Example pgm that is 3X slower on SS2 than on SS1+

[Mark G. Johnson]

Here is a short C program that runs 3 times faster on a SS1+ than on a
SS2.  Mesured with /bin/time,

    Compiled with               SPARCstation 2     SPARCstation 1+
 --------------------------------------------------------------------
    cc -O prog.c -lm             29.3 user sec       9.5 user sec


/* ************* code follows *************** */
#include <stdio.h>
#include <math.h>
/* compute the first 1000 digits of PI = 4arctan(1) */
main()
{
  long d = 4, r = 10000, n = 251, m = 3.322*n*d;
  long i, j, k, q;
  static long a[3340];

  for (i = 0; i <= m; i++) a[i] = 2;
  a[m] = 4;

  for (i = 1; i <= n; i++) {
    q = 0;
    for (k = m; k > 0; k--) {
      a[k] = a[k]*r+q;
      q = a[k]/(2*k+1);
      a[k] -= (2*k+1)*q;
      q *= k;
    }
    a[0] = a[0]*r+q;
    q = a[0]/r;
    a[0] -= q*r;
    printf("%04d%s", q, i & 7 ? "  " : "\n");
  }
}
/* ************* end of code *************** */

The reason for this unexpected slowdown is rather obscure: the subroutines
that implement multiplication and division get placed in a very unlucky
spot in the SS2's cache.  In the SS1 they get plopped in a less dangerous
area.

This can be seen by having the compiler and/or the OS move the
multiplication and division subroutines to new positions.  Or, by
finagling the *other* subroutines (e.g. fp math) that might get in the way
of the mult and div subroutines.  When you do this, the SS2 becomes faster
than the SS1+.

    Compiled with               SPARCstation 2     SPARCstation 1+
 --------------------------------------------------------------------
    cc -O prog.c -lm             29.3 user sec       9.5 user sec
    cc -O -Bstatic prog.c -lm     5.2 user sec       8.9 user sec
    cc -O prog.c                  5.5 user sec       9.5 user sec

Mark Johnson	
 	MIPS Computer Systems, 930 E. Arques M/S 2-02, Sunnyvale, CA 94086
	(408) 524-8308    mark at mips.com  {or ...!decwrl!mips!mark}