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

[law at redhat dot com]

In message <20030225154852 dot GA746 at tornado dot toronto dot redhat dot com>, Diego Novillo wri
tes:
 >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'.
This should have been caught *much* earlier.


 >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.
Then you should find out why that wasn't folded earlier (ie, before ccp).


 >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().
I don't think you're going to find any significant factorable code. 
Sharing code was my intent from the start, but reality quickly set in.