[Bug c++/27257] New: Error integer compare in g++ 4.1.0

[Bug c++/27257] New: Error integer compare in g++ 4.1.0

[johnzhang at tencent dot com]

0X80000000 - 10 < 0 and 10 - 0X80000000 < 0
gcc version 3.3.4 gets a right result of comparing between 10 and 0X80000000,
but gcc version 4.1.0 fails.

the test code followed:

#include <stdio.h>
int main()
{
        int b = 0X80000000;
        if ((10 -  b) < 0)
        {
                printf("(10 - b) = %d < 0 is True\n", 10 - b);
        }
        else
        {
                printf("(10 - b) = %d < 0 is False\n", 10 - b);
        }
        if ((b - 10) < 0)
        {
                printf("(b - 10) = %d < 0 is True\n", b - 10);
        }
        else
        {
                printf("(b - 10) = %d < 0 is False\n", b - 10);
        }

        return 0;
}

I had tested the code and got the result:
bingo@sttest:~/tmp> ../bin/g++ t.cpp
bingo@sttest:~/tmp> a.out
(10 - b) = -2147483638 < 0 is True
(b - 10) = 2147483638 < 0 is True


-- 
           Summary: Error integer compare in g++ 4.1.0
           Product: gcc
           Version: 4.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: johnzhang at tencent dot com


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=27257

[Bug c++/27257] Error integer compare in g++ 4.1.0

[pluto at agmk dot net]

------- Comment #1 from pluto at agmk dot net  2006-04-22 08:23 -------
(In reply to comment #0)

>         int b = 0X80000000;

>         if ((b - 10) < 0)

b - 10 is 0xff(...)7ffffff6 and this is an integer overflow.

[ cite: c++ standard / $5.5 ]

if during the evaluation of an expression the result isn't mathematically
defined nor in the range of representable values for its type the behaviuor
is undefined, unless such an expression is a constant expression, in which
case the program is ill-formed. (...)

[ /cite ]

so, use -fwrapv (man g++) to get not quite correct behaviour.


-- 

pluto at agmk dot net changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |pluto at agmk dot net


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=27257

[Bug c++/27257] Error integer compare in g++ 4.1.0

[pinskia at gcc dot gnu dot org]

------- Comment #2 from pinskia at gcc dot gnu dot org  2006-04-22 15:35 -------
This is signed int overflow being undefined, in fact I should I know because I
was the one who write the patch to optimize this.


-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |INVALID


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=27257

[Bug c++/27257] Error integer compare in g++ 4.1.0

[johnzhang at tencent dot com]

------- Comment #3 from johnzhang at tencent dot com  2006-04-28 06:36 -------
Ok, you are right. it would be nice if g++ 4.1.0 acts as what g++ 3.3.4 does.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=27257

[Bug c++/27257] Error integer compare in g++ 4.1.0

[pinskia at gcc dot gnu dot org]

------- Comment #4 from pinskia at gcc dot gnu dot org  2006-04-28 16:29 -------
(In reply to comment #3)
> Ok, you are right. it would be nice if g++ 4.1.0 acts as what g++ 3.3.4 does.

Use -fwrapv if you want defined wrapping.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=27257

[Bug c++/27257] Error integer compare in g++ 4.1.0

[pinskia at gcc dot gnu dot org]

------- Comment #5 from pinskia at gcc dot gnu dot org  2007-01-15 20:04 -------
*** Bug 30475 has been marked as a duplicate of this bug. ***


-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |felix-gcc at fefe dot de


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=27257