[Bug tree-optimization/122261] New: Missed vectorisation with wrapper struct

[sjames at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=122261

            Bug ID: 122261
           Summary: Missed vectorisation with wrapper struct
           Product: gcc
           Version: 16.0
            Status: UNCONFIRMED
          Keywords: alias, missed-optimization
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: sjames at gcc dot gnu.org
  Target Milestone: ---

This comes from https://github.com/llvm/llvm-project/issues/54646:

```
#ifdef FOO

struct S { long int a; long int b; };
void f(S& s1, const S& s2 ) {
    s1.a = s2.a;
    s1.b = s2.b;
}

#else

struct Wrapper { long int t; };
struct S1 { Wrapper a; Wrapper b; };

void f(S1& s1, const S1& s2 ) {
    s1.a = s2.a;
    s1.b = s2.b;
}

#endif
```

At -O2 on trunk (with -UFOO; -DFOO is fine), we get:
```
"f(S1&, S1 const&)":
        mov     rax, QWORD PTR [rsi]
        mov     QWORD PTR [rdi], rax
        mov     rax, QWORD PTR [rsi+8]
        mov     QWORD PTR [rdi+8], rax
        ret
```

Clang with -O2 -Xclang -new-struct-path-tbaa gives:
```
f(S1&, S1 const&):
        movups  xmm0, xmmword ptr [rsi]
        movups  xmmword ptr [rdi], xmm0
        ret
```