Re: in place swap of float types

From	Keith Thompson <kst-u@mib.org>
Newsgroups	comp.lang.c
Subject	Re: in place swap of float types
Date	2019-08-28 13:01 -0700
Organization	None to speak of
Message-ID	<lnk1axxdnu.fsf@kst-u.example.com> (permalink)
References	<slrnqmaoja.4b2.3queens@cpe-65-25-200-198.new.res.rr.com> <qk5auh$iso$1@dont-email.me> <ybt9F.186029$Nw3.39237@fx40.am4> <f4eedf3e-7145-4c21-9e52-c2ec96a04727@googlegroups.com>

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James Kuyper <jameskuyper@alumni.caltech.edu> writes:
> On Wednesday, August 28, 2019 at 7:03:05 AM UTC-4, Bart wrote:
>> On 28/08/2019 08:36, David Brown wrote:
> ...
>> > No, such tricks are not useful - if space is really tight, write clear
>> > sensible code and work /with/ your compiler, rather than trying to
>> > outsmart it.
>> 
>> I don't understand why anyone would think this xor trick was any faster 
>> than using the conventional swap idiom using a temporary.
>
> The xor trick has always been about minimizing memory usage (including
> registers) - not minimizing execution time.
>
> And as David pointed out, it's of doubtful usefulness, even for that
> purpose, in high-level languages like C. The compiler generally knows
> better than the programmer how to best use the available registers, and
> languages like C don't provide the level of detailed control needed to
> do a significantly better job. Only the best assembly language
> programmers can do a significantly better job than the best compilers -
> because the expertise of the best assembly language programmers has, to
> the extent possible, been built into the best compilers.

Another problem that I don't think anyone has pointed out in this thread
is that it fails if you try to swap an object with itself -- for example
swapping *p and *q, where p and q are pointers to the same object, or
swapping arr[i] and arr[j] where i==j.

Most algorithms, if implemented correctly, won't try to swap an object
with itself, but it's something to consider.

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
Will write code for food.
void Void(void) { Void(); } /* The recursive call of the void */

Thread

in place swap of float types 3queens@shabbyroad.com - 2019-08-27 17:03 +0000
  Re: in place swap of float types Jorgen Grahn <grahn+nntp@snipabacken.se> - 2019-08-27 19:22 +0000
    Re: in place swap of float types Andrew Smallshaw <andrews@sdf.org> - 2019-09-02 15:25 +0000
      Re: in place swap of float types Jorgen Grahn <grahn+nntp@snipabacken.se> - 2019-09-02 19:37 +0000
        Re: in place swap of float types Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2019-09-03 06:37 -0700
          Re: in place swap of float types David Brown <david.brown@hesbynett.no> - 2019-09-03 16:13 +0200
            Re: in place swap of float types Bart <bc@freeuk.com> - 2019-09-03 16:54 +0100
              Re: in place swap of float types David Brown <david.brown@hesbynett.no> - 2019-09-04 11:19 +0200
                Re: in place swap of float types Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2019-09-04 02:35 -0700
                Re: in place swap of float types David Brown <david.brown@hesbynett.no> - 2019-09-04 12:31 +0200
                Re: in place swap of float types Bart <bc@freeuk.com> - 2019-09-04 12:12 +0100
                Re: in place swap of float types David Brown <david.brown@hesbynett.no> - 2019-09-04 15:53 +0200
                Re: in place swap of float types Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2019-09-04 07:17 -0700
                Re: in place swap of float types David Brown <david.brown@hesbynett.no> - 2019-09-05 15:12 +0200
                Re: in place swap of float types Ben Bacarisse <ben.usenet@bsb.me.uk> - 2019-09-05 21:11 +0100
                Re: in place swap of float types Bart <bc@freeuk.com> - 2019-09-04 16:30 +0100
                Re: in place swap of float types jadill33@gmail.com - 2019-09-04 10:26 -0700
                Re: in place swap of float types Ike Naar <ike@iceland.freeshell.org> - 2019-09-05 06:00 +0000
                Re: in place swap of float types Bart <bc@freeuk.com> - 2019-09-05 11:14 +0100
                Re: in place swap of float types David Brown <david.brown@hesbynett.no> - 2019-09-05 14:54 +0200
                Re: in place swap of float types Bart <bc@freeuk.com> - 2019-09-05 15:48 +0100
                Re: in place swap of float types David Brown <david.brown@hesbynett.no> - 2019-09-06 10:35 +0200
                Re: in place swap of float types Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2019-09-06 02:52 -0700
                Re: in place swap of float types David Brown <david.brown@hesbynett.no> - 2019-09-06 13:18 +0200
                Re: in place swap of float types Bart <bc@freeuk.com> - 2019-09-06 12:14 +0100
                Re: in place swap of float types David Brown <david.brown@hesbynett.no> - 2019-09-06 15:08 +0200
                Re: in place swap of float types Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2019-09-06 06:23 -0700
                Re: in place swap of float types Keith Thompson <kst-u@mib.org> - 2019-09-04 14:16 -0700
                Re: in place swap of float types Bart <bc@freeuk.com> - 2019-09-04 22:42 +0100
                Re: in place swap of float types Keith Thompson <kst-u@mib.org> - 2019-09-04 15:21 -0700
                Re: in place swap of float types David Brown <david.brown@hesbynett.no> - 2019-09-05 15:15 +0200
              Re: in place swap of float types Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2019-09-04 08:05 -0600
                Re: in place swap of float types David Brown <david.brown@hesbynett.no> - 2019-09-05 15:17 +0200
                Re: in place swap of float types Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2019-09-05 20:28 -0600
                Re: in place swap of float types David Brown <david.brown@hesbynett.no> - 2019-09-06 10:39 +0200
                Re: in place swap of float types Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2019-09-06 08:11 -0600
                Re: in place swap of float types scott@slp53.sl.home (Scott Lurndal) - 2019-09-06 18:31 +0000
          Re: in place swap of float types Jorgen Grahn <grahn+nntp@snipabacken.se> - 2019-09-04 19:02 +0000
      Re: in place swap of float types David Brown <david.brown@hesbynett.no> - 2019-09-02 22:16 +0200
  Re: in place swap of float types Robert Wessel <robertwessel2@yahoo.com> - 2019-08-27 17:59 -0500
    Re: in place swap of float types Richard Damon <Richard@Damon-Family.org> - 2019-08-27 17:45 -0700
    Re: in place swap of float types James Kuyper <jameskuyper@alumni.caltech.edu> - 2019-08-27 20:48 -0400
      Re: in place swap of float types Tim Rentsch <tr.17687@z991.linuxsc.com> - 2019-09-03 06:17 -0700
    Re: in place swap of float types Ben Bacarisse <ben.usenet@bsb.me.uk> - 2019-08-28 02:09 +0100
    Re: in place swap of float types Siri Cruise <chine.bleu@yahoo.com> - 2019-08-27 21:26 -0700
    Re: in place swap of float types David Brown <david.brown@hesbynett.no> - 2019-08-28 09:22 +0200
    Re: in place swap of float types Andrey Tarasevich <andreytarasevich@hotmail.com> - 2019-08-28 16:36 -0700
  Re: in place swap of float types David Brown <david.brown@hesbynett.no> - 2019-08-28 09:36 +0200
    Re: in place swap of float types Bart <bc@freeuk.com> - 2019-08-28 12:02 +0100
      Re: in place swap of float types James Kuyper <jameskuyper@alumni.caltech.edu> - 2019-08-28 05:08 -0700
        Re: in place swap of float types Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2019-08-28 05:26 -0700
        Re: in place swap of float types David Brown <david.brown@hesbynett.no> - 2019-08-28 14:40 +0200
        Re: in place swap of float types Keith Thompson <kst-u@mib.org> - 2019-08-28 13:01 -0700
    Re: in place swap of float types Philipp Klaus Krause <pkk@spth.de> - 2019-09-04 11:18 +0200
      Re: in place swap of float types David Brown <david.brown@hesbynett.no> - 2019-09-04 12:57 +0200
        Re: in place swap of float types Philipp Klaus Krause <pkk@spth.de> - 2019-09-04 13:24 +0200
  Re: in place swap of float types Bonita Montero <Bonita.Montero@gmail.com> - 2019-09-05 08:18 +0200
    Re: in place swap of float types David Brown <david.brown@hesbynett.no> - 2019-09-05 15:19 +0200
      Re: in place swap of float types Bonita Montero <Bonita.Montero@gmail.com> - 2019-09-05 16:18 +0200
        Re: in place swap of float types Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2019-09-05 07:32 -0700
          Re: in place swap of float types Bonita Montero <Bonita.Montero@gmail.com> - 2019-09-05 17:32 +0200
          Re: in place swap of float types David Brown <david.brown@hesbynett.no> - 2019-09-06 10:54 +0200
            Re: in place swap of float types Bonita Montero <Bonita.Montero@gmail.com> - 2019-09-06 14:23 +0200

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