🚀 go-pugleaf

I wrote a program for my VAX assembly class' programming assignment
and it solves simple equations by iteration.  C version is following;
-------------------------------------------------------------------------------
#include <stdio.h>
#include <math.h>

main()
{
  double x = 1.0, px;
  double y = 2.0, py;

  do {
    px = x;
    py = y;
    x = 1.0 + sqrt(px * px + py * py) / 10.0 * cos(py);
    y = 2.0 + (sin(px) + sin(py) - px) / 20.0;
  } while (abs((x - px) / x) > 1.0e-16 ||
	   abs((y - py) / y) > 1.0e-16);
  printf("x = %17.15lf\ny = %17.15lf\n", x, y);
}
-------------------------------------------------------------------------------
My VAX assembly program agrees with the above C program compiled by cc
on SunOS 4.x.x (sorry don't remember);

x = 0.899475740409558
y = 2.038799794467183

However, the same C program compiled by two versions of gcc (2.4.3 and
2.3.1) don't agree with the above results;

x = 0.906946738485910
y = 2.037538420581679

Gcc 2.4.3 is running on a DECstation/Ultrix and 2.3.1 is running on
Sun Sparc/SunOS 4.x.x (Same program compiled by cc on Ultrix doesn't
get result in reasonable amount of time).

Emacs Calc running on Emacs 18.59 also agrees with my VAX assembly
program (I fed two results to the equations to see if same results
come up).

Why?  Is this a bug of gcc?
Thanks in advance.

Ken Nakata
-- 
* I apologize if there are misused or impolite words or phrases in this
mail or post.   They are not intended; I don't fully understand certain
words or nuances of phrasing.    Any corrections of English composition
or content would be greatly appreciated.        Ken Nakata, CIS, NJIT *