[Bug go/64021] New: Empty struct vs libffi

[rth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64021

            Bug ID: 64021
           Summary: Empty struct vs libffi
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: go
          Assignee: ian at airs dot com
          Reporter: rth at gcc dot gnu.org
                CC: cmang at google dot com

In go_struct_to_ffi, an empty struct is maped to ffi_type_void.

On i386, some ABIs return a struct pop 4 bytes of stack.  If one
calls a function returning a structure without preparing for this,
the stack will be corrupted.

On sparc32, functions returning a structure return to a different
address.  If one calls a function returning a structure without
preparing for this, the instruction after the call will be skipped.

In both cases, this causes reflect/all_test.go returnEmpty to fail.

I wonder what's better: follow C++ and map empty structures to a
single byte (i.e. no more zero-sized structures), or enhance libffi
to be able to deal with empty structures so that we don't have to
lie to libffi about the type being manipulated?