdraw this idea.  Not good.


> A general note -- from looking at your patches it appeared that you were
> trying to get the two folders to always return the same results.  That's
> not a particularly interesting thing to do.
> 
>   The (poorly named) nondestructive folder is used for constant discovery,
>   and as a result stuff like simplifying var + 0 to var isn't really
>   interesting.
> 
Yes.  I got caught by this.  I see now that you had something else in
mind for the nd-folder.  Maybe it would make sense to rename it or
simply document its intended use?


> I don't think it's worth the effort to check in your ENABLE_CHECKING
> stuff to verify the two folders produce the same constant results.  The
> only places I see them differing is in address arithmetic.    The
> differences between their results are not significant in terms of allowing
> us to find more constants.
> 
OK.  No problem.


Diego.