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Re: how much computers can do in one second

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Marko Rauhamaa <marko@pacujo.net> writes:
<snip>
> Somewhat related. Here's a similar program to measure how fast the CPU's
> 32-bit integers wrap around:
>
>
> ========================================================================
> #include <stdlib.h>
>
> int main(int argc, char **argv)
> {
>     int i;
>     for (i = 0; i < i + 1; i++)
>         ;
>     return 0;
> }
> ========================================================================

(You can write this as

  int main(void)
  {
      for (int i = 0; i < i + 1; i++)
          ;
  }

in modern C.)

<snip>
> BTW, is your 2-gigahertz processor executing two instructions per cycle?

Modern machines don't work as simply as that (as I'm sure you know).
The above compiles to an inner loop of 5 instructions and takes (on my
machine) 4.34s for 2^31 iterations.  That's a bit more than 2.4
instructions per ns.  If I simplify the loop to

  for (int i = 0; i >= 0; i++);

it compiles to 3 instructions but takes almost the same time (4.32s).
The lack of optimisation means that i is not in a register so cache
accesses are probably a limiting factor.

-- 
Ben.

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how much computers can do in one second RS Wood <rsw@therandymon.com> - 2017-03-27 14:43 +0300
  Re: how much computers can do in one second Marko Rauhamaa <marko@pacujo.net> - 2017-03-27 15:06 +0300
    Re: how much computers can do in one second Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-03-29 01:55 +0100
      Re: how much computers can do in one second Marko Rauhamaa <marko@pacujo.net> - 2017-03-29 10:25 +0300
      Re: how much computers can do in one second Richard Kettlewell <invalid@invalid.invalid> - 2017-03-29 08:52 +0100
        Re: how much computers can do in one second Marko Rauhamaa <marko@pacujo.net> - 2017-03-29 12:43 +0300
          Re: how much computers can do in one second Caecilius <nospam@spamless.invalid> - 2017-03-30 16:07 +0100
            Re: how much computers can do in one second Marko Rauhamaa <marko@pacujo.net> - 2017-03-30 19:09 +0300
              Re: how much computers can do in one second Bob Eager <news0006@eager.cx> - 2017-03-30 21:36 +0000
                Re: how much computers can do in one second Marko Rauhamaa <marko@pacujo.net> - 2017-03-31 09:56 +0300
  Re: how much computers can do in one second Huge <Huge@nowhere.much.invalid> - 2017-03-27 13:51 +0000
  Re: how much computers can do in one second Sylvia Else <sylvia@not.at.this.address> - 2017-03-28 11:57 +1100
    Re: how much computers can do in one second Huge <Huge@nowhere.much.invalid> - 2017-03-28 09:51 +0000
      Re: how much computers can do in one second Bob Eager <news0006@eager.cx> - 2017-03-28 11:12 +0000

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