https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88148
--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> --- So the issue is that VN done after unrolling removes SSA defs that are still referenced from loop->nb_iterations. CFG cleanup makes sure to update it when doing replace_uses_by (which is a quite expensive operation!). VN doesn't do anything like that. For regions, when we are in loop-closed SSA form we would know which loops to eventually update. When we are not in loop-closed SSA the affected loops are all downstream ones (blowing away the region-VN complexity guarantees). niter "uses" do not count towards SSA uses. We cannot easily substitute into loop info during eliminate_cleanup since we need to substitute available names. We could in theory substitute into loop info during the main VN walk where we perform region elimination when we visit a loop header. But we'd have to eliminate a GENERIC tree here... There is already a helper besides substitute_in_loop_info, namely the specific simplify_replace_tree which we could outfit with a valueization hook rather than a 1:1 name/value pair. Let me see if I can cook up sth like that.
