(fwd) Bug in gcc4 initialisers suspected

[Marcel van Kervinck <marcelk at unox dot hak dot nl>]

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Sender: Marcel van Kervinck <marcelk@unox.bitpit.net>
From: Marcel van Kervinck <marcelk@unox.hak.nl>
Subject: Bug in gcc4 initialisers suspected
Newsgroups: comp.std.c,comp.lang.c
Cc: gcc-bugs@gcc.gnu.org
Organization: bitpit
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Date: 17 May 2005 15:37:00 GMT
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Dear all,

I would like to confirm my suspicion of a compiler
bug in gcc4 for MacOSX. The code example below expects
that the initializer of 'v' sets all elements in
v.u.bytes[] to zero, as specified by the C99 standard:

"""21 If there are fewer initializers in a brace-enclosed
      list than there are elements or members of an
      aggregate, [...] the remainder of the aggregate shall
      be initialized implicitly the same as objects that
      have static storage duration."""
      (ref: ANSI+ISO+IEC+8999-1999, 6.7.8 Initialization, p.127)

However, the code only works as expected on some platforms:

system        cpu             compiler    result
------        ---             --------    ------
SunOS-5.8     UltraSPARC-IIe  gcc-2.95.3  returns OK
FreeBSD-4.11  Pentium2        gcc-2.95.4  returns OK
SunOS-5.8     UltraSPARC-IIi  gcc-3.1.1   returns OK
Darwin-7.9.0  PowerPC-G4      gcc-3.3     returns OK
Linux-2.4.29  Celeron         gcc-3.3.5   returns OK
FreeBSD-5.4   Pentium4        gcc-3.4.2   returns OK
Darwin-8.0.0  PowerPC-G4      gcc-4.0.0   failed assertion `v.u.bytes[1] == 0'
Darwin-8.0.0  PowerPC-G5      gcc-4.0.0   failed assertion `v.u.bytes[1] == 0'

Interestingly, removing the intermediate
level 'union u' makes the code pass.

The questions I'm seeking an answer to:
1. Is this a compiler bug or a display of undefined behaviour?
2. Is this gcc4-specific or only in combination with PowerPC?

Best regards,

        Marcel
--  _   _
  _| |_|_|
 |_    |_     Marcel van Kervinck
         |_|  marcelk@bitpit.net

----------------------------------------------------------------
#include <assert.h>

typedef struct {
        union {
                unsigned char bytes[2];
        } u;
} vector_t;

int main(void)
{
        int i;
        for (i=0; i<2; i++) {
                vector_t v = {{{0,}}};
                assert(v.u.bytes[1] == 0);
                v.u.bytes[1]++;
        }
}
----------------------------------------------------------------
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