[Bug rtl-optimization/114258] New: repeat store operation when copying a union

["absoler at smail dot nju.edu.cn" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114258

            Bug ID: 114258
           Summary: repeat store operation when copying a union
           Product: gcc
           Version: 13.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: absoler at smail dot nju.edu.cn
  Target Milestone: ---

Hi, here's the code and generated binary compiled with gcc-13.2.0 -O2:

```
// https://godbolt.org/z/jsMhcbKnh

union U6 {
   int  f0;
   long long  f1;
   int  f2;
};

union U8 {
   int  f0;
   const long long  f1;
   const long long  f2;
};
union U6 g_13 = {.f0 = 0xE13F5E3DL};
union U8 g_189 = {.f0 = 0x959411BBL};

int d = 0;
int a;
void func_1() {
  const union U6 b[] = {{.f0 = 2}, {.f0 = 2}};
  for (; d;)
    a = 0;
  g_189.f0 &= (g_13 = b[1], 5);
}
```

```
func_1:
 mov    0x2f02(%rip),%eax        # 404028 <d>
 test   %eax,%eax
 je     401130 <func_1+0x10>
 jmp    40112a <func_1+0xa>
 nopl   0x0(%rax)
 movq   $0x0,0x2edd(%rip)        # 404018 <g_13>
 andl   $0x5,0x2ece(%rip)        # 404010 <g_189>
 movl   $0x2,0x2ecc(%rip)        # 404018 <g_13>
 ret
```

we can see that the first write to `g_13` could be removed to improve
performance.