[Bug c/40366] New: Array + XOR swap fails

[Bug c/40366] New: Array + XOR swap fails

[ashutosh dot sharma dot 0204 at gmail dot com]

Compiler Version:
gcc (GCC) 4.2.4 (Ubuntu 4.2.4-1ubuntu3)
Copyright (C) 2007 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.


Sample code:

#include <stdio.h>

int main() {
        int a[2];
        a[0]=10;
        a[1]=20;
        printf("a[0] = %d a[1] = %d\n", a[0], a[1]);
        a[0]^=a[1]^=a[0]^=a[1];
        printf("a[0] = %d a[1] = %d\n", a[0], a[1]);
        return 0;
}

gcc test.c -o test -O2

a[0] = 10 a[1] = 20
a[0] = 20 a[1] = 10

gcc test.c -o test -O0

a[0] = 10 a[1] = 20
a[0] = 0 a[1] = 10

Bug: When compiled with -O0 swap fails.
Swap only fails in case of array. Following code works fine with -O0:

#include <stdio.h>

int main() {
        int a, b;
        a=10;
        b=20;
        printf("a = %d b = %d\n", a, b);
        a^=b^=a^=b;
        printf("a = %d b = %d\n", a, b);
        return 0;
}


-- 
           Summary: Array + XOR swap fails
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: major
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: ashutosh dot sharma dot 0204 at gmail dot com


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40366

[Bug c/40366] Array + XOR swap fails

[pinskia at gmail dot com]

------- Comment #1 from pinskia at gmail dot com  2009-06-07 07:23 -------
Subject: Re:   New: Array + XOR swap fails

Both of these are undefined. Look up about sequence point rules.

Sent from my iPhone

On Jun 7, 2009, at 12:15 AM, "ashutosh dot sharma dot 0204 at gmail  
dot com" <gcc-bugzilla@gcc.gnu.org> wrote:

> Compiler Version:
> gcc (GCC) 4.2.4 (Ubuntu 4.2.4-1ubuntu3)
> Copyright (C) 2007 Free Software Foundation, Inc.
> This is free software; see the source for copying conditions.  There  
> is NO
> warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR  
> PURPOSE.
>
>
> Sample code:
>
> #include <stdio.h>
>
> int main() {
>        int a[2];
>        a[0]=10;
>        a[1]=20;
>        printf("a[0] = %d a[1] = %d\n", a[0], a[1]);
>        a[0]^=a[1]^=a[0]^=a[1];
>        printf("a[0] = %d a[1] = %d\n", a[0], a[1]);
>        return 0;
> }
>
> gcc test.c -o test -O2
>
> a[0] = 10 a[1] = 20
> a[0] = 20 a[1] = 10
>
> gcc test.c -o test -O0
>
> a[0] = 10 a[1] = 20
> a[0] = 0 a[1] = 10
>
> Bug: When compiled with -O0 swap fails.
> Swap only fails in case of array. Following code works fine with -O0:
>
> #include <stdio.h>
>
> int main() {
>        int a, b;
>        a=10;
>        b=20;
>        printf("a = %d b = %d\n", a, b);
>        a^=b^=a^=b;
>        printf("a = %d b = %d\n", a, b);
>        return 0;
> }
>
>
> -- 
>           Summary: Array + XOR swap fails
>           Product: gcc
>           Version: unknown
>            Status: UNCONFIRMED
>          Severity: major
>          Priority: P3
>         Component: c
>        AssignedTo: unassigned at gcc dot gnu dot org
>        ReportedBy: ashutosh dot sharma dot 0204 at gmail dot com
>
>
> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40366
>


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40366

Re: [Bug c/40366] New: Array + XOR swap fails

[Andrew Thomas Pinski]

Both of these are undefined. Look up about sequence point rules.

Sent from my iPhone

On Jun 7, 2009, at 12:15 AM, "ashutosh dot sharma dot 0204 at gmail  
dot com" <gcc-bugzilla@gcc.gnu.org> wrote:

> Compiler Version:
> gcc (GCC) 4.2.4 (Ubuntu 4.2.4-1ubuntu3)
> Copyright (C) 2007 Free Software Foundation, Inc.
> This is free software; see the source for copying conditions.  There  
> is NO
> warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR  
> PURPOSE.
>
>
> Sample code:
>
> #include <stdio.h>
>
> int main() {
>        int a[2];
>        a[0]=10;
>        a[1]=20;
>        printf("a[0] = %d a[1] = %d\n", a[0], a[1]);
>        a[0]^=a[1]^=a[0]^=a[1];
>        printf("a[0] = %d a[1] = %d\n", a[0], a[1]);
>        return 0;
> }
>
> gcc test.c -o test -O2
>
> a[0] = 10 a[1] = 20
> a[0] = 20 a[1] = 10
>
> gcc test.c -o test -O0
>
> a[0] = 10 a[1] = 20
> a[0] = 0 a[1] = 10
>
> Bug: When compiled with -O0 swap fails.
> Swap only fails in case of array. Following code works fine with -O0:
>
> #include <stdio.h>
>
> int main() {
>        int a, b;
>        a=10;
>        b=20;
>        printf("a = %d b = %d\n", a, b);
>        a^=b^=a^=b;
>        printf("a = %d b = %d\n", a, b);
>        return 0;
> }
>
>
> -- 
>           Summary: Array + XOR swap fails
>           Product: gcc
>           Version: unknown
>            Status: UNCONFIRMED
>          Severity: major
>          Priority: P3
>         Component: c
>        AssignedTo: unassigned at gcc dot gnu dot org
>        ReportedBy: ashutosh dot sharma dot 0204 at gmail dot com
>
>
> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40366
>

[Bug c/40366] Array + XOR swap fails

[schwab at linux-m68k dot org]

------- Comment #2 from schwab at linux-m68k dot org  2009-06-07 09:04 -------


*** This bug has been marked as a duplicate of 11751 ***


-- 

schwab at linux-m68k dot org changed:

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


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40366