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| From | Joe Pfeiffer <pfeiffer@cs.nmsu.edu> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: Which operation is more expensive?(a <= b and a != 0) |
| Date | 2012-02-27 19:18 -0700 |
| Organization | A noiseless patient Spider |
| Message-ID | <1bvcmrem55.fsf@pfeifferfamily.net> (permalink) |
| References | <jigsb4$rdc$1@speranza.aioe.org> |
Ping Cheu <nospam@nospam.com> writes:
> 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?
For compilers and architectures for which this is true, the reason is
that the condition codes are set as a side effect of the decrement, so
you don't need to actually perform the comparison in the decrement
version.
It isn't true for all architectures because (1) not all architectures
let you get away with skipping the comparison, and (2) many
implementations will manage to combine the increment, compare, and
branch in u-ops that get executed out of order and don't end up taking
any extra time.
It isn't true for all compilers because (assuming it's true for the
architecture in the first place!) the compiler may figure out it can run
the loop backwards all by itself.
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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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