[Bug target/89592] FAIL: tmpdir-g++.dg-struct-layout-1/t025 cp_compat_x_alt.o-cp_compat_y_tst.o execute against GCC4.8

[rguenth at gcc dot gnu.org]

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.