Clang compiles this C function:

struct foo {
    char member[];
};

long test(struct foo zst, long t, long u, long v, long w, long x, long y,
          long z) {
    return z;
}

To this assembly code on x86-64 Linux (Godbolt):

test:
        mov     rax, qword ptr [rsp + 16]
        ret

In the LLVM IR, struct foo zst is passed by pointer, despite being zero-sized.

In contrast, GCC emits:

test:
        mov     rax, qword ptr [rsp + 8]
        ret

GCC's assembly for this function matches what both GCC and clang emit for this:

struct foo {
    char member[0];
};

long test(struct foo zst, long t, long u, long v, long w, long x, long y,
          long z) {
    return z;
}

The same issue exists for unions.

This is inconvenient for Rust, where it is common practice to map both C flexible array members and C zero-length array members to the same Rust construct (a zero-length Rust array).