Re: Optimisation questions

From	James Kuyper <jameskuyper@verizon.net>
Newsgroups	comp.lang.c
Subject	Re: Optimisation questions
Date	2012-02-04 18:44 -0500
Organization	A noiseless patient Spider
Message-ID	<jgkfso$1bh$2@dont-email.me> (permalink)
References	<jgkbiu$432$1@speranza.aioe.org>

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On 02/04/2012 05:30 PM, Raj Pashwar wrote:
> Hello :
> 
> My current program runs too slow. It uses x/8 and x%8 many times, where x 
> is an int in both cases.
> 
> I have replaced x/8 with x>>3, as this is faster (pretty sure).
> However, I am not sure whether this is faster than x%8 :
> x-=(x>>3)<<3;

As a general rule, your compiler should generate exactly the same code
for x/8 and x>>3, because they're required to have exactly the same
behavior. If it doesn't, get a better one. What that code will be may be
different on different machines, it might be called a division
instruction on one machine, and a shift instruction on a different
machine, but you can generally trust the compiler to know which one is
faster. If it doesn't, get a better one.

> 
> Is this generally a good optimisation to use?
> 
> Also I've got a memory region that I constantly need to access, so I 
> would like it to be in cache. I know that I can use 'register' in 
> variable declarations, to speed up it's access.

That's unlikely to be the case; if putting it in a register would speed
up, you can be reasonably sure the compiler already puts it in a
register, even if you don't specifically tell it to do so. If it didn't
put it in a register, it probably had a good reason for not doing so; in
that case, if it paid attention to your "register" declaration, it could
actually slow your program down. For precisely this reason, most
compilers ignore the keyword "register", and the way the standard is
written, this is perfectly acceptable behavior.
-- 
James Kuyper

Thread

Optimisation questions Raj Pashwar <raj121190@hotmail.NOSPAM.com> - 2012-02-04 22:30 +0000
  Re: Optimisation questions Ian Collins <ian-news@hotmail.com> - 2012-02-05 11:44 +1300
    Re: Optimisation questions Keith Thompson <kst-u@mib.org> - 2012-02-04 14:52 -0800
      Re: Optimisation questions Ian Collins <ian-news@hotmail.com> - 2012-02-05 11:55 +1300
      Re: Optimisation questions Phil Carmody <thefatphil_demunged@yahoo.co.uk> - 2012-02-21 18:41 +0200
  Re: Optimisation questions "BartC" <bc@freeuk.com> - 2012-02-04 22:44 +0000
    Re: Optimisation questions "BartC" <bc@freeuk.com> - 2012-02-04 23:41 +0000
  Re: Optimisation questions Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-02-04 17:51 -0500
  Re: Optimisation questions Keith Thompson <kst-u@mib.org> - 2012-02-04 14:52 -0800
  Re: Optimisation questions James Kuyper <jameskuyper@verizon.net> - 2012-02-04 18:44 -0500
    Re: Optimisation questions Stephen Sprunk <stephen@sprunk.org> - 2012-02-04 18:00 -0600
      Re: Optimisation questions James Kuyper <jameskuyper@verizon.net> - 2012-02-04 21:18 -0500
        Re: Optimisation questions Kaz Kylheku <kaz@kylheku.com> - 2012-02-05 03:04 +0000
          Re: Optimisation questions Dann Corbit <dcorbit@connx.com> - 2012-02-06 09:40 -0800
            Re: Optimisation questions Willem <willem@toad.stack.nl> - 2012-02-06 18:51 +0000
            Re: Optimisation questions Markus Wichmann <nullplan@gmx.net> - 2012-02-07 14:19 +0100
              Re: Optimisation questions Dann Corbit <dcorbit@connx.com> - 2012-02-07 14:25 -0800
          Re: Optimisation questions Stephen Sprunk <stephen@sprunk.org> - 2012-02-06 17:29 -0600
            Re: Optimisation questions Kaz Kylheku <kaz@kylheku.com> - 2012-02-06 23:52 +0000
            Re: Optimisation questions pete <pfiland@mindspring.com> - 2012-02-06 19:10 -0500
    Re: Optimisation questions Raj Pashwar <raj121190@hotmail.NOSPAM.com> - 2012-02-05 10:51 +0000
      Re: Optimisation questions Ian Collins <ian-news@hotmail.com> - 2012-02-06 00:08 +1300
        Re: Optimisation questions Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-02-05 08:13 -0500
      Re: Optimisation questions "BartC" <bc@freeuk.com> - 2012-02-05 11:33 +0000
      Re: Optimisation questions Zoltan Kocsi <zoltan@bendor.com.au> - 2012-02-05 23:24 +1100
        Re: Optimisation questions Raj Pashwar <raj121190@hotmail.NOSPAM.com> - 2012-02-05 22:28 +0000
          Re: Optimisation questions Kaz Kylheku <kaz@kylheku.com> - 2012-02-05 22:37 +0000
          Re: Optimisation questions Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-02-05 17:48 -0500
          Re: Optimisation questions Keith Thompson <kst-u@mib.org> - 2012-02-05 15:57 -0800
      Re: Optimisation questions "BartC" <bc@freeuk.com> - 2012-02-05 12:53 +0000
  Re: Optimisation questions Geoff <geoff@invalid.invalid> - 2012-02-04 16:07 -0800
  Re: Optimisation questions "christian.bau" <christian.bau@cbau.wanadoo.co.uk> - 2012-02-04 15:37 -0800
    Re: Optimisation questions Nick Keighley <nick_keighley_nospam@hotmail.com> - 2012-02-05 04:26 -0800
      Re: Optimisation questions lawrence.jones@siemens.com - 2012-02-07 10:56 -0500
  Re: Optimisation questions Nick Keighley <nick_keighley_nospam@hotmail.com> - 2012-02-05 04:29 -0800
  Re: Optimisation questions Michael Angelo Ravera <maravera@prodigy.net> - 2012-02-05 23:24 -0800

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