Re: Optimisation questions

Message-ID	<4F306BFB.2561@mindspring.com> (permalink)
Date	2012-02-06 19:10 -0500
From	pete <pfiland@mindspring.com>
Organization	PF
Newsgroups	comp.lang.c
Subject	Re: Optimisation questions
References	(1 earlier) <jgkfso$1bh$2@dont-email.me> <jgkgra$886$1@dont-email.me> <jgkou6$g96$1@dont-email.me> <20120205034114.797@kylheku.com> <jgpnon$qm7$1@dont-email.me>

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Stephen Sprunk wrote:
> 
> On 04-Feb-12 21:04, Kaz Kylheku wrote:
> > On 2012-02-05, James Kuyper <jameskuyper@verizon.net> wrote:
> >> On 02/04/2012 07:00 PM, Stephen Sprunk wrote:
> >>> On 04-Feb-12 17:44, James Kuyper wrote:
> >>>> 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.
> >>>
> >>> That only holds if x is non-negative.
> >>
> >> You're right
> >> - the result is implementation-defined if x is negative.
> >
> > Yes and the point is that this is what helps to optimize it better.
> 
> You missed the point.  If x is negative,
> they are not guaranteed to have the same behavior,
> so the compiler is not allowed to make such an optimization.  

I think Kaz was talking about a different optimization.

The value of (x>>3) being implementation defined 
for negative values of x,
suggests that the resulting opcodes for (x>>3)
should be simpler than they would be
if (x>>3) was defined as always being equal to (x/8).


-- 
pete

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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