[Bug tree-optimization/26406] Fowardprop does harm for VRP to figure out if a point is non zero

[law at redhat dot com]

------- Comment #12 from law at redhat dot com  2006-02-22 21:45 -------
Subject: Re:  Fowardprop does harm for VRP to
        figure out if a point is non zero

On Wed, 2006-02-22 at 20:55 +0000, rguenth at gcc dot gnu dot org wrote:
> 
> ------- Comment #11 from rguenth at gcc dot gnu dot org  2006-02-22 20:55 -------
> So I suppose VRP cannot see "backwards" for
> 
>   i_2 = j_1;
>   if (i_2 == 0)
>     return j_1;
> 
> ?  (of course copyprop would clean this up, but suppose for a moment this
> gets to VRP)
Nope, it can't.  It's not just the lack of backwards propagation,
but also the fact that i is unused in the subgraphs after the
conditional, so VRP won't record any information for either i or j
in this kind of example.

Fixing VRP to gather data for "i" in this example would result in
a pretty significant compile-time hit.

I'll note this was one of the reasons why we moved copyprop to
run immediately before VRP -- copies in the IL were hiding a
nontrivial number of detectable and useful ranges.

jeff


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26406