Re: Short-circuiting in C

From	Nobody <nobody@nowhere.com>
Subject	Re: Short-circuiting in C
Date	2011-05-27 07:41 +0100
Message-Id	<pan.2011.05.27.06.41.10.672000@nowhere.com>
Newsgroups	comp.lang.c
References	<irm4et$66d$1@speranza.aioe.org> <pan.2011.05.26.23.06.27.516000@nowhere.com> <irmv36$4o3$1@speranza.aioe.org>
Organization	Zen Internet

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On Thu, 26 May 2011 21:34:24 -0400, Billy Mays wrote:

>> How does GPU programming relate to C programming?
>>
>> Sure, GLSL/HLSL/Cg are "C-like" languages, but they aren't C.
> 
> I'm using OpenCL's library which then invokes the GPU.  The programming 
> is done in a subset of C.

OpenCL may be closer to C than e.g. GLSL, but it still isn't C. And even
if it was, C doesn't define the execution time of any given construct, nor
treat it as significant (i.e. a compiler make whatever optimisations it
likes so long as the code produces the correct result, regardless of the
effect upon execution time).

AFAICT, the only reliable way to prevent short-circuting is to either
ensure that the later expressions have side-effects such that
short-circuiting would change the observable behaviour, or to "hide" their
implementation in a different translation unit so that the compiler cannot
determine whether short-circuiting will change the observable behaviour.

Either approach is likely to impose an unnecessary performance penalty on
either or both implementations (CPU and GPU). I don't know whether the
latter approach is even possible for OpenCL.

Actually, with modern CPUs performing speculative execution, I'm not sure
that the "hiding" approach will work. If the CPU hasn't performed the
calculation by the time it discovers that the result isn't needed, it
will just cancel it.

Thread

Short-circuiting in C Billy Mays <noway@nohow.com> - 2011-05-26 13:58 -0400
  Re: Short-circuiting in C Ben Pfaff <blp@cs.stanford.edu> - 2011-05-26 11:09 -0700
    Re: Short-circuiting in C Thomas Richter <thor@math.tu-berlin.de> - 2011-05-26 20:22 +0200
      Re: Short-circuiting in C Ben Pfaff <blp@cs.stanford.edu> - 2011-05-26 11:35 -0700
        Re: Short-circuiting in C falk@rahul.net (Edward A. Falk) - 2011-05-31 18:45 +0000
          Re: Short-circuiting in C Nizumzen <chicken@mcnuggets.com> - 2011-05-31 20:31 +0100
      Re: Short-circuiting in C Billy Mays <noway@nohow.com> - 2011-05-26 14:51 -0400
        Re: Short-circuiting in C Billy Mays <noway@nohow.com> - 2011-05-26 15:10 -0400
          Re: Short-circuiting in C Billy Mays <noway@nohow.com> - 2011-05-26 21:35 -0400
            Re: Short-circuiting in C Keith Thompson <kst-u@mib.org> - 2011-05-26 23:38 -0700
              Re: Short-circuiting in C Shao Miller <sha0.miller@gmail.com> - 2011-05-27 09:28 -0400
          Re: Short-circuiting in C falk@rahul.net (Edward A. Falk) - 2011-05-31 19:01 +0000
            Re: Short-circuiting in C falk@rahul.net (Edward A. Falk) - 2011-05-31 19:02 +0000
        Re: Short-circuiting in C Keith Thompson <kst-u@mib.org> - 2011-05-26 12:55 -0700
          Re: Short-circuiting in C Billy Mays <noway@nohow.com> - 2011-05-26 16:17 -0400
            Re: Short-circuiting in C Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-05-26 21:28 -0400
              Re: Short-circuiting in C Seebs <usenet-nospam@seebs.net> - 2011-05-27 05:06 +0000
              Re: Short-circuiting in C Noob <root@127.0.0.1> - 2011-05-27 17:48 +0200
                Re: Short-circuiting in C Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-05-27 21:31 -0400
              Re: Short-circuiting in C blmblm@myrealbox.com <blmblm.myrealbox@gmail.com> - 2011-05-28 03:28 +0000
                Re: Short-circuiting in C Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-05-27 23:43 -0400
                Re: Short-circuiting in C blmblm@myrealbox.com <blmblm.myrealbox@gmail.com> - 2011-05-28 19:31 +0000
                Re: Short-circuiting in C Michael Press <rubrum@pacbell.net> - 2011-05-30 00:28 -0700
                Re: Short-circuiting in C BGB <cr88192@hotmail.com> - 2011-05-30 23:08 -0700
                Re: Short-circuiting in C Michael Press <rubrum@pacbell.net> - 2011-05-31 18:30 -0700
                Re: Short-circuiting in C BGB <cr88192@hotmail.com> - 2011-06-02 09:08 -0700
                Re: Short-circuiting in C Nobody <nobody@nowhere.com> - 2011-05-28 14:08 +0100
                Re: Short-circuiting in C BGB <cr88192@hotmail.com> - 2011-05-28 12:52 -0700
              Re: Short-circuiting in C falk@rahul.net (Edward A. Falk) - 2011-05-31 19:22 +0000
                Re: Short-circuiting in C Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-05-31 21:07 -0400
                Re: Short-circuiting in C Dr Nick <3-nospam@temporary-address.org.uk> - 2011-06-11 17:07 +0100
                Re: Short-circuiting in C Shao Miller <sha0.miller@gmail.com> - 2011-06-11 12:57 -0500
  Re: Short-circuiting in C "Morris Keesan" <mkeesan@post.harvard.edu> - 2011-05-26 14:36 -0400
  Re: Short-circuiting in C "Scott Fluhrer" <sfluhrer@ix.netcom.com> - 2011-05-26 14:39 -0400
    Re: Short-circuiting in C Keith Thompson <kst-u@mib.org> - 2011-05-26 13:28 -0700
      Re: Short-circuiting in C Urist Sethkab <uristqwerty@gmail.com> - 2011-05-26 15:34 -0700
        Re: Short-circuiting in C "Scott Fluhrer" <sfluhrer@ix.netcom.com> - 2011-05-27 10:08 -0400
          Re: Short-circuiting in C Keith Thompson <kst-u@mib.org> - 2011-05-27 07:56 -0700
          Re: Short-circuiting in C Geoff <geoff@invalid.invalid> - 2011-05-27 20:14 -0700
            Re: Short-circuiting in C Keith Thompson <kst-u@mib.org> - 2011-05-27 20:26 -0700
              Re: Short-circuiting in C Geoff <geoff@invalid.invalid> - 2011-05-27 20:44 -0700
  Re: Short-circuiting in C Nobody <nobody@nowhere.com> - 2011-05-27 00:06 +0100
    Re: Short-circuiting in C Billy Mays <noway@nohow.com> - 2011-05-26 21:34 -0400
      Re: Short-circuiting in C Nobody <nobody@nowhere.com> - 2011-05-27 07:41 +0100
  Re: Short-circuiting in C Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-05-26 21:21 -0400
  Re: Short-circuiting in C Shao Miller <sha0.miller@gmail.com> - 2011-05-26 20:41 -0500
    Re: Short-circuiting in C Shao Miller <sha0.miller@gmail.com> - 2011-05-26 21:03 -0500
      Re: Short-circuiting in C Shao Miller <sha0.miller@gmail.com> - 2011-05-26 21:37 -0500
  Re: Short-circuiting in C ImpalerCore <jadill33@gmail.com> - 2011-05-27 09:01 -0700
    Re: Short-circuiting in C Willem <willem@toad.stack.nl> - 2011-05-27 16:08 +0000
      Re: Short-circuiting in C ImpalerCore <jadill33@gmail.com> - 2011-05-27 09:46 -0700
        Re: Short-circuiting in C BGB <cr88192@hotmail.com> - 2011-05-28 13:27 -0700
  Re: Short-circuiting in C James Kuyper <jameskuyper@verizon.net> - 2011-05-28 09:44 -0400
    Re: Short-circuiting in C Keith Thompson <kst-u@mib.org> - 2011-05-28 12:15 -0700
      Re: Short-circuiting in C James Kuyper <jameskuyper@verizon.net> - 2011-05-28 17:07 -0400
  Re: Short-circuiting in C "Herbert Rosenau" <os2guy@pc-rosenau.de> - 2011-05-29 07:25 +0000
  Re: Short-circuiting in C falk@rahul.net (Edward A. Falk) - 2011-05-31 18:40 +0000
    Re: Short-circuiting in C BGB <cr88192@hotmail.com> - 2011-06-02 09:48 -0700
      Re: Short-circuiting in C Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-06-02 21:05 -0400

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