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Re: The container abstraction and parallel programming

From jacob navia <jacob@spamsink.net>
Newsgroups comp.lang.c
Subject Re: The container abstraction and parallel programming
Date 2012-01-08 11:48 +0100
Organization lccwin
Message-ID <jebs9q$dd2$1@speranza.aioe.org> (permalink)
References <je7uhf$hjj$1@speranza.aioe.org> <9mpj60Fdr5U5@mid.individual.net> <je8vto$bi6$1@speranza.aioe.org> <je9hu7$he1$2@dont-email.me>

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Le 07/01/12 14:39, Stephen Sprunk a écrit :
>
> There are no "dependencies" between array members.  There is a possible
> aliasing problem when dealing with pointers rather than arrays, but that
> is easily enough solved with "restrict" qualifiers.
>
> I don't understand what you think the problem is.
>
> S
>

The problem, Stephen, is as follows:

gcc will vectorize this:

void doADD(double *L, double *R, double *result,size_t n)
{
      for(size_t i=0; i<n;i++) result[i]=L[i]+R[i];
}

but will NOT vectorize this:

void doADD(double *L, double *R, double *result,size_t n)
{
      for(size_t i=0; i<n;i++) result[n-i]=L[n-i]+R[n-i];
}

Why?

As you (correctly) say:

 > There are no "dependencies" between array members.

All this huge machinery for automatic vectorizing has never worked
correctly for dozens of years since requires an incredible
amount of program analysis. My proposition is:

Why make complicated something that you can do in a simple way?

Let the programmer declare the data that should be handled in an SIMD
way.

If I write:

	c = (a+b)/2;
	if (c&1) fn(1);

You could deduce that c must be of integer type isn't it?
LET THE COMPILER DO THAT. Let's eliminate declarations for
integer and let the compiler deduce from the program text
the type of each variable.

Looks ridiculous isn't it?

Well, for SIMD the problem is the same.

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Thread

The container abstraction and parallel programming jacob navia <jacob@spamsink.net> - 2012-01-07 00:02 +0100
  Re: The container abstraction and parallel programming blp@cs.stanford.edu (Ben Pfaff) - 2012-01-06 15:08 -0800
    Re: The container abstraction and parallel programming jacob navia <jacob@spamsink.net> - 2012-01-07 00:27 +0100
      Re: The container abstraction and parallel programming blp@cs.stanford.edu (Ben Pfaff) - 2012-01-06 15:30 -0800
        Re: The container abstraction and parallel programming jacob navia <jacob@spamsink.net> - 2012-01-07 00:41 +0100
  Re: The container abstraction and parallel programming Kaz Kylheku <kaz@kylheku.com> - 2012-01-07 00:04 +0000
    Re: The container abstraction and parallel programming jacob navia <jacob@spamsink.net> - 2012-01-07 09:06 +0100
    Re: The container abstraction and parallel programming Nick Keighley <nick_keighley_nospam@hotmail.com> - 2012-01-08 03:42 -0800
  Re: The container abstraction and parallel programming Ian Collins <ian-news@hotmail.com> - 2012-01-07 13:50 +1300
    Re: The container abstraction and parallel programming jacob navia <jacob@spamsink.net> - 2012-01-07 09:31 +0100
      Re: The container abstraction and parallel programming Ian Collins <ian-news@hotmail.com> - 2012-01-07 22:03 +1300
      Re: The container abstraction and parallel programming Stephen Sprunk <stephen@sprunk.org> - 2012-01-07 07:39 -0600
        Re: The container abstraction and parallel programming Gene <gene.ressler@gmail.com> - 2012-01-07 09:27 -0800
          Re: The container abstraction and parallel programming Stephen Sprunk <stephen@sprunk.org> - 2012-01-08 11:37 -0600
            Re: The container abstraction and parallel programming jacob navia <jacob@spamsink.net> - 2012-01-08 18:42 +0100
        Re: The container abstraction and parallel programming jacob navia <jacob@spamsink.net> - 2012-01-08 11:48 +0100
          Re: The container abstraction and parallel programming ec429 <ec429@cam.ac.uk> - 2012-01-08 11:16 +0000
          Re: The container abstraction and parallel programming Stephen Sprunk <stephen@sprunk.org> - 2012-01-08 11:52 -0600
            Re: The container abstraction and parallel programming Keith Thompson <kst-u@mib.org> - 2012-01-08 14:17 -0800
              Re: The container abstraction and parallel programming pete <pfiland@mindspring.com> - 2012-01-08 22:08 -0500
              Re: The container abstraction and parallel programming Stephen Sprunk <stephen@sprunk.org> - 2012-01-09 11:07 -0600
                Re: The container abstraction and parallel programming Keith Thompson <kst-u@mib.org> - 2012-01-09 14:55 -0800
            Re: The container abstraction and parallel programming jacob navia <jacob@spamsink.net> - 2012-01-09 00:22 +0100
          Re: The container abstraction and parallel programming Nobody <nobody@nowhere.com> - 2012-01-09 14:19 +0000
            Re: The container abstraction and parallel programming Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-01-09 16:40 +0000
            Re: The container abstraction and parallel programming Kaz Kylheku <kaz@kylheku.com> - 2012-01-09 17:53 +0000
            Re: The container abstraction and parallel programming Gene <gene.ressler@gmail.com> - 2012-01-10 16:58 -0800
  Re: The container abstraction and parallel programming Robert Wessel <robertwessel2@yahoo.com> - 2012-01-07 01:28 -0600
  Re: The container abstraction and parallel programming Jens Gustedt <jens.gustedt@loria.fr> - 2012-01-07 09:39 +0100
    Re: The container abstraction and parallel programming jacob navia <jacob@spamsink.net> - 2012-01-07 09:54 +0100
      Re: The container abstraction and parallel programming Jens Gustedt <jens.gustedt@loria.fr> - 2012-01-07 11:36 +0100
  Re: The container abstraction and parallel programming Gareth Owen <gwowen@gmail.com> - 2012-01-08 08:08 +0000
    Re: The container abstraction and parallel programming jacob navia <jacob@spamsink.net> - 2012-01-08 11:41 +0100
      Re: The container abstraction and parallel programming jacob navia <jacob@spamsink.net> - 2012-01-08 11:53 +0100
      Re: The container abstraction and parallel programming jacob navia <jacob@spamsink.net> - 2012-01-08 11:59 +0100
      Re: The container abstraction and parallel programming ec429 <ec429@cam.ac.uk> - 2012-01-08 11:05 +0000
        Re: The container abstraction and parallel programming jacob navia <jacob@spamsink.net> - 2012-01-08 12:29 +0100
          Re: The container abstraction and parallel programming ec429 <ec429@cam.ac.uk> - 2012-01-08 13:35 +0000
      Re: The container abstraction and parallel programming gwowen <gwowen@gmail.com> - 2012-01-09 01:09 -0800
      Re: The container abstraction and parallel programming Tim Rentsch <txr@alumni.caltech.edu> - 2012-01-26 09:23 -0800
        Re: The container abstraction and parallel programming jacob navia <jacob@spamsink.net> - 2012-01-26 23:41 +0100
          Re: The container abstraction and parallel programming Tim Rentsch <txr@alumni.caltech.edu> - 2012-03-07 16:18 -0800

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