[Bug middle-end/114258] 2 stores happen when copying from a const union (array) to an union

["pinskia at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

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

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2024-03-06
          Component|rtl-optimization            |middle-end
           Severity|normal                      |enhancement
            Summary|repeat store operation when |2 stores happen when
                   |copying a union             |copying from a const union
                   |                            |(array) to an union
     Ever confirmed|0                           |1
           Keywords|                            |missed-optimization

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Reduced testcase:
```
union U6 {
   short  f0;
   int f1;
};
union U6 g_13;
void func_1() {
  static const union U6 b[] = {{.f0 = 2}, {.f0 = 2}};
  g_13 = b[1];
}
```

It comes from the expansion of the assignment.


For aarch64 we have:
```
;; g_13 = b[1];

(insn 5 4 6 (set (reg:DI 101)
        (high:DI (symbol_ref:DI ("*.LANCHOR0") [flags 0x182])))
"/app/example.cpp":9:8 -1
     (nil))

(insn 6 5 7 (set (reg/f:DI 100)
        (lo_sum:DI (reg:DI 101)
            (symbol_ref:DI ("*.LANCHOR0") [flags 0x182])))
"/app/example.cpp":9:8 -1
     (expr_list:REG_EQUAL (symbol_ref:DI ("*.LANCHOR0") [flags 0x182])
        (nil)))

(insn 7 6 8 (set (mem/c:SI (reg/f:DI 100) [1 g_13D.4435+0 S4 A64])
        (const_int 0 [0])) "/app/example.cpp":9:8 -1
     (nil))

(insn 8 7 9 (set (reg:HI 102)
        (const_int 2 [0x2])) "/app/example.cpp":9:8 -1
     (nil))

(insn 9 8 10 (set (mem/c:HI (reg/f:DI 100) [2 g_13D.4435+0 S2 A64])
        (reg:HI 102)) "/app/example.cpp":9:8 -1
     (nil))

(insn 10 9 11 (set (mem/c:SI (reg/f:DI 100) [1 g_13D.4435+0 S4 A64])
        (const_int 0 [0])) "/app/example.cpp":9:8 -1
     (nil))

(insn 11 10 12 (set (reg:HI 103)
        (const_int 2 [0x2])) "/app/example.cpp":9:8 -1
     (nil))

(insn 12 11 0 (set (mem/c:HI (reg/f:DI 100) [2 g_13D.4435+0 S2 A64])
        (reg:HI 103)) "/app/example.cpp":9:8 -1
     (nil))
```

GCC could do better here in the expansion.