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

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

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

--- Comment #2 from Richard Biener  ---
Huh, very strange RTL we emit for the union assignment.

void func_1(union U6 *a) {
  g_13 = *a;
}

works OK though.

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

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

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

Andrew Pinski  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  ---
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.