[Bug c/26824] New: optimisation bug with -0x80000000

[Bug c/26824] New: optimisation bug with -0x80000000

[simonmar at microsoft dot com]

The following code is compiled incorrectly with -O2:

#include <stdlib.h>

void f(int x) {
    if (x < 0) {
        if (-x > 0) {
            exit(1);
        }
    }
}

int main() {
        f(-0x80000000);
        exit(0);
}

$ gcc foo.c; ./a.out         
$ gcc -O2 foo.c; ./a.out 
zsh: 4407 exit 1     ./a.out
$


-- 
           Summary: optimisation bug with -0x80000000
           Product: gcc
           Version: 4.1.0
            Status: UNCONFIRMED
          Severity: major
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: simonmar at microsoft dot com
 GCC build triplet:  x86_64-redhat-linux
  GCC host triplet:  x86_64-redhat-linux
GCC target triplet:  x86_64-redhat-linux


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26824

[Bug middle-end/26824] optimisation bug with -0x80000000

[pinskia at gcc dot gnu dot org]

------- Comment #1 from pinskia at gcc dot gnu dot org  2006-03-23 14:12 -------
-0x80000000 is going to overflow and for signed integers overflow is undefined.
 If you want signed integers to be defined to be wrapping use -fwrapv.


-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
          Component|c                           |middle-end
         Resolution|                            |INVALID


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26824

[Bug middle-end/26824] optimisation bug with -0x80000000

[falk at debian dot org]

------- Comment #2 from falk at debian dot org  2006-03-23 14:44 -------
Wait a minute. 0x80000000 is unsigned. So -0x80000000 is well-defined, and
is 0x80000000 (unsigned). This is then converted to signed. Since 0x80000000
cannot be represented in signed, the result is implementation specific.
We document it to be -0x80000000. So the call is just fine.


-- 

falk at debian dot org changed:

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


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26824

[Bug middle-end/26824] optimisation bug with -0x80000000

[pinskia at gcc dot gnu dot org]

------- Comment #3 from pinskia at gcc dot gnu dot org  2006-03-23 14:49 -------
It is the -x in f where x = 0x80000000 which is undefined as it overflows.


-- 

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=26824

[Bug middle-end/26824] optimisation bug with -0x80000000

[falk at debian dot org]

------- Comment #4 from falk at debian dot org  2006-03-23 14:57 -------
(In reply to comment #3)
> It is the -x in f where x = 0x80000000 which is undefined as it overflows.

Oh. I see. Right.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26824

[Bug c/26824] optimisation bug with -0x80000000

[simonmar at microsoft dot com]

------- Comment #5 from simonmar at microsoft dot com  2006-03-23 15:10 -------
I see your point, but I still think there's a bug.  Let me change the code
slightly:

#include <stdlib.h>
#include <stdio.h>

void f(int x) {
    long y;
    if (x < 0) {
        y = -x;
        if (y > 0) {
            printf("%d\n",y);
        }
    }
}

int main() {
        f(-0x80000000);
        exit(0);
}

$ gcc -O2 foo.c
$ ./a.out      
-2147483648

so, we're in the y > 0 branch, but y is clearly < 0.


-- 

simonmar at microsoft dot com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |UNCONFIRMED
          Component|middle-end                  |c
         Resolution|INVALID                     |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26824

[Bug c/26824] optimisation bug with -0x80000000

[rearnsha at gcc dot gnu dot org]

------- Comment #6 from rearnsha at gcc dot gnu dot org  2006-03-23 15:14 -------
Pinskia is right, and this is just as undefined as your previous example
(negating INT_MIN is just undefined, however you do it).  All you've done is
obfuscated things a bit more.


-- 

rearnsha at gcc dot gnu dot org changed:

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


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26824

[Bug c/26824] optimisation bug with -0x80000000

[pinskia at gcc dot gnu dot org]

------- Comment #7 from pinskia at gcc dot gnu dot org  2006-03-23 15:15 -------
(In reply to comment #5)
> I see your point, but I still think there's a bug.  Let me change the code
> slightly:
The code is still undefined, as you are comparing an overflowed variable to
something.

-x is still undefined for x being 0x80000000 as it overflows and overflow
signed integer is undefined runtime code.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26824

[Bug c/26824] optimisation bug with -0x80000000

[pinskia at gcc dot gnu dot org]

------- Comment #8 from pinskia at gcc dot gnu dot org  2006-03-23 15:17 -------
Reopening to ... 


-- 

pinskia at gcc dot gnu dot org changed:

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


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26824

[Bug c/26824] optimisation bug with -0x80000000

[pinskia at gcc dot gnu dot org]

------- Comment #9 from pinskia at gcc dot gnu dot org  2006-03-23 15:17 -------
Mark as a dup of bug 25329.

*** This bug has been marked as a duplicate of 25329 ***


-- 

pinskia at gcc dot gnu dot org changed:

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


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26824