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Re: Odd benchmark behavior

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"Nils M Holm" <news2009@t3x.org> wrote in message
news:9talg3F84gU1@mid.individual.net...

> The goal was to find out how bad exactly the code generated by my
> toy compiler is, so I passed the following program to GNU C (GCC),
> Portable C (PCC), and my toy compiler (let's call it SCC):

> #include <stdio.h>
>
> #define MAX 20000
>
> main() {
>        int     i, j, p[MAX], k;
>
>        for (k=0, i=3; k<MAX; i+=2) {
>                for (j=0; j<k; j++)
>                        if (i % p[j] == 0)
>                                break;
>                if (j < k) continue;
>                p[k++] = i;
>        }
>        printf("%d\n", p[k-1]);
> }

> gcc -O2 p.c: 12.39s
> pcc -O p.c: 12.42s
> scc p.c: 12.27s
>
> I can explain the difference between GCC/PCC and SCC by lower
> startup overhead, but shouldn't the better code quality of PCC
> and GCC compensate for this over a run time of 12 seconds?
>
> I am rather puzzled now. Any ideas?

I tried it with my compiler (not of C, but similar), which while not a toy,
does also generate some very bad code. And while it was slower than the Cs,
all the results were in quite a narrow range: 3.1 to 3.4 seconds for various
C compilers, and 3.5 seconds for my compiler (all x86-32).

The 3.1 seconds was for gcc -O3.

However, with the conventional sieve benchmark, gcc -O3 was over 3 times as
fast as my compiler. So perhaps there's something peculiar with your
benchmark.

(The ratio between gcc -O3 and gcc -O0 was 1.13:1 with your benchmark, but
4:1 with the regular sieve, however the latter does repeat the main loop
thousands of times, which I suspect -O3 takes advantage of. You need a
different benchmark which isn't repetitive, and doesn't use too much memory
either, otherwise the timings will be dominated by memory accesses.)

-- 
Bartc 

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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

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