https://gcc.gnu.org/bugzilla/show_bug.cgi?id=122969
--- Comment #4 from Tamar Christina <tnfchris at gcc dot gnu.org> --- All the no-options result in duplicate PHI nodes that use the same values just have different RHS >>> p debug (loop->header) <bb 21> [local count: 958878292]: # a.2_29 = PHI <0(18), _34(25)> # a_lsm.8_31 = PHI <0(18), _34(25)> _34 = a.2_29 + -1; if (_34 != 1) goto <bb 25>; [94.50%] else goto <bb 32>; [5.50%] $11 = void >>> p debug (update_bb) <bb 33> [local count: 958878292]: # a.2_6 = PHI <a_lsm.8_36(32), _41(34)> # a_lsm.8_18 = PHI <a_lsm.8_36(32), _41(34)> _41 = a.2_6 + -1; if (_41 != 1) goto <bb 34>; [94.50%] else goto <bb 26>; [5.50%] $12 = void So one of the PHIs is completely useless. But that makes the update loop visit the PHI twice, but the first time we already updated them so we end up with a non SSA_NAME there. In the old code we'd just re-update the PHI with the same statement. So I'll put back that code as fallback. testing a patch.
