[Bug c++/120689] New: Codegen optimization regression passing struct in register in gcc 10+ on x86-64.

[amohr at amohr dot org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=120689

            Bug ID: 120689
           Summary: Codegen optimization regression passing struct in
                    register in gcc 10+ on x86-64.
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: amohr at amohr dot org
  Target Milestone: ---

struct s { char a, b, c; };

void t2(s);

void t1(s x) {
    t2(x);
}

GCC 9.5 on x86-64 at -O3 generates:

t1(s):
        jmp     t2(s)

GCC 10.1 (and newer) at -O3 generates:

t1(s):
        mov     edx, edi
        mov     eax, edi
        movzx   edi, dil
        shr     eax, 16
        and     edx, 65280
        or      rdx, rdi
        movzx   edi, al
        sal     rdi, 16
        or      rdi, rdx
        jmp     t2(s)

I tested all newer major versions thru 15 & trunk, and they do similarly.