[Bug tree-optimization/18694] [4.0 regression] loop miscompilation at -O1 (-ftree-ch)

["kazu at cs dot umass dot edu" <gcc-bugzilla at gcc dot gnu dot org>]

------- Additional Comments From kazu at cs dot umass dot edu  2004-12-10 18:53 -------
Subject: Re:  [4.0 regression] loop
 miscompilation at -O1 (-ftree-ch)

Hi Zdenek,

> > >  or simply use dominated_by_p, which is not too expensive -
> > > only a couple of "if" statements, assuming the dominator infomatino is
> > > available.
> > EDGE_DFS_BACK would have been more efficient.  But I don't think it's
> > 100% safe right now.
> 
> not that much.  Since you do not change cfg in the progress of dominator
> optimizations, you should have fast query structure for dominators
> up-to-date; i.e.  testing whether a basic block dominates other takes only
> two comparisons of integers.

Right.  If we have dominator information available,
mark_dfs_back_edges does nothing but duplicates a part of what the
dominator information has to offer.  (It's not that
mark_dfs_back_edges is very flow, but ....)

Kazu Hirata


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=18694