http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60510
--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> --- The issue is t.f:7:0: note: Analyze phi: prephitmp_32 = PHI <pretmp_31(4), _18(7)> t.f:7:0: note: swapping oprnds: _18 = _13 + _16; t.f:7:0: note: reduction: detected reduction chain: _18 = _16 + _13; t.f:7:0: note: Detected reduction. in the non-working vs. t.f:7:0: note: Analyze phi: prephitmp_32 = PHI <pretmp_31(4), _18(7)> t.f:7:0: note: detected reduction: _18 = _17 + prephitmp_32; t.f:7:0: note: Detected reduction. in the working case (thus in the non-working case we detect a SLP reduction which this really isn't - which means we fail to detect it as regular reduction, I think because of association issues). Working: <bb 5>: # i_1 = PHI <1(4), i_20(7)> # prephitmp_32 = PHI <pretmp_31(4), _18(7)> ... _18 = _17 + prephitmp_32; non-working: # prephitmp_32 = PHI <pretmp_31(4), _18(7)> ... _13 = _12 + prephitmp_32; _18 = _13 + _16; there is tree reassoc code to make sure the reduction is properly associated but it doesn't seem to trigger here? See swap_ops_for_binary_stmt and the comment before it.
