From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <gcc-bugs-return-247557-listarch-gcc-bugs=gcc.gnu.org@gcc.gnu.org>
Received: (qmail 12115 invoked by alias); 14 Mar 2008 20:11:52 -0000
Received: (qmail 11998 invoked by uid 48); 14 Mar 2008 20:11:09 -0000
Date: Fri, 14 Mar 2008 20:11:00 -0000
Subject: [Bug c++/35593]  New: spurious warning "array subscript is below array bounds" with void* function argument plus -O2
X-Bugzilla-Reason: CC
Message-ID: <bug-35593-1937@http.gcc.gnu.org/bugzilla/>
Reply-To: gcc-bugzilla@gcc.gnu.org
To: gcc-bugs@gcc.gnu.org
From: "niemayer at isg dot de" <gcc-bugzilla@gcc.gnu.org>
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: 2008-03/txt/msg01183.txt.bz2

When compiling the following tiny example .cxx source with "-Wall -O2", I get
spurious warnings:

------------- cut example test.cxx here --------------
extern void function(void * x);

struct A {
        long x;
        char d[0];
};


void test(A * a) {
        function((char *)a - 4); // gcc emits warning, here - why?
}

// ------------

struct B {
        char d[0];
};

void test(B * b) {
        char * c = (char *)b;
        void * v = c-1;
        function(v);  // gcc emits warning, here - why?
}
------------- cut here --------------

The output of
> gcc -Wall -c test.cxx -O2
is:
/tmp/test.cxx: In function &#8216;void test(B*)&#8217;:
/tmp/test.cxx:22: warning: array subscript is below array bounds
/tmp/test.cxx: In function &#8216;void test(A*)&#8217;:
/tmp/test.cxx:10: warning: array subscript is below array bounds

I have not found a sane way in my non-example, real-world code to avoid those
warnings.


-- 
           Summary: spurious warning "array subscript is below array bounds"
                    with void* function argument plus -O2
           Product: gcc
           Version: 4.3.0
            Status: UNCONFIRMED
          Severity: minor
          Priority: P3
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: niemayer at isg dot de
 GCC build triplet: i686-pc-linux-gnu
  GCC host triplet: i686-pc-linux-gnu
GCC target triplet: i686-pc-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=35593