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

[law@redhat.com]

In message <20030225155039.GB746@tornado.toronto.redhat.com>, Diego Novillo wri
tes:
 >On Tue, 25 Feb 2003, Jeff Law wrote:
 >
 >> In message <20030225152722.GA32721@tornado.toronto.redhat.com>, Diego Novil
 >lo w
 >> rites:
 >>  >Oops.  This means that:
 >>  >
 >>  >(a) evaluate_stmt() has to be careful when returning UNDEFINED.
 >>  >    The optimistic evaluation of PHI nodes may cause problems
 >>  >    like the one above.  I fixed this particular one by copying
 >>  >    more code from the destructive folder into the
 >>  >    non-destructive folder.
 >> Copying or better yet sharing code between the folders is a good thing.
 >> 
 >> 
 >>  >(b) The non-destructive and destructive folders must always
 >>  >    return the same values.  I've added a (moderately expensive)
 >>  >    check for this in the final fold pass.
 >> No.  They do not.  This is wrong.  
 >> 
 >> The check is mildly interesting in that it can point out places where
 >> the non-destructive folder could be optimizing, but if we have a
 >> requirement that they return the same results, then something is
 >> wrong elsewhere (as I pointed out in my last message).
 >> 
 >You pointed out, but you didn't explain why this is wrong.
 >Explain please.
 >
 >How is it OK for the two versions of fold() to return different
 >values for the exact same expression?
As long as the return values will run-time evaluate to the same value,
then it is safe.  ie, it is safe for one to simplify an expression that
the other does not.

Jeff