c/8395: gcc 2.95.4 and 3.2 generate wrong code for double on intel

[Bruce Allen]

http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view%20audit-trail&database=gcc&pr=8395

There's nothing wrong here.  It's very reasonable for this code to 
produce ~10^-16 for double.
The reason is tha the number 1.2 can not be exactly represented as an 
IEEE754 floating point number.
The numbers 5 and -6 CAN be exactly represented.

I suggest that you read the first 15 or so pages of "Numerical Recipes 
in C" by Press, Teukolsky (and two others).
They discuss this question.  Then look at 
http://www.etsimo.uniovi.es/~antonio/uned/ieee754/IEEE-754.html

The closest IEEE754 double to 1.2 is (in binary)
1.0011001100110011001100110011001100110011001100110011
=
1.1999999999999999555910790149937383830547332763671875......
in decimal.

If we multiply this by 5 (which is exactly represented by an IEEE754 
double) one gets
5.9999999999999997779553950749686919152736663818359375

which is not 6.  The difference is what you are seeing.

#include <stdio.h>
int main(void)
{
    double x, y, z;
    long double lx, ly, lz;

    x = -6.0;
    y = -1.2;
    z = 5;
    printf("%g %g %g %g\n",
           x,
           y,
           z,
           x - y * z);
    lx = -6.0L;
    ly = -1.2L;
    lz = 5L;
    printf("%Lg %Lg %Lg %Lg\n",
           lx,
           ly,
           lz,
           lx - ly * lz);
    return(0);
}