From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <gcc-bugs-return-403493-listarch-gcc-bugs=gcc.gnu.org@gcc.gnu.org>
Received: (qmail 12912 invoked by alias); 12 Oct 2012 10:11:20 -0000
Received: (qmail 12821 invoked by uid 48); 12 Oct 2012 10:10:54 -0000
From: "yangzhe1990 at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/54907] New: post increasing a value pointed by p in subexpression of an expression modifying p saves the increased value in the wrong place
Date: Fri, 12 Oct 2012 10:11:00 -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: normal
X-Bugzilla-Who: yangzhe1990 at gmail dot com
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-54907-4@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: 2012-10/txt/msg01169.txt.bz2


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54907

             Bug #: 54907
           Summary: post increasing a value pointed by p in subexpression
                    of an expression modifying p saves the increased value
                    in the wrong place
    Classification: Unclassified
           Product: gcc
           Version: 4.7.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: yangzhe1990@gmail.com


#include <stdio.h>

int main() {
    char s[] = "axxxxx";
    char *p = s;

    printf("s = %s in the beginning.\n"
           "p is pointed at the %d-th char.\n", s, p - s);
    //p = p + (*p)++ * 3 + 2 - 'a' * 3; // (1)
    p += (*p)++ * 3 + 2 - 'a' * 3; // (2)
    printf("p is moved ahead by %d steps\n", p - s);
    printf("s = %s after the operation.\n", s);
    return 0;
}

The expected result is "bxxxxx". But the output is "axbxxx".

Maybe in the wrong code, when it saves the value, it lookups the address again
by *p, but p is modified in the expression.

As discussed in stackoverflow,
http://stackoverflow.com/questions/12823663/would-p-p-p-3-c-cause-an-undefined-behavior?answertab=votes#tab-top
most people think it's a bug of gcc.

Bug found in gcc 4.4.6, 4.7.1, g++ 4.4.6. g++ 4.7.1 produces the correct
result.