Re: Learning functions; roots of the quadratic; two return values

From	Melzzzzz <mel@zzzzz.com>
Newsgroups	comp.lang.c
Subject	Re: Learning functions; roots of the quadratic; two return values
Date	2015-10-08 00:53 +0200
Organization	albasani.net
Message-ID	<20151008005326.47e5ac1f@maxa-pc> (permalink)
References	(8 earlier) <muvg2s$ncq$1@speranza.aioe.org> <mv0n91$v9l$1@dont-email.me> <mv1ksu$q4n$1@speranza.aioe.org> <20151007051930.3973122b@maxa-pc> <lnmvvuk68e.fsf@kst-u.example.com>

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On Wed, 07 Oct 2015 09:12:33 -0700
Keith Thompson <kst-u@mib.org> wrote:

> Melzzzzz <mel@zzzzz.com> writes:
> > On Tue, 6 Oct 2015 23:18:54 +0000 (UTC)
> > glen herrmannsfeldt <gah@ugcs.caltech.edu> wrote:
> >
> >> 
> >>     delta=b*b-4*a*c;
> >>     if(delta<0) exit(123);
> >>     x1=(-b-sqrt(delta))/(2*a);
> >>     x2=(-b+sqrt(delta))/(2*a);
> >> 
> >> note that this avoids the double use of delta, and is otherwise
> >> likely more readable.  But it has two calls to sqrt(), and 
> >> everyone knows that sqrt() is a slow function. Because of this,
> >> we should hand optimize to the form above, either with one or
> >> two variables.  (not seriously)
> >> 
> >> But okay, we should trust the compiler to figure out that only
> >> one sqrt() is needed. And sqrt() really isn't all that slow if
> >> we are only doing millions of sqrt(), not billions or trillions.
> >
> > How can compiler figure out that? In C++ there is constexpr
> > annotation which helps compiler in such cases and other languages
> > have similar (eg D pure), but not C.
> 
> The behavior of the sqrt() function is defined by the standard.  A
> compiler can take advantage of the fact that calling sqrt() twice with
> the same argument will yield the same result both times, and will not
> have side effects.
> 

Yes, I see that compiler (gcc) takes advantage of knowing contents of
sqrt, but there is some weird constraint that make it generate calls
for several sqrts anyway. That is it does not calls function first,
rather it executes sqrt instruction directly, but then it checks
result with itself (ucomisd) and if parity flag is set (unordered) it
calls sqrt then checks result again and if parity calls sqrt again.
This code is not present if -ffast-math or -Ofast flag is present.

Thread

Learning functions; roots of the quadratic; two return values Alla _ <modelling.data@gmail.com> - 2015-10-03 10:43 -0700
  Re: Learning functions; roots of the quadratic; two return values Andrew Cooper <amc96@cam.ac.uk> - 2015-10-03 19:00 +0100
    Re: Learning functions; roots of the quadratic; two return values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-10-03 19:34 +0100
      Re: Learning functions; roots of the quadratic; two return values regis <regis.barbanchon@free.fr> - 2015-10-03 21:29 +0200
        Re: Learning functions; roots of the quadratic; two return values regis <regis.barbanchon@free.fr> - 2015-10-03 21:36 +0200
        Re: Learning functions; roots of the quadratic; two return values regis <regis.barbanchon@free.fr> - 2015-10-03 21:43 +0200
        Re: Learning functions; roots of the quadratic; two return values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-10-03 20:46 +0100
          Re: Learning functions; roots of the quadratic; two return values regis <regis.barbanchon@free.fr> - 2015-10-03 22:46 +0200
            Re: Learning functions; roots of the quadratic; two return values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-10-03 23:18 +0100
              Re: Learning functions; roots of the quadratic; two return values regis <regis.barbanchon@free.fr> - 2015-10-04 01:08 +0200
              Re: Learning functions; roots of the quadratic; two return values regis <regis.barbanchon@free.fr> - 2015-10-04 01:36 +0200
                Re: Learning functions; roots of the quadratic; two return values Ian Collins <ian-news@hotmail.com> - 2015-10-04 12:52 +1300
                Re: Learning functions; roots of the quadratic; two return values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-10-04 15:42 +0100
          Re: Learning functions; roots of the quadratic; two return values jacobnavia <jacob@jacob.remcomp.fr> - 2015-10-03 23:05 +0200
            Re: Learning functions; roots of the quadratic; two return values regis <regis.barbanchon@free.fr> - 2015-10-03 23:41 +0200
              Re: Learning functions; roots of the quadratic; two return values jacobnavia <jacob@jacob.remcomp.fr> - 2015-10-04 00:14 +0200
            Re: Learning functions; roots of the quadratic; two return values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-10-03 23:07 +0100
              Re: Learning functions; roots of the quadratic; two return values jacobnavia <jacob@jacob.remcomp.fr> - 2015-10-04 00:32 +0200
              Re: Learning functions; roots of the quadratic; two return values Keith Thompson <kst-u@mib.org> - 2015-10-04 12:56 -0700
                Re: Learning functions; roots of the quadratic; two return values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-10-04 23:41 +0100
                Re: Learning functions; roots of the quadratic; two return values Keith Thompson <kst-u@mib.org> - 2015-10-04 18:24 -0700
              Re: Learning functions; roots of the quadratic; two return values James Kuyper <jameskuyper@verizon.net> - 2015-10-04 16:36 -0400
                Re: Learning functions; roots of the quadratic; two return values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-10-05 00:01 +0100
                Re: Learning functions; roots of the quadratic; two return values James Kuyper <jameskuyper@verizon.net> - 2015-10-04 19:24 -0400
          Re: Learning functions; roots of the quadratic; two return values Ian Collins <ian-news@hotmail.com> - 2015-10-04 10:18 +1300
            Re: Learning functions; roots of the quadratic; two return values jacobnavia <jacob@jacob.remcomp.fr> - 2015-10-03 23:27 +0200
              Re: Learning functions; roots of the quadratic; two return values Ian Collins <ian-news@hotmail.com> - 2015-10-04 11:08 +1300
                Re: Learning functions; roots of the quadratic; two return values Richard Damon <Richard@Damon-Family.org> - 2015-10-03 18:38 -0400
                Re: Learning functions; roots of the quadratic; two return values Ian Collins <ian-news@hotmail.com> - 2015-10-04 11:47 +1300
          Re: Learning functions; roots of the quadratic; two return values raltbos@xs4all.nl (Richard Bos) - 2015-10-04 14:07 +0000
            Re: Learning functions; roots of the quadratic; two return values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-10-04 15:48 +0100
              Re: Learning functions; roots of the quadratic; two return values regis <regis.barbanchon@free.fr> - 2015-10-04 18:59 +0200
                Re: Learning functions; roots of the quadratic; two return values regis <regis.barbanchon@free.fr> - 2015-10-04 20:06 +0200
              Re: Learning functions; roots of the quadratic; two return values Tim Rentsch <txr@alumni.caltech.edu> - 2015-10-04 15:21 -0700
                Re: Learning functions; roots of the quadratic; two return values glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-10-05 05:25 +0000
        Re: Learning functions; roots of the quadratic; two return values regis <regis.barbanchon@free.fr> - 2015-10-03 21:48 +0200
          Re: Learning functions; roots of the quadratic; two return values regis <regis.barbanchon@free.fr> - 2015-10-04 00:31 +0200
    Re: Learning functions; roots of the quadratic; two return values alla.rashitova@gmail.com - 2015-10-05 08:40 -0700
      Re: Learning functions; roots of the quadratic; two return values Richard Heathfield <rjh@cpax.org.uk> - 2015-10-05 17:10 +0100
      Re: Learning functions; roots of the quadratic; two return values Barry Schwarz <schwarzb@dqel.com> - 2015-10-05 10:41 -0700
  Re: Learning functions; roots of the quadratic; two return values "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-10-03 12:04 -0700
    Re: Learning functions; roots of the quadratic; two return values Philip Lantz <prl@canterey.us> - 2015-10-03 12:48 -0700
    Re: Learning functions; roots of the quadratic; two return values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-10-03 20:55 +0100
      Re: Learning functions; roots of the quadratic; two return values "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-10-03 13:27 -0700
      Re: Learning functions; roots of the quadratic; two return values jacobnavia <jacob@jacob.remcomp.fr> - 2015-10-03 23:08 +0200
        Re: Learning functions; roots of the quadratic; two return values Richard Heathfield <rjh@cpax.org.uk> - 2015-10-03 22:52 +0100
    Re: Learning functions; roots of the quadratic; two return values Rosario19 <Ros@invalid.invalid> - 2015-10-04 20:46 +0200
      Re: Learning functions; roots of the quadratic; two return values Keith Thompson <kst-u@mib.org> - 2015-10-04 15:47 -0700
      Re: Learning functions; roots of the quadratic; two return values regis <regis.barbanchon@free.fr> - 2015-10-05 03:06 +0200
        Re: Learning functions; roots of the quadratic; two return values Rosario19 <Ros@invalid.invalid> - 2015-10-05 05:31 +0200
          Re: Learning functions; roots of the quadratic; two return values Rosario19 <Ros@invalid.invalid> - 2015-10-05 11:58 +0200
            Re: Learning functions; roots of the quadratic; two return values Rosario19 <Ros@invalid.invalid> - 2015-10-05 12:00 +0200
              Re: Learning functions; roots of the quadratic; two return values Rosario19 <Ros@invalid.invalid> - 2015-10-05 12:06 +0200
          Re: Learning functions; roots of the quadratic; two return values regis <regis.barbanchon@free.fr> - 2015-10-05 12:39 +0200
            Re: Learning functions; roots of the quadratic; two return values Bartc <bc@freeuk.com> - 2015-10-05 12:20 +0100
              Re: Learning functions; roots of the quadratic; two return values regis <regis.barbanchon@free.fr> - 2015-10-05 13:50 +0200
              Re: Learning functions; roots of the quadratic; two return values Richard Heathfield <rjh@cpax.org.uk> - 2015-10-05 12:57 +0100
        Re: Learning functions; roots of the quadratic; two return values glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-10-05 05:40 +0000
          Re: Learning functions; roots of the quadratic; two return values regis <regis.barbanchon@free.fr> - 2015-10-05 08:30 +0200
          Re: Learning functions; roots of the quadratic; two return values Philip Lantz <prl@canterey.us> - 2015-10-05 10:03 -0700
            Re: Learning functions; roots of the quadratic; two return values glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-10-05 22:25 +0000
              Re: Learning functions; roots of the quadratic; two return values James Kuyper <jameskuyper@verizon.net> - 2015-10-05 19:26 -0400
                Re: Learning functions; roots of the quadratic; two return values glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-10-06 03:44 +0000
                Re: Learning functions; roots of the quadratic; two return values James Kuyper <jameskuyper@verizon.net> - 2015-10-06 10:55 -0400
                Re: Learning functions; roots of the quadratic; two return values regis <regis.barbanchon@free.fr> - 2015-10-06 23:07 +0200
                Re: Learning functions; roots of the quadratic; two return values James Kuyper <jameskuyper@verizon.net> - 2015-10-06 18:35 -0400
                Re: Learning functions; roots of the quadratic; two return values raltbos@xs4all.nl (Richard Bos) - 2015-10-14 11:08 +0000
                Re: Learning functions; roots of the quadratic; two return values James Kuyper <jameskuyper@verizon.net> - 2015-10-14 11:11 -0400
                Re: Learning functions; roots of the quadratic; two return values regis <regis.barbanchon@free.fr> - 2015-10-14 22:22 +0200
                Re: Learning functions; roots of the quadratic; two return values James Kuyper <jameskuyper@verizon.net> - 2015-10-14 17:17 -0400
                Re: Learning functions; roots of the quadratic; two return values regis <regis.barbanchon@free.fr> - 2015-10-14 23:45 +0200
                Re: Learning functions; roots of the quadratic; two return values glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-10-15 00:11 +0000
                Re: Learning functions; roots of the quadratic; two return values Bartc <bc@freeuk.com> - 2015-10-15 01:37 +0100
                Re: Learning functions; roots of the quadratic; two return values glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-10-15 05:52 +0000
                Re: Learning functions; roots of the quadratic; two return values James Kuyper <jameskuyper@verizon.net> - 2015-10-14 21:54 -0400
                Re: Learning functions; roots of the quadratic; two return values asetofsymbols@gmail.com - 2015-10-14 22:40 -0700
                Re: Learning functions; roots of the quadratic; two return values asetofsymbols@gmail.com - 2015-10-15 05:00 -0700
                Re: Learning functions; roots of the quadratic; two return values supercat@casperkitty.com - 2015-10-15 11:58 -0700
                Re: Learning functions; roots of the quadratic; two return values glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-10-15 06:04 +0000
                Re: Learning functions; roots of the quadratic; two return values glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-10-06 23:18 +0000
                Re: Learning functions; roots of the quadratic; two return values supercat@casperkitty.com - 2015-10-06 16:31 -0700
                Re: Learning functions; roots of the quadratic; two return values glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-10-07 00:38 +0000
                Re: Learning functions; roots of the quadratic; two return values supercat@casperkitty.com - 2015-10-06 21:19 -0700
                Re: Learning functions; roots of the quadratic; two return values James Kuyper <jameskuyper@verizon.net> - 2015-10-06 22:15 -0400
                Re: Learning functions; roots of the quadratic; two return values glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-10-07 08:45 +0000
                Re: Learning functions; roots of the quadratic; two return values Melzzzzz <mel@zzzzz.com> - 2015-10-07 05:19 +0200
                Re: Learning functions; roots of the quadratic; two return values Keith Thompson <kst-u@mib.org> - 2015-10-07 09:12 -0700
                Re: Learning functions; roots of the quadratic; two return values Melzzzzz <mel@zzzzz.com> - 2015-10-08 00:53 +0200
                Re: Learning functions; roots of the quadratic; two return values Keith Thompson <kst-u@mib.org> - 2015-10-07 16:23 -0700
              Re: Learning functions; roots of the quadratic; two return values regis <regis.barbanchon@free.fr> - 2015-10-06 02:16 +0200
                Re: Learning functions; roots of the quadratic; two return values supercat@casperkitty.com - 2015-10-06 15:43 -0700
                Re: Learning functions; roots of the quadratic; two return values regis <regis.barbanchon@free.fr> - 2015-10-07 02:48 +0200
                Re: Learning functions; roots of the quadratic; two return values Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-10-14 14:00 -0700
                Re: Learning functions; roots of the quadratic; two return values regis <regis.barbanchon@free.fr> - 2015-10-14 23:23 +0200
  Re: Learning functions; roots of the quadratic; two return values Geoff <geoff@invalid.invalid> - 2015-10-03 12:50 -0700
  Re: Learning functions; roots of the quadratic; two return values Bartc <bc@freeuk.com> - 2015-10-03 21:16 +0100
    Re: Learning functions; roots of the quadratic; two return values alla.rashitova@gmail.com - 2015-10-05 09:02 -0700
      Re: Learning functions; roots of the quadratic; two return values Richard Heathfield <rjh@cpax.org.uk> - 2015-10-05 17:13 +0100
  Re: Learning functions; roots of the quadratic; two return values Barry Schwarz <schwarzb@dqel.com> - 2015-10-03 15:37 -0700
  Re: Learning functions; roots of the quadratic; two return values Rosario19 <Ros@invalid.invalid> - 2015-10-05 23:44 +0200
    Re: Learning functions; roots of the quadratic; two return values Geoff <geoff@invalid.invalid> - 2015-10-05 18:37 -0700
    Re: Learning functions; roots of the quadratic; two return values Barry Schwarz <schwarzb@dqel.com> - 2015-10-05 19:24 -0700
      Re: Learning functions; roots of the quadratic; two return values Rosario19 <Ros@invalid.invalid> - 2015-10-06 07:36 +0200
    Re: Learning functions; roots of the quadratic; two return values Rosario19 <Ros@invalid.invalid> - 2015-10-06 07:34 +0200

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