https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110381
--- Comment #8 from Richard Biener <rguenth at gcc dot gnu.org> --- OK, so we transform the in-order FOLD_LEFT_REDUCTION as # sum_22 = PHI <sum_15(5), 0.0(2)> # vectp_foos.7_25 = PHI <vectp_foos.7_23(5), foos_12(D)(2)> ... vect_foo_c_8.9_21 = MEM <vector(2) double> [(double *)vectp_foos.7_25]; vectp_foos.7_20 = vectp_foos.7_25 + 16; vect_foo_c_8.10_7 = MEM <vector(2) double> [(double *)vectp_foos.7_20]; vectp_foos.7_6 = vectp_foos.7_25 + 32; vect_foo_c_8.11_5 = MEM <vector(2) double> [(double *)vectp_foos.7_6]; stmp_sum_13.12_4 = BIT_FIELD_REF <vect_foo_c_8.9_21, 64, 0>; stmp_sum_13.12_31 = sum_22 + stmp_sum_13.12_4; stmp_sum_13.12_32 = BIT_FIELD_REF <vect_foo_c_8.9_21, 64, 64>; stmp_sum_13.12_33 = stmp_sum_13.12_31 + stmp_sum_13.12_32; stmp_sum_13.12_34 = BIT_FIELD_REF <vect_foo_c_8.10_7, 64, 0>; stmp_sum_13.12_35 = stmp_sum_13.12_33 + stmp_sum_13.12_34; stmp_sum_13.12_36 = BIT_FIELD_REF <vect_foo_c_8.10_7, 64, 64>; stmp_sum_13.12_37 = stmp_sum_13.12_35 + stmp_sum_13.12_36; stmp_sum_13.12_38 = BIT_FIELD_REF <vect_foo_c_8.11_5, 64, 0>; stmp_sum_13.12_39 = stmp_sum_13.12_37 + stmp_sum_13.12_38; stmp_sum_13.12_40 = BIT_FIELD_REF <vect_foo_c_8.11_5, 64, 64>; foo$a_11 = _3->a; foo$b_9 = _3->b; foo$c_8 = _3->c; sum_13 = stmp_sum_13.12_39 + stmp_sum_13.12_40; sum_14 = foo$b_9 + sum_13; sum_15 = foo$a_11 + sum_14; where you can see the final step updates one but not the last scalar stmt in the SLP reduction group. That causes us to keep the scalar reads live and apply some elements multiple times.
