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Re: Which operation is more expensive?(a <= b and a != 0)

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On 2/27/12 4:24 PM, Ping Cheu wrote:
> From a web-site that teaches optimization for c,
> 
> I saw bellow optimization rule.
> 
>     int fact1_func (int n)
>     {
>         int i, fact = 1;
>         for (i = 1; i <= n; i++)
>           fact *= i;
>         return(fact);
>     }
> 
>     int fact2_func(int n)
>     {
>         int i, fact = 1;
>         for (i = n; i != 0; i--)
>            fact *= i;
>         return(fact);
>     }
> 
> The author say 'fact2_func' is faster than 'fact1_func'.
> 
> That means 'i <= n' operation is more expensive than 'i != 0'.
> 
> So far, I thought all condition statements have same cost.
> 
> So I want to know the cost of condition states from the fast one to slow 
> one.
> 
> Can anyone tell me? 

The difference is that the second compares to a constant 0, which MAY be
faster than comparing to a variable n.

Many processors can quickly test the value of a register to see if it is
negative or positive (sign bit test) or zero or non-zero without running
an explicit comparison operation.

Note that the second version will behave differently in the case of n<0,
likely taking a significantly longer time than the first, finally
returning a value that is probably 0 (after numerous instances of
undefined behavior of overflow), while the first will rapidly return 1.

If you change the second to use i > 0, they will both return rapidly,
but you have possibly lost some of the advantage, as this relationship
is less often a built in automatic, but may still be slightly faster due
to the constant 0 instead of the variable n being used.

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Which operation is more expensive?(a <= b and a != 0) Ping Cheu <nospam@nospam.com> - 2012-02-27 21:24 +0000
  Re: Which operation is more expensive?(a <= b and a != 0) James Kuyper <jameskuyper@verizon.net> - 2012-02-27 16:47 -0500
    Re: Which operation is more expensive?(a <= b and a != 0) Michael Angelo Ravera <maravera@prodigy.net> - 2012-03-07 04:23 -0800
  Re: Which operation is more expensive?(a <= b and a != 0) Keith Thompson <kst-u@mib.org> - 2012-02-27 13:54 -0800
    Re: Which operation is more expensive?(a <= b and a != 0) falk@rahul.net (Edward A. Falk) - 2012-02-27 23:47 +0000
  Re: Which operation is more expensive?(a <= b and a != 0) Shao Miller <sha0.miller@gmail.com> - 2012-02-27 16:57 -0500
    Re: Which operation is more expensive?(a <= b and a != 0) Stephen Sprunk <stephen@sprunk.org> - 2012-02-28 13:43 -0600
  Re: Which operation is more expensive?(a <= b and a != 0) Richard Damon <news.x.richarddamon@xoxy.net> - 2012-02-27 16:58 -0500
  Re: Which operation is more expensive?(a <= b and a != 0) Malcolm McLean <malcolm.mclean5@btinternet.com> - 2012-02-27 13:55 -0800
    Re: Which operation is more expensive?(a <= b and a != 0) Kenneth Brody <kenbrody@spamcop.net> - 2012-03-03 15:58 -0500
  Re: Which operation is more expensive?(a <= b and a != 0) Keith Thompson <kst-u@mib.org> - 2012-02-27 14:17 -0800
  Re: Which operation is more expensive?(a <= b and a != 0) Barry Schwarz <schwarz45@yahoo.com> - 2012-02-27 14:52 -0800
    Re: Which operation is more expensive?(a <= b and a != 0) Peter Nilsson <airia@acay.com.au> - 2012-02-27 18:16 -0800
      Re: Which operation is more expensive?(a <= b and a != 0) qarnos <darrencubitt@gmail.com> - 2012-02-29 12:20 -0800
      Re: Which operation is more expensive?(a <= b and a != 0) Barry Schwarz <schwarzb@dqel.com> - 2012-03-13 22:07 -0700
  Re: Which operation is more expensive?(a <= b and a != 0) Kaz Kylheku <kaz@kylheku.com> - 2012-02-28 00:56 +0000
  Re: Which operation is more expensive?(a <= b and a != 0) Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2012-02-27 19:18 -0700
    Re: Which operation is more expensive?(a <= b and a != 0) China Blue Sea <chine.bleu@yahoo.com> - 2012-02-27 18:30 -0800
  Re: Which operation is more expensive?(a <= b and a != 0) "BartC" <bc@freeuk.com> - 2012-02-28 13:22 +0000
  Re: Which operation is more expensive?(a <= b and a != 0) Stephen Sprunk <stephen@sprunk.org> - 2012-02-28 13:38 -0600
  Re: Which operation is more expensive?(a <= b and a != 0) jgk@panix.com (Joe keane) - 2012-02-28 22:35 +0000
    Re: Which operation is more expensive?(a <= b and a != 0) Phil Carmody <thefatphil_demunged@yahoo.co.uk> - 2012-03-03 01:36 +0200
      Re: Which operation is more expensive?(a <= b and a != 0) jgk@panix.com (Joe keane) - 2012-03-05 20:03 +0000
        Re: Which operation is more expensive?(a <= b and a != 0) Phil Carmody <thefatphil_demunged@yahoo.co.uk> - 2012-03-12 01:08 +0200
          Re: Which operation is more expensive?(a <= b and a != 0) Harald van Dijk <haraldvdijk@gmail.com> - 2012-03-12 00:05 -0700
            Re: Which operation is more expensive?(a <= b and a != 0) James Kuyper <jameskuyper@verizon.net> - 2012-03-12 08:59 -0400
              Re: Which operation is more expensive?(a <= b and a != 0) gwowen <gwowen@gmail.com> - 2012-03-12 06:24 -0700
                Re: Which operation is more expensive?(a <= b and a != 0) James Kuyper <jameskuyper@verizon.net> - 2012-03-12 10:13 -0400
                Re: Which operation is more expensive?(a <= b and a != 0) Keith Thompson <kst-u@mib.org> - 2012-03-12 10:44 -0700
                Re: Which operation is more expensive?(a <= b and a != 0) Phil Carmody <thefatphil_demunged@yahoo.co.uk> - 2012-03-13 17:12 +0200
          Re: Which operation is more expensive?(a <= b and a != 0) Keith Thompson <kst-u@mib.org> - 2012-03-12 11:20 -0700

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