From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <gcc-bugs-return-468039-listarch-gcc-bugs=gcc.gnu.org@gcc.gnu.org>
Received: (qmail 31120 invoked by alias); 22 Nov 2014 08:51:00 -0000
Mailing-List: contact gcc-bugs-help@gcc.gnu.org; run by ezmlm
Precedence: bulk
List-Id: <gcc-bugs.gcc.gnu.org>
List-Archive: <http://gcc.gnu.org/ml/gcc-bugs/>
List-Post: <mailto:gcc-bugs@gcc.gnu.org>
List-Help: <mailto:gcc-bugs-help@gcc.gnu.org>
Sender: gcc-bugs-owner@gcc.gnu.org
Received: (qmail 31083 invoked by uid 48); 22 Nov 2014 08:50:56 -0000
From: "rth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug go/64021] New: Empty struct vs libffi
Date: Sat, 22 Nov 2014 08:51:00 -0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: new
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: go
X-Bugzilla-Version: unknown
X-Bugzilla-Keywords:
X-Bugzilla-Severity: normal
X-Bugzilla-Who: rth at gcc dot gnu.org
X-Bugzilla-Status: UNCONFIRMED
X-Bugzilla-Priority: P3
X-Bugzilla-Assigned-To: ian at airs dot com
X-Bugzilla-Target-Milestone: ---
X-Bugzilla-Flags:
X-Bugzilla-Changed-Fields: bug_id short_desc product version bug_status bug_severity priority component assigned_to reporter cc
Message-ID: <bug-64021-4@http.gcc.gnu.org/bugzilla/>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: 7bit
X-Bugzilla-URL: http://gcc.gnu.org/bugzilla/
Auto-Submitted: auto-generated
MIME-Version: 1.0
X-SW-Source: 2014-11/txt/msg02511.txt.bz2

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?