Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > comp.lang.c > #19064

Re: Odd benchmark behavior

Show all headers | View raw

BartC <bc@freeuk.com> wrote:
> I suspect the % operation (which needs a divide) dominates the proceedings. 
> I put in an extra dummy % op, and it nearly doubled the runtime (3.5 to 6.6 
> seconds).

Yes, division is certainly an expensive operation, but I like your
initial guess better (memory access dominates the results). So I
rewrote the program slightly to give the optimizer a chance to do
better register allocation and voila:

scc p2.c:	30s
gcc -O2 p2.c:	15s

Here is the new program (using an even more brute-force algorithm):

----------------------------------------------------------------
#include <stdio.h>

#define	MAX 10000

main() {
	int	i, j, k, n;

	for (k=0, i=3; k<MAX; i+=2) {
		for (j=2; j<i/2; j++)
			if (i % j == 0)
				break;
		if (j < k) continue;
		n = i;
		k++;
	}
	printf("%d\n", n);
}
----------------------------------------------------------------

I also tried a dummy remainder and it indeed doubled the run time
of the SCC-generated executable while the GCC-generated executable
was not affected. I guess that GCC simply removes the dead code.

-- 
Nils M Holm  < n m h @ t 3 x . o r g >  www.t3x.org

Back to comp.lang.c | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread

Thread

Odd benchmark behavior Nils M Holm <news2009@t3x.org> - 2012-03-26 07:51 +0000
  Re: Odd benchmark behavior "io_x" <a@b.c.invalid> - 2012-03-26 11:26 +0200
    Re: Odd benchmark behavior "BartC" <bc@freeuk.com> - 2012-03-26 10:40 +0100
      Re: Odd benchmark behavior gwowen <gwowen@gmail.com> - 2012-03-26 03:37 -0700
      Re: Odd benchmark behavior "io_x" <a@b.c.invalid> - 2012-03-26 14:13 +0200
    Re: Odd benchmark behavior "io_x" <a@b.c.invalid> - 2012-03-26 11:55 +0200
      Re: Odd benchmark behavior "io_x" <a@b.c.invalid> - 2012-03-27 09:17 +0200
        Re: Odd benchmark behavior "io_x" <a@b.c.invalid> - 2012-03-27 09:20 +0200
          Re: Odd benchmark behavior "Scott Fluhrer" <sfluhrer@ix.netcom.com> - 2012-03-27 13:39 -0400
          Re: Odd benchmark behavior pete <pfiland@mindspring.com> - 2012-03-27 21:49 -0500
            Re: Odd benchmark behavior "io_x" <a@b.c.invalid> - 2012-04-05 12:18 +0200
              Re: Odd benchmark behavior pete <pfiland@mindspring.com> - 2012-04-05 06:48 -0400
                Re: Odd benchmark behavior "io_x" <a@b.c.invalid> - 2012-04-05 14:12 +0200
                Re: Odd benchmark behavior pete <pfiland@mindspring.com> - 2012-04-05 09:03 -0400
                Re: Odd benchmark behavior "io_x" <a@b.c.invalid> - 2012-04-05 16:18 +0200
                Re: Odd benchmark behavior "io_x" <a@b.c.invalid> - 2012-04-07 13:26 +0200
                Re: Odd benchmark behavior "io_x" <a@b.c.invalid> - 2012-04-06 09:31 +0200
        Re: Odd benchmark behavior "io_x" <a@b.c.invalid> - 2012-03-27 19:01 +0200
  Re: Odd benchmark behavior "BartC" <bc@freeuk.com> - 2012-03-26 10:36 +0100
    Re: Odd benchmark behavior Nils M Holm <news2009@t3x.org> - 2012-03-26 10:02 +0000
  Re: Odd benchmark behavior Ian Collins <ian-news@hotmail.com> - 2012-03-26 22:58 +1300
  Re: Odd benchmark behavior "BartC" <bc@freeuk.com> - 2012-03-26 15:16 +0100
    Re: Odd benchmark behavior Nils M Holm <news2009@t3x.org> - 2012-03-27 06:20 +0000
      Re: Odd benchmark behavior "BartC" <bc@freeuk.com> - 2012-03-27 10:13 +0100
        Re: Odd benchmark behavior Nils M Holm <news2009@t3x.org> - 2012-03-27 11:59 +0000

csiph-web