[Bug ipa/118907] New: ICF optimises bit selection differently based on declaration order

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=118907

            Bug ID: 118907
           Summary: ICF optimises bit selection differently based on
                    declaration order
           Product: gcc
           Version: 15.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: ipa
          Assignee: unassigned at gcc dot gnu.org
          Reporter: sjames at gcc dot gnu.org
                CC: arsen at gcc dot gnu.org
  Target Milestone: ---

This comes from
https://old.reddit.com/r/cpp_questions/comments/1ilww3q/gcc_bit_manipulation_code_generation_seems_buggy/.

godbolt: https://godbolt.org/z/GnbKzd33s

```
#include <cstdint>
#include <bit>

using namespace std;

using Bitboard = uint64_t;

template <typename ...Squares>
Bitboard setSquare(Bitboard bitboard, Squares... squares) { 

    return bitboard | ((1ull << squares) | ...);
}

Bitboard setSquare1(Bitboard bitboard, uint32_t a, uint32_t b, uint32_t c,
uint32_t d) {
    return setSquare(bitboard, a, b, c, d);
}

Bitboard setSquare2(Bitboard bitboard, uint32_t a, uint32_t b, uint32_t c,
uint32_t d) {
    bitboard |= 1ull << a;
    bitboard |= 1ull << b;
    bitboard |= 1ull << c;
    bitboard |= 1ull << d;
    return bitboard;
}

Bitboard setSquare3(Bitboard bitboard, uint32_t a, uint32_t b, uint32_t c,
uint32_t d) {
    return bitboard | ((1ULL << a) | (1ULL << b) | (1ULL << c) | (1ULL << d));
}

Bitboard setSquare4(Bitboard bitboard, uint32_t a, uint32_t b, uint32_t c,
uint32_t d) {
    return bitboard | (1ULL << a) | (1ULL << b) | (1ULL << c) | (1ULL << d);
}

Bitboard setSquare5(Bitboard bitboard, uint32_t a, uint32_t b, uint32_t c,
uint32_t d) {
    Bitboard result = bitboard | ((1ULL << a) | (1ULL << b) | (1ULL << c) |
(1ULL << d));
    return result;
}

Bitboard setSquare6(Bitboard bitboard, uint32_t a, uint32_t b, uint32_t c,
uint32_t d) {
    Bitboard result =  bitboard | (1ULL << a) | (1ULL << b) | (1ULL << c) |
(1ULL << d);
    return result;
}
```

With `-std=c++20 -O2`, ICF makes a mess in that `setSquare3` and `setSquare4`
get optimised differently.