[Bug c/13421] New: IA32 bigmem pointer subtraction and –ftrapv option causes unjustified program abort

["vik dot heyndrickx at pandora dot be" <gcc-bugzilla@gcc.gnu.org>]

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kernel-2.4.22-1.2115.nptl, glibc-2.3.2-101.1
Configured with: ../configure --prefix=/usr --mandir=/usr/share/man --
infodir=/usr/share/info --enable-shared --enable-threads=posix --disable-
checking --with-system-zlib --enable-__cxa_atexit --host=i386-redhat-linux
Thread model: posix
gcc version 3.3.2 20031022 (Red Hat Linux 3.3.2-1)

Circumstances: “-ftrapv” is a gcc compiler option used to detect signed integer 
overflow conditions, and as such is frequently used to debug programs. This 
flag is by default disabled.
On IA32 these days adressible memory for a process can be larger than 2^31 
octets. It is possible for a memory block whose lower bound address is less 
than 2^31 en whose upper bound address is larger than 2^31, to be assigned. 
Subtracting these two addresses is a normal operation to determine the size of 
that block. It appears however that when “-ftrapv” is used to compile a 
program, execution of this program is aborted when subtracting these pointers. 
In my opinion this should not happen, because there is nothing illegal about.

Example of failing program test.c (it looks "manufactured", but is short, the 
real program I encountered this problem with is less manufactured, I got the 
addresses from malloc(), and that program was long):

<cut>
long signed diff = 0;

void setdiff (unsigned char *a, unsigned char *b) {
        diff = b - a;
}

int main (void) {
        unsigned char *a, *b;

        a = (unsigned char*)0x7FFFF000u;
        b = (unsigned char*)0x80000001u;
        setdiff (a, b);
        return 0;
}
</cut>

Compiler command line:
gcc -ftrapv test.c

execution of the resulting program a.out:
Aborted

-- 
           Summary: IA32 bigmem pointer subtraction and –ftrapv option
                    causes unjustified program abort
           Product: gcc
           Version: 3.3.2
            Status: UNCONFIRMED
          Severity: critical
          Priority: P2
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: vik dot heyndrickx at pandora dot be
                CC: gcc-bugs at gcc dot gnu dot org
  GCC host triplet: gcc version 3.3.2 20031022 (Red Hat Linux 3.3.2-1)


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=13421