[Bug c++/108586] New: Using std::array instead of a plain array results in different (worse?) code in array zero initialization

[technicallyanonymous at proton dot me via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108586

            Bug ID: 108586
           Summary: Using std::array instead of a plain array results in
                    different (worse?) code in array zero initialization
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: technicallyanonymous at proton dot me
  Target Milestone: ---

This code, compiled with GCC 12.2 (-O2 -std=c++20):

#include <array>

int foo(char *arr);

int square(int num) {
    std::array<char,96> arr = {};
    return foo(arr.data());
}

results in this:

square(int):
        sub     rsp, 104
        xor     eax, eax
        mov     ecx, 12
        mov     rdi, rsp
        rep stosq
        mov     rdi, rsp
        call    foo(char*)
        add     rsp, 104
        ret

However, this very similar code:

int foo(char *arr);

int square(int num) {
    char arr[96] = {};
    return foo(arr);
}

is compiled as:

square(int):
        sub     rsp, 104
        pxor    xmm0, xmm0
        mov     rdi, rsp
        movaps  XMMWORD PTR [rsp], xmm0
        movaps  XMMWORD PTR [rsp+16], xmm0
        movaps  XMMWORD PTR [rsp+32], xmm0
        movaps  XMMWORD PTR [rsp+48], xmm0
        movaps  XMMWORD PTR [rsp+64], xmm0
        movaps  XMMWORD PTR [rsp+80], xmm0
        call    foo(char*)
        add     rsp, 104
        ret

As you can see, SSE instructions are emitted only with a plain array. I also
tested clang and in both cases it emits movaps instructions.