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

[Diego Novillo <dnovillo at redhat dot com>]

On Tue, 25 Feb 2003, Jeff Law wrote:

>  >My recent change to the flowgraph exposed a couple of problems in
>  >CCP:
>  >
>  >(1) ccp_fold() and fold() are not always returning the same
>  >    value.   For instance, strlen("abc") is folded to 3 by
>  >    fold(), but not folded by ccp_fold().
> This *should* be OK.  If it isn't then we've got a design issue in
> CCP.
> 
What should be OK?  The two versions of fold() returning
different values?  How is that OK?  I don't follow.

> Err, that's the problem.  If none of the operands are undefined, but 
> the expression does not fold, then the expression *must* be marked as
> varying.
> 
Not necessarily.  I agree in cases like 'foo (1, 3)'.  But
sometimes the expression does not fold because ccp_fold() didn't
know how to fold it.  This also happened to things like '1 && 1'.

If we mark it VARYING due to limitations in the nondestructive
folder, we may end up blocking further propagation of constants.
In the case of '1 && 1', the variable that the expression was
being assigned to was never considered constant.

The bigger problem here is actually the two diverging
implementations of fold().  I think we are making a big mistake.
We should factor more code in the two flavours of fold().


Diego.