https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67221
--- Comment #5 from Richard Biener <rguenth at gcc dot gnu.org> --- Ok, so we have e_17 = e_3 + 1; and e_3 value-numbers to e_17 (for some reason). match-and-simplify then recursively matches ((e_3 + 1) + 1) + 1 ... Visiting BB 8 SCC consists of: e_3 e_17 Starting iteration 1 Value numbering e_3 stmt = e_3 = PHI <0(5), e_17(8)> Setting value number of e_3 to c_2 (changed) Value numbering e_17 stmt = e_17 = e_3 + 1; Setting value number of e_17 to e_17 (changed) Starting iteration 2 Value numbering e_3 stmt = e_3 = PHI <0(5), e_17(8)> Setting value number of e_3 to e_17 (changed) Oops. This isn't supposed to happen. Ok - so the only executable edge is the backedge here (should have catched that block as unexecutable, sth to fix as well). Hmm, we only forbid a VARING -> non-VARYING lattice transition, not a transition from one value to another. Fishy. Of course even with the old PHI value-numbering we should have arrived at e_3 == e_17 and the very same problem (problem being using the backedge value as "same val" and the entry edge not executable). Value numbering e_17 stmt = e_17 = e_3 + 1; So the workaround I am installing is to mark backedges into unreachable blocks unexecutable as well. Still the reason the def of e_17 is not found unreachable is (again) in sub-optimal visiting order of the DOM walk :(
