copied from: http://gcc.gnu.org/ml/gcc-patches/2006-02/msg00812.html
Hi Paolo, with your patch: http://gcc.gnu.org/ml/gcc-patches/2005-11/msg01718.html simplify_plus_minus isn't able anymore to simplify things like (r1 + 8) - r1. The function is left at: if (!canonicalized) { int n_constants = 0; for (i = 0; i < n_ops; i++) if (GET_CODE (ops[i].op) == CONST_INT) n_constants++; if (n_constants <= 1) return NULL_RTX; } because only a single constant was found and no canonicalization took place. Your patch changed: if (n_ops <= 2 && !force) return NULL_RTX; to: gcc_assert (n_ops >= 2); if (!canonicalized) return NULL_RTX; Which was later on changed by Alan Modra with: http://gcc.gnu.org/ml/gcc-patches/2005-12/msg00015.html to the version mentioned above. The reason here was that expressions like reg + c1 + c2 were not simplified anymore. So the new version exits always if the initial loop couldn't do any canonicalization work and only 0 or 1 constants are left. What prevents simplification in all those cases. ... As Roger Sayle pointed out the patch I've proposed in the email isn't the proper fix for this issue. Paolo Bonzini already proposed a solution in: http://gcc.gnu.org/ml/gcc-patches/2006-02/msg00884.html Thats why I'll assign the bug to him. -- Summary: Missed optimization in simplify_plus_minus - regression against 4.1.0 Product: gcc Version: 4.2.0 Status: UNCONFIRMED Keywords: missed-optimization Severity: normal Priority: P3 Component: rtl-optimization AssignedTo: bonzini at gnu dot org ReportedBy: krebbel at gcc dot gnu dot org GCC build triplet: s390x-ibm-linux-gnu GCC host triplet: s390x-ibm-linux-gnu GCC target triplet: s390x-ibm-linux-gnu http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26847
