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| From | blmblm@myrealbox.com <blmblm.myrealbox@gmail.com> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: Short-circuiting in C |
| Date | 2011-05-28 03:28 +0000 |
| Organization | None |
| Message-ID | <94b8ekFsnU1@mid.individual.net> (permalink) |
| References | <irm4et$66d$1@speranza.aioe.org> <ln62oxw829.fsf@nuthaus.mib.org> <irmcij$ro9$1@speranza.aioe.org> <irmuo5$u9t$1@dont-email.me> |
In article <irmuo5$u9t$1@dont-email.me>,
Eric Sosman <esosman@ieee-dot-org.invalid> wrote:
> On 5/26/2011 4:17 PM, Billy Mays wrote:
> > On 05/26/2011 03:55 PM, Keith Thompson wrote:
> >> Billy Mays<noway@nohow.com> writes:
> >>> On 05/26/2011 02:22 PM, Thomas Richter wrote:
> >>>> On 26.05.2011 20:09, Ben Pfaff wrote:
> >>>>> Billy Mays<noway@nohow.com> writes:
> >>>>>
> >>>>>> I was wondering if there is a way to avoid the C short circuiting for
> >>>>>> boolean expressions such as the following:
> >>>>>>
> >>>>>> if(a&& b&& c) {
> >>>>>> /* Do stuff */
> >>>>>> }
> > [...]
> > The processor is SIMD so each thread runs the same code, but they modify
> > their own data. As long as each thread executes the same instruction,
> > the processor can simultaneously work on each thread's data.
> >
> > However, if one of the threads branches then the processor degrades
> > (SISD?) and loses a lot of the parallelism.
> >
> > The goal here is to ensure each thread does exactly the same thing, even
> > if it is slightly wasteful.
>
> C has no way to express the computation you desire. C's abstract
> machine has only one flow of control, if we ignore the rather mysterious
> effects of asynchronous signals and `volatile' variable. There is no
> C construct that will evaluate a,b,c in parallel execution paths and
> then somehow combine the three results into a single test. You've
> chosen the wrong implementation language.
Hesitant though I am to argue with someone who generally seems
far more expert than I am ....
I don't get the impression at all that the OP wants a, b, and
c evaluated in parallel. As I understand it, he just wants to
be sure that there is no branching involved in the evaluation
of the condition (a && b && c). I admit I don't have a clear
understanding of the platform he's targeting, but it sounds like
it somehow involves multiple processing elements [*] executing the
same compiled code, and under those circumstances I can vaguely
imagine that performance might be better if all these processing
elements execute exactly the same instructions (i.e., no branches).
[*] The ones that put the "MD" in "SIMD" is what I'm thinking here,
though admittedly not with as much clarity as I might like.
--
B. L. Massingill
ObDisclaimer: I don't speak for my employers; they return the favor.
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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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