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| From | Shao Miller <sha0.miller@gmail.com> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: Which operation is more expensive?(a <= b and a != 0) |
| Date | 2012-02-27 16:57 -0500 |
| Organization | A noiseless patient Spider |
| Message-ID | <jigu8d$79a$1@dont-email.me> (permalink) |
| References | <jigsb4$rdc$1@speranza.aioe.org> |
On 2/27/2012 16:24, Ping Cheu wrote:
> From a web-site that teaches optimization for c,
>
Please share the source. With no follow-up to your last thread, the
probability of this happening seems low.
> 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?
Please don't take a claim out of context and suppose it applies
universally. Please share the context.
C doesn't define "the speed" of evaluation of anything that I'm aware
of, so my answer would be "no, 'fact2_func' is not defined to always be
faster than 'fact1_func'."
However, if you consider that 'i <= n' involves _two_ reads and 'i != 0'
involves _one_ read, then unless you can read both at the same time or
deduce an optimization, 2 reads > 1 read.
Enjoy.
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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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