https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66610
--- Comment #2 from David Malcolm <dmalcolm at gcc dot gnu.org> --- Created attachment 35820 --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=35820&action=edit A less minimal example FWIW, here's a less minimal example, better showing what the code was doing (interpreter values were a union plus a type code). As in the previous example, fre2 is able to do this optimization of the copy of arr[1] to arr[0] when doing the copy field-wise: Replaced arr_6(D)->union_field.int_field with i_1 in all uses of _10 = arr_6(D)->union_field.int_field; Replaced arr_6(D)->type_code with 0 in all uses of _13 = arr_6(D)->type_code; turning the loop body into: <bb 3>: arr_6(D)->union_field.int_field = i_1; arr_6(D)->type_code = 0; MEM[(struct value *)arr_6(D) + 16B].union_field.int_field = i_1; MEM[(struct value *)arr_6(D) + 16B].type_code = 0; i_15 = i_1 + 1; allowing the loop to later be optimized away by cddce2.
