[Bug tree-optimization/25243] [4.1/4.2 Regression] Jump threading opportunity missed in tree-ssa but caught in jump1

["law at redhat dot com" <gcc-bugzilla at gcc dot gnu dot org>]

------- Comment #6 from law at redhat dot com  2005-12-03 18:27 -------
Subject: Re:  [4.1/4.2 Regression] Jump
        threading opportunity missed in tree-ssa but caught in jump1

On Sat, 2005-12-03 at 17:46 +0000, steven at gcc dot gnu dot org wrote:
> 
> ------- Comment #5 from steven at gcc dot gnu dot org  2005-12-03 17:46 -------
> Actually, it's more related to Bug 21488.  What happens is that we record a
> value for the left hand side of a single-argument PHI node (i.e. for
> "rhs=PHI(lhs)" we record an equivalence rhs==lhs), but the left hand side also
> already has a value (in this case, lhs==4).
> 
> Later on we don't copy propagate lhs into the uses of rhs because lhs is
> defined in a loop and we don't copy propagate out of loops, so we never see
> that rhs==4 too.
> 
> My hack in comment #3 propagates the value "4" by following SSA_NAME_VALUE
> links as deeply as possible.  What we should probably do instead is look
> through SSA_NAME_VALUE chains in record_equivalences_from_phis.
The reason we don't generally walk through those chains is it is
possible to get loops in the value chain.  I've always considered
this a semi-bug in DOM.

Jeff


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=25243