Re: Function pointer vs conditional branch

From	Dr Nick <3-nospam@temporary-address.org.uk>
Newsgroups	comp.lang.c
Subject	Re: Function pointer vs conditional branch
Date	2012-01-06 06:44 +0000
Message-ID	<8739bt49em.fsf@temporary-address.org.uk> (permalink)
References	<je2jb2$886$1@speranza.aioe.org> <pan.2012.01.05.21.29.55.1000@nowhere.com>

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Nobody <nobody@nowhere.com> writes:

> On Wed, 04 Jan 2012 22:20:18 +0000, Edward Rutherford wrote:
>
>> As a general rule-of-thumb: Are function pointers more efficient than if 
>> statements, if the same path will be taken throughout the course of the 
>> program?
>
> I suspect that the conditionals will be more efficient in most cases.

I suspect we don't know enough.  I'm sure this is the case if it's a
choice between doing a simple "if x" do this within a 20 iterations
loop, or do that on one hand and doing the same thing once before the
loop to set a function pointer, the if will win.

If it's a nest of 50 if statements to pick one of 17 different
operations, and some of the ifs involve calculating the same thing each
time, and there are billions of iterations of the loop, the function
pointer might win.

Then, of course, you pull the nest of ifs out of the loop, case it
inside the loop and let the compiler build a table if it thinks it's the
best thing.

The if is almost certainly easier to read, and that's a form of
efficiency.
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Thread

Function pointer vs conditional branch Edward Rutherford <edward.p.rutherford79@REMOVETHIS.gmail.com> - 2012-01-04 22:20 +0000
  Re: Function pointer vs conditional branch Kaz Kylheku <kaz@kylheku.com> - 2012-01-04 22:46 +0000
  Re: Function pointer vs conditional branch Ian Collins <ian-news@hotmail.com> - 2012-01-05 11:47 +1300
  Re: Function pointer vs conditional branch Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-01-04 20:15 -0500
    Re: Function pointer vs conditional branch Kaz Kylheku <kaz@kylheku.com> - 2012-01-05 01:25 +0000
      Re: Function pointer vs conditional branch Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-01-04 20:36 -0500
        Re: Function pointer vs conditional branch Kaz Kylheku <kaz@kylheku.com> - 2012-01-05 06:02 +0000
  Re: Function pointer vs conditional branch "BartC" <bc@freeuk.com> - 2012-01-05 01:25 +0000
  Re: Function pointer vs conditional branch Gene <gene.ressler@gmail.com> - 2012-01-04 17:57 -0800
  Re: Function pointer vs conditional branch gordonb.vj0cm@burditt.org (Gordon Burditt) - 2012-01-04 23:54 -0600
  Re: Function pointer vs conditional branch 88888 Dihedral <dihedral88888@googlemail.com> - 2012-01-04 23:25 -0800
  Re: Function pointer vs conditional branch 88888 Dihedral <dihedral88888@googlemail.com> - 2012-01-04 23:38 -0800
  Re: Function pointer vs conditional branch Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2012-01-05 13:47 -0700
  Re: Function pointer vs conditional branch Nobody <nobody@nowhere.com> - 2012-01-05 21:29 +0000
    Re: Function pointer vs conditional branch Dr Nick <3-nospam@temporary-address.org.uk> - 2012-01-06 06:44 +0000
  Re: Function pointer vs conditional branch BGB <cr88192@hotmail.com> - 2012-01-06 11:41 -0600
    Re: Function pointer vs conditional branch Kaz Kylheku <kaz@kylheku.com> - 2012-01-06 18:33 +0000
      Re: Function pointer vs conditional branch Willem <willem@toad.stack.nl> - 2012-01-06 21:41 +0000
        Re: Function pointer vs conditional branch Stephen Sprunk <stephen@sprunk.org> - 2012-01-06 16:08 -0600
        Re: Function pointer vs conditional branch Kaz Kylheku <kaz@kylheku.com> - 2012-01-06 22:35 +0000
          Re: Function pointer vs conditional branch BGB <cr88192@hotmail.com> - 2012-01-06 17:00 -0600
            Re: Function pointer vs conditional branch Kaz Kylheku <kaz@kylheku.com> - 2012-01-06 23:44 +0000
          Re: Function pointer vs conditional branch Stephen Sprunk <stephen@sprunk.org> - 2012-01-06 17:08 -0600
            Re: Function pointer vs conditional branch Kaz Kylheku <kaz@kylheku.com> - 2012-01-06 23:49 +0000
              Re: Function pointer vs conditional branch jacob navia <jacob@spamsink.net> - 2012-01-07 00:55 +0100
                Re: Function pointer vs conditional branch Kaz Kylheku <kaz@kylheku.com> - 2012-01-07 00:07 +0000
        Re: Function pointer vs conditional branch "christian.bau" <christian.bau@cbau.wanadoo.co.uk> - 2012-01-07 06:14 -0800
          Re: Function pointer vs conditional branch "BartC" <bc@freeuk.com> - 2012-01-07 14:44 +0000
            Re: Function pointer vs conditional branch Robert Wessel <robertwessel2@yahoo.com> - 2012-01-07 14:37 -0600
              Re: Function pointer vs conditional branch Kaz Kylheku <kaz@kylheku.com> - 2012-01-07 20:42 +0000
                Re: Function pointer vs conditional branch Stephen Sprunk <stephen@sprunk.org> - 2012-01-07 16:47 -0600
                Re: Function pointer vs conditional branch Robert Wessel <robertwessel2@yahoo.com> - 2012-01-08 01:49 -0600
                Re: Function pointer vs conditional branch Kaz Kylheku <kaz@kylheku.com> - 2012-01-08 08:35 +0000
            Re: Function pointer vs conditional branch Stephen Sprunk <stephen@sprunk.org> - 2012-01-07 17:02 -0600
    Re: Function pointer vs conditional branch "BartC" <bc@freeuk.com> - 2012-01-06 21:29 +0000
      Re: Function pointer vs conditional branch BGB <cr88192@hotmail.com> - 2012-01-06 17:15 -0600
  Re: Function pointer vs conditional branch Michael Angelo Ravera <maravera@prodigy.net> - 2012-01-06 23:00 -0800
  Re: Function pointer vs conditional branch jgk@panix.com (Joe keane) - 2012-01-07 21:04 +0000

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