Strange g+ 3.1.1 double behavior

[Gunther Weber <weber at informatik dot uni-kl dot de>]

Hello,

while searching for a problem in one of my programs I ran across this 
strange gcc 3.1.1 beahvior:

--- TestFloat.cc ---
#include <iostream>

double torus(double x, double y, double z)
{
 const double R = .7;
 const double r = .1;
 return (x*x + y*y)*(x*x + y*y) - 2*(R*R + (r*r - z*z))*(x*x + y*y) +
   (R*R - (r*r - z*z))*(R*R - (r*r - z*z));
}


int main()
{
 std::cout << torus(0.0,0.0,10.0) -  torus(0.0,0.0,10.0) << std::endl;
}
--- Test Float.cc ---

--- Log ---.
[weber@stella Test]$ g++3.1.1 TestFloat.cc 
-Wl,--rpath,/usr/global/packages/languages/gcc-3.1.1/lib
[weber@stella Test]$ ./a.out
3.85469e-13
[weber@stella Test]$ g++3.1.1 --version
g++3.1.1 (GCC) 3.1.1
Copyright (C) 2002 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.
gcc3.1.1 -v
Reading specs from 
/usr/global/packages/languages/gcc-3.1.1/lib/gcc-lib/i686-pc-linux-gnu/3.1.1/specs
Configured with: ./configure 
--prefix=/usr/global/packages/languages/gcc-3.1.1 --enable-languages=c,c++
Thread model: single
gcc version 3.1.1
[weber@stella Test]$ ldd a.out
       libstdc++.so.4 => 
/usr/global/packages/languages/gcc-3.1.1/lib/libstdc++.so.4 (0x40014000)
       libm.so.6 => /lib/i686/libm.so.6 (0x400d4000)
       libgcc_s.so.1 => 
/usr/global/packages/languages/gcc-3.1.1/lib/libgcc_s.so.1 (0x400f6000)
       libc.so.6 => /lib/i686/libc.so.6 (0x42000000)
       /lib/ld-linux.so.2 => /lib/ld-linux.so.2 (0x40000000)
--- Log ---

All this on a Celeron system running RedHat 7.3. Does anyone what could 
be causing this?  I would expect that both function calls with identical 
arguments would result in the same double value and thus in the programm 
printing "0".

Thank you for any help
 Gunther H Weber