[Bug c/47732] New: counter decremented to zero not detected when other counter overflows

[Bug c/47732] New: counter decremented to zero not detected when other counter overflows

[J.K.Annot.at.Infor at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47732

           Summary: counter decremented to zero not detected when other
                    counter overflows
           Product: gcc
           Version: 4.3.3
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: J.K.Annot.at.Infor@gmail.com


Created attachment 23336
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=23336
preprocessor output

gcc version 4.3.3 [gcc-4_3-branch revision 147732] (SUSE Linux)

Target: x86_64-suse-linux

Configured with: ../configure --prefix=/usr --infodir=/usr/share/info
--mandir=/usr/share/man --libdir=/usr/lib64 --libexecdir=/usr/lib64
--enable-languages=c,c++,objc,fortran,obj-c++,java,ada
--enable-checking=release --with-gxx-include-dir=/usr/include/c++/4.3
--enable-ssp --disable-libssp --with-bugurl=http://bugs.opensuse.org/
--with-pkgversion='SUSE Linux' --disable-libgcj --disable-libmudflap
--with-slibdir=/lib64 --with-system-zlib --enable-__cxa_atexit
--enable-libstdcxx-allocator=new --disable-libstdcxx-pch
--enable-version-specific-runtime-libs --program-suffix=-4.3
--enable-linux-futex --without-system-libunwind --with-cpu=generic
--build=x86_64-suse-linux

COLLECT_GCC_OPTIONS='-v' '-save-temps' '-O2' '-mtune=generic'
 /usr/lib64/gcc/x86_64-suse-linux/4.3/cc1 -E -quiet -v verify_counter_nonzero.c
-mtune=generic -O2 -fpch-preprocess -o verify_counter_nonzero.i

A local counter (named lCounter) is decremented in a while-loop, and the loop
must stop when the counter reaches zero.
In the same loop, a second local counter (lVeryHighCounter) is incremented and
overflows in the same cycle where the first counter reaches zero.
The overflow of the second counter seems to trigger that it is not detected
that the first counter reaches zero: (lCounter != 0) remains false.
A static counter (named sCounter) is used to observe what happens and to stop
the program as soon as the bug is observed.

The used -O2 option is needed to reproduce the bug.
Using -O1 or -O3 does not reproduce the bug.

extern int printf(const char *, ...);
extern void exit(int);

static int    sCounter    = 3;

void    verify_counter_nonzero(
    int    aDummy
)
{
    printf("sCounter value: %d\n", sCounter);

    if (sCounter == 0) {
        printf("Compiler bug detected! This function should not be called when
counter reached 0.\n");

        exit(0);
    }
}

int    main()
{
    int    lCounter        = 3;
    int    lVeryHighCounter    = 0x80000000 - 3;

    while (lCounter != 0) {
        verify_counter_nonzero(lVeryHighCounter);

        sCounter        -= 1;
        lCounter        -= 1;
        lVeryHighCounter    += 1;

        /* In the same cycle where lCounter is decremented to 0, 
           lVeryHighCounter is incremented (and overflows) from 0x7FFFFFFF to
0x80000000.
           This seems to trigger the bug: the loop condition (lCounter != 0)
should become false,
           but remains true.
        */
    }

    printf("Compiler bug not detected! Loop terminated correctly.\n");
}

[Bug c/47732] counter decremented to zero not detected when other counter overflows

[pinskia at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47732

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

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

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> 2011-02-14 18:22:17 UTC ---
"lVeryHighCounter is incremented (and overflows) from 0x7FFFFFFF to
0x80000000."

And overflow is undefined in C/C++ so you are invoking undefined behavior here.

[Bug c/47732] counter decremented to zero not detected when other counter overflows

[J.K.Annot.at.Infor at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47732

--- Comment #2 from Jan Koen Annot <J.K.Annot.at.Infor at gmail dot com> 2011-02-15 14:21:16 UTC ---
(In reply to comment #1)
> And overflow is undefined in C/C++ so you are invoking undefined behavior here.

Wow! This is an eye-opener for me! I am quite accustomed to assume two's
complement arithmetic with implicit wrap-around for signed integers.

After your remark I googled a bit and found several articles about undefined
behavior, e.g. John Regehr's "A Guide to Undefined Behavior in C and C++, Part
1".
Very instructive.