https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61515
--- Comment #24 from Richard Biener <rguenth at gcc dot gnu.org> --- (In reply to Jeffrey A. Law from comment #23) > The piece you're missing in this is that when we seen an assignment to some > SSA_NAME, call it LHS and we've traversed a backedge, then we have to walk > through all the equivalences and see if there's anything that's been marked > as equivalent to LHS and invalidate those. Then we also ahve to invalidate > LHS. > > for (unsigned int i = 1; i < num_ssa_names; i++) > if (ssa_name (i) && SSA_NAME_VALUE (ssa_name (i)) == lhs) > record_temporary_equivalence (ssa_name (i), NULL_TREE, stack); > > if (SSA_NAME_VALUE (lhs)) > record_temporary_equivalence (lhs, NULL_TREE, stack); > > > The loop finds handles things equivalent to LHS, the second handles LHS > itself. Both are required for correctness. > > In the past there was a map similar to your implementation, but it's not > sufficient. See pr61289.C and pr61289-2.C. > > The problem is you may need to invalidate equivalences that are created > *outside* tree-ssa-threadedge.c. So any attempts to track inside > tree-ssa-threadedge are not sufficient for correctness. > > What's still not clear to me here is *why* we're calling the invalidation > code in the first place. It's only supposed to be called when we encounter > a statement which produces an output that we can't handle *and* we've > traversed a backedge. The combination of those two events is supposed to be > very rare. Otherwise the invalidation is obviously too expensive. That's > why I suggested in c#12 that we need to look at *why* we're calling the > invalidation code at all. Well, I don't know the code at all so I can just try to fix the complexity in the present algorithm. Please have a look here during stage3.
