[Bug libstdc++/124121] Missed optimization in initialization of std::inplace_vector with std::initializer_list

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=124121

--- Comment #23 from Tomasz Kamiński <tkaminsk at gcc dot gnu.org> ---
Part of it seem to be related, by the fact that runtime path does not zero all
the elements of the underlying array, i.e. _M_elems[4] in this case. That's
prevents `inplace_vector<char, 4>` from being constant expressions (until:
P3726R1)

Modifying the example to initialize all elements (see
https://compiler-explorer.com/z/jYcb3rxzP):
#include <inplace_vector>
std::inplace_vector<char, 4> f() {
  return {'a', 'b', 'c', 'd'};
}
std::inplace_vector<char, 4> g() {
  constexpr std::inplace_vector<char, 4> result{'a', 'b', 'c', 'd'};
  return result;
}

Gives for GCC:
"f()":
        lea     rax, [rsp-10]
        mov     DWORD PTR [rsp-10], 0
        mov     BYTE PTR [rsp-6], 0
        mov     eax, DWORD PTR "C.0.0"[rip]
        bts     rax, 34
        ret
"g()":
        movabs  rax, 18864104033
        ret
"C.0.0":
        .byte   97
        .byte   98
        .byte   99
        .byte   100

And clang:
f():
        mov     byte ptr [rsp - 12], 0
        lea     rax, [rsp - 16]
        mov     qword ptr [rsp - 8], rax
        mov     dword ptr [rsp - 16], 1684234849
        mov     byte ptr [rsp - 12], 4
        movabs  rax, 18864104033
        ret

g():
        movabs  rax, 18864104033
        ret