Re: harder case for optimization

From	Melzzzzz <mel@zzzzz.com>
Newsgroups	comp.lang.c
Subject	Re: harder case for optimization
Date	2014-11-28 21:31 +0100
Organization	albasani.net
Message-ID	<20141128213141.61a5083c@maxa-pc> (permalink)
References	(9 earlier) <e3485f9b-51d4-4c36-b3cc-4e6f10dd395a@googlegroups.com> <m5ai1a$mqi$1@dont-email.me> <20141128114639.902@kylheku.com> <20141128210320.53d6e8c2@maxa-pc> <20141128120935.82@kylheku.com>

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On Fri, 28 Nov 2014 20:11:25 +0000 (UTC)
Kaz Kylheku <kaz@kylheku.com> wrote:

> On 2014-11-28, Melzzzzz <mel@zzzzz.com> wrote:
> > gcc does integer division by multiplication, but it does not
> > do that for floats.
> 
> With good reason: it wouldn't be equivalent in terms of the result,
> and other considerations, like perhaps the exact ranges of inputs
> under which an overflow would occur.
> 
> However, GCC has an option for looser floating-point math:
> -ffast-math. Try that; see if it still refuses.

You are right! I compiled with -O3 and gcc generates divsd, but
with -Ofast it generates mulsd ;)
So -Ofast (and -ffast-math particularly) is for inaccurate
optimizations.

Thread

harder case for optimization fir <profesor.fir@gmail.com> - 2014-11-27 00:42 -0800
  Re: harder case for optimization "BartC" <bc@freeuk.com> - 2014-11-27 11:34 +0000
    Re: harder case for optimization David Brown <david.brown@hesbynett.no> - 2014-11-27 13:29 +0100
    Re: harder case for optimization Malcolm McLean <malcolm.mclean5@btinternet.com> - 2014-11-27 05:58 -0800
      Re: harder case for optimization Malcolm McLean <malcolm.mclean5@btinternet.com> - 2014-11-27 06:36 -0800
    Re: harder case for optimization fir <profesor.fir@gmail.com> - 2014-11-27 12:26 -0800
  Re: harder case for optimization "BartC" <bc@freeuk.com> - 2014-11-27 22:07 +0000
    Re: harder case for optimization fir <profesor.fir@gmail.com> - 2014-11-27 14:49 -0800
      Re: harder case for optimization "BartC" <bc@freeuk.com> - 2014-11-28 00:05 +0000
    Re: harder case for optimization David Brown <david.brown@hesbynett.no> - 2014-11-28 00:46 +0100
      Re: harder case for optimization "BartC" <bc@freeuk.com> - 2014-11-28 00:55 +0000
        Re: harder case for optimization David Brown <david.brown@hesbynett.no> - 2014-11-28 07:20 +0100
          Re: harder case for optimization Malcolm McLean <malcolm.mclean5@btinternet.com> - 2014-11-28 00:41 -0800
            Re: harder case for optimization fir <profesor.fir@gmail.com> - 2014-11-28 01:54 -0800
              Re: harder case for optimization Malcolm McLean <malcolm.mclean5@btinternet.com> - 2014-11-28 02:19 -0800
                Re: harder case for optimization fir <profesor.fir@gmail.com> - 2014-11-28 04:05 -0800
                Closed captioning (Was: harder case for something or other) gazelle@shell.xmission.com (Kenny McCormack) - 2014-11-28 12:15 +0000
                Re: harder case for optimization fir <profesor.fir@gmail.com> - 2014-11-28 06:11 -0800
              Re: harder case for optimization gazelle@shell.xmission.com (Kenny McCormack) - 2014-11-28 12:18 +0000
                Re: harder case for optimization fir <profesor.fir@gmail.com> - 2014-11-28 04:42 -0800
                Re: harder case for optimization gazelle@shell.xmission.com (Kenny McCormack) - 2014-11-28 16:37 +0000
                Re: harder case for optimization fir <profesor.fir@gmail.com> - 2014-11-28 08:48 -0800
                Re: harder case for optimization Kaz Kylheku <kaz@kylheku.com> - 2014-11-28 16:51 +0000
                Re: harder case for optimization fir <profesor.fir@gmail.com> - 2014-11-28 10:38 -0800
            Re: harder case for optimization David Brown <david.brown@hesbynett.no> - 2014-11-28 11:34 +0100
          Re: harder case for optimization "BartC" <bc@freeuk.com> - 2014-11-28 11:21 +0000
            Re: harder case for optimization fir <profesor.fir@gmail.com> - 2014-11-28 04:23 -0800
            Re: harder case for optimization David Brown <david.brown@hesbynett.no> - 2014-11-28 13:58 +0100
              Re: harder case for optimization "BartC" <bc@freeuk.com> - 2014-11-28 13:46 +0000
                Re: harder case for optimization David Brown <david.brown@hesbynett.no> - 2014-11-28 16:25 +0100
                Re: harder case for optimization fir <profesor.fir@gmail.com> - 2014-11-28 08:18 -0800
                Re: harder case for optimization David Brown <david.brown@hesbynett.no> - 2014-11-28 20:17 +0100
                Re: harder case for optimization Malcolm McLean <malcolm.mclean5@btinternet.com> - 2014-11-28 08:54 -0800
                Re: harder case for optimization David Brown <david.brown@hesbynett.no> - 2014-11-28 20:22 +0100
                Re: harder case for optimization Malcolm McLean <malcolm.mclean5@btinternet.com> - 2014-11-28 11:29 -0800
                Re: harder case for optimization Kaz Kylheku <kaz@kylheku.com> - 2014-11-28 19:51 +0000
                Re: harder case for optimization David Brown <david.brown@hesbynett.no> - 2014-11-28 20:58 +0100
                Re: harder case for optimization Melzzzzz <mel@zzzzz.com> - 2014-11-28 21:03 +0100
                Re: harder case for optimization Kaz Kylheku <kaz@kylheku.com> - 2014-11-28 20:11 +0000
                Re: harder case for optimization Melzzzzz <mel@zzzzz.com> - 2014-11-28 21:31 +0100
                Re: harder case for optimization "BartC" <bc@freeuk.com> - 2014-11-28 20:06 +0000
                Re: harder case for optimization "BartC" <bc@freeuk.com> - 2014-11-28 17:14 +0000
                Re: harder case for optimization David Brown <david.brown@hesbynett.no> - 2014-11-28 20:54 +0100
                Re: harder case for optimization "BartC" <bc@freeuk.com> - 2014-11-28 20:16 +0000
                Re: harder case for optimization David Brown <david.brown@hesbynett.no> - 2014-11-28 21:56 +0100
        Re: harder case for optimization fir <profesor.fir@gmail.com> - 2014-11-27 23:37 -0800
          Re: harder case for optimization fir <profesor.fir@gmail.com> - 2014-11-27 23:54 -0800
          Re: harder case for optimization BartC <bc@freeuk.com> - 2014-11-28 11:01 +0000
            Re: harder case for optimization fir <profesor.fir@gmail.com> - 2014-11-28 23:52 -0800
              Re: harder case for optimization Morris Dovey <mrdovey@iedu.com> - 2014-11-29 02:03 -0600
                Re: harder case for optimization fir <profesor.fir@gmail.com> - 2014-11-29 00:38 -0800
  Re: harder case for optimization fir <profesor.fir@gmail.com> - 2014-11-29 03:12 -0800
    Re: harder case for optimization BartC <bc@freeuk.com> - 2014-11-29 12:09 +0000
      Re: harder case for optimization fir <profesor.fir@gmail.com> - 2014-11-29 04:18 -0800
      Re: harder case for optimization fir <profesor.fir@gmail.com> - 2014-11-29 05:22 -0800
        Re: harder case for optimization BartC <bc@freeuk.com> - 2014-11-29 16:58 +0000
          Re: harder case for optimization fir <profesor.fir@gmail.com> - 2014-11-29 09:35 -0800
          Re: harder case for optimization Malcolm McLean <malcolm.mclean5@btinternet.com> - 2014-11-29 10:55 -0800
            Re: harder case for optimization BartC <bc@freeuk.com> - 2014-11-29 21:00 +0000
              Re: harder case for optimization Ian Collins <ian-news@hotmail.com> - 2014-11-30 11:25 +1300
                Re: harder case for optimization BartC <bc@freeuk.com> - 2014-11-29 22:56 +0000
                Re: harder case for optimization Ian Collins <ian-news@hotmail.com> - 2014-11-30 12:00 +1300
                Re: harder case for optimization fir <profesor.fir@gmail.com> - 2014-11-29 16:01 -0800
                Re: harder case for optimization fir <profesor.fir@gmail.com> - 2014-11-29 16:21 -0800
                Re: harder case for optimization fir <profesor.fir@gmail.com> - 2014-11-29 16:52 -0800
                Re: harder case for optimization fir <profesor.fir@gmail.com> - 2014-11-29 17:14 -0800
            Re: harder case for optimization glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2014-11-30 00:20 +0000
              Re: harder case for optimization Malcolm McLean <malcolm.mclean5@btinternet.com> - 2014-11-29 16:43 -0800
        Re: harder case for optimization Kaz Kylheku <kaz@kylheku.com> - 2014-11-29 17:29 +0000
  Re: harder case for optimization fir <profesor.fir@gmail.com> - 2014-12-05 02:12 -0800

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