https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89592
--- Comment #6 from Richard Biener <rguenth at gcc dot gnu.org> ---
I think it only makes sense to pass aggregates with a flexarray member by
reference and pass the return slot by reference as well. Only the caller
can know the complete type used here. This means passing by value needs
to do a copy at the caller side.
OTOH
struct S { int i; char c[]; };
struct S foo (struct S val) { return val; }
is compiled to
foo:
.LFB0:
.cfi_startproc
movl %edi, %eax
ret
and the C FE warns:
t.c: In function ‘foo’:
t.c:2:22: note: the ABI of passing struct with a flexible array member has
changed in GCC 4.4
struct S foo (struct S val) { return val; }
before that ABI change we had
foo:
.LFB2:
movl 8(%rsp), %eax
movl %eax, (%rdi)
movq %rdi, %rax
ret
so it was passed on the stack and there was a return slot? The C++ compiler
still produces the above with GCC 8, even when marking foo extern "C",
so that's another inconsistency / bug.
