[Bug c++/85720] New: bad codegen for looped assignment of primitives at -O2

[redbeard0531 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85720

            Bug ID: 85720
           Summary: bad codegen for looped assignment of primitives at -O2
           Product: gcc
           Version: 8.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redbeard0531 at gmail dot com
  Target Milestone: ---

https://godbolt.org/g/qp19Cv

using SIZE_T = decltype(sizeof(0));
void fill(char* p, SIZE_T n) {
    for (SIZE_T i = 0; i < n; i++){
        p[i] = -1;
    }
}

fill(char*, unsigned long):
        test    rsi, rsi
        je      .L1
        add     rsi, rdi
.L3:
        mov     BYTE PTR [rdi], -1
        add     rdi, 1
        cmp     rdi, rsi
        jne     .L3
.L1:
        ret

At -O3 it basically just tail-calls memset.

Also applies to other types than char, but char is most egregious.

This ticket is spun out of from
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85707