Re: Short-circuiting in C

From	Billy Mays <noway@nohow.com>
Newsgroups	comp.lang.c
Subject	Re: Short-circuiting in C
Date	2011-05-26 14:51 -0400
Organization	Aioe.org NNTP Server
Message-ID	<irm7h1$ea0$1@speranza.aioe.org> (permalink)
References	<irm4et$66d$1@speranza.aioe.org> <87lixttjt7.fsf@blp.benpfaff.org> <irm5op$43$1@news.belwue.de>

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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 */
>>> }
>>
>> x = a;
>> y = b;
>> z = c;
>> if (x&& y&& z) {
>> /* Do stuff */
>> }
>
> Sorry, I fail to see how this should work. Actually, a sufficiently
> smart compiler would reduce this to just the above, and in fact, a
> compiler can modify the code always on the basis of the "as-if" rule.
>
> So the question goes back to the OP what the intent of disabling the
> short-circuiting is. If it is to avoid two branches, then if a,b and c
> are always 0 or 1 (or any other number, identical for all three
> variables), you could simply write:
>
> if (a & b & c) {
> }
>
> If the intent is that an actual access of the variables happen, probably
> because they are hardware registers, then you would also suggest to
> declare them as volatile.
>
> However, typically the impact of such micro-optimizations is negligible.
> Question to the OP: Did you actually measure?
>
> Greetings,
> Thomas


I did not, but I suspect using an & instead of an && would fail if a was 
1 and b was 2.  The programming guide I was using mentioned that the 
short-circuiting behavior would force all the threads to serialize 
rather than run in lockstep.

--
Bill

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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