From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <gcc-bugs-return-381512-listarch-gcc-bugs=gcc.gnu.org@gcc.gnu.org>
Received: (qmail 8830 invoked by alias); 27 Jan 2012 18:05:40 -0000
Received: (qmail 8819 invoked by uid 22791); 27 Jan 2012 18:05:39 -0000
X-SWARE-Spam-Status: No, hits=-2.9 required=5.0	tests=ALL_TRUSTED,AWL,BAYES_00
X-Spam-Check-By: sourceware.org
Received: from localhost (HELO gcc.gnu.org) (127.0.0.1)    by sourceware.org (qpsmtpd/0.43rc1) with ESMTP; Fri, 27 Jan 2012 18:05:26 +0000
From: "eggert at gnu dot org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/52023] New: _Alignof (double) yields wrong value on x86
Date: Fri, 27 Jan 2012 20:06:00 -0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: new
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: c
X-Bugzilla-Keywords:
X-Bugzilla-Severity: major
X-Bugzilla-Who: eggert at gnu dot org
X-Bugzilla-Status: UNCONFIRMED
X-Bugzilla-Priority: P3
X-Bugzilla-Assigned-To: unassigned at gcc dot gnu.org
X-Bugzilla-Target-Milestone: ---
X-Bugzilla-Changed-Fields:
Message-ID: <bug-52023-4@http.gcc.gnu.org/bugzilla/>
X-Bugzilla-URL: http://gcc.gnu.org/bugzilla/
Auto-Submitted: auto-generated
Content-Type: text/plain; charset="UTF-8"
MIME-Version: 1.0
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
X-SW-Source: 2012-01/txt/msg03187.txt.bz2

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52023

             Bug #: 52023
           Summary: _Alignof (double) yields wrong value on x86
    Classification: Unclassified
           Product: gcc
           Version: 4.7.0
            Status: UNCONFIRMED
          Severity: major
          Priority: P3
         Component: c
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: eggert@gnu.org
              Host: x86-64
            Target: x86
             Build: x86-64


Created attachment 26482
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=26482
gcc -v output

On x86, alignof (double) should be 4, since that's the alignment
requirement used for 'double' when laying out structures.  But with
GCC alignof incorrectly returns 8.  This causes some gnulib tests to fail
and (more importantly) it may cause problems with programs such as
conservative garbage collectors that use alignof to determine which
addresses might be of valid objects.

The problem also occurs with other fundamental types, such as 'long long'.

Here's a simple test case:

  #include <stddef.h>
  #include <stdalign.h>
  int a = alignof (double);
  int b = offsetof (struct { char c; double d; }, d);
  int main (void) { return a == b; }

Here's the preprocessed output:

  int a = _Alignof (double);
  int b = __builtin_offsetof (struct { char c; double d; }, d);
  int main (void) { return a == b; }

'main' should return 1, but it returns 0 due to the bug, which causes
'a' to be 8 instead of 4 as it should be.

I'm using GCC on the trunk (r183603), built on Fedora 16 x86-64 with
no special flags, and I'm compiling with 'gcc -v -m32 test.c' and
running with './a.out'.  I'm attaching the output of 'gcc -v'.