Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > comp.lang.c > #23085

Re: Pre And Post Increment Operator Output

Show all headers | View raw

On 7/4/2012 4:27 PM, Richard Damon wrote:
> On 6/22/12 9:34 AM, Ben Bacarisse wrote:
>>
>> I've never liked the view that anything can happen with undefined code;
>> it sounds too much like a cheap pedagogical trick to say that the above
>> could format your drive.
>
> One issue with C, is that when C was first being defined, computer
> architectures could have some strange rules with limitations on things
> like writing to and reading from a given location sequentially could
> cause weird results, sometimes totally random numbers, sometimes bus
> faults. Hardware designers didn't feel bad about putting restrictions on
> "silly" combinations of operations, or making the software have to wait
> for results to be done.
>
> C, in an effort to be as efficient as possible, choose to make some of
> these corner cases "undefined behavior" rather than make the compiler
> add test to the code to detect these. While in the example, it looks
> obvious of the double access, it real code, it might be better hidden
> with something like:
>
>
> int foo(int *a, int *b) {
>    return ++*a + ++*b;
> }
>
> int main() {
>    int i = 0;
>    i = foo(&i, &i);
>    printf("%d\n", i);
> }
>
> Where the code doing the dual access has two pointers, which happen to
> be set to point to the same spot elsewhere. Note that the function foo
> has well define behavior as long as a != b.
>
> Now, it is more common that the processor has logic to detect these sort
> of conflicts and rather than "getting it wrong", they stall the
> processor a bit so the answer can get there, but that took too much
> logic to be worth it in the earlier days.

     Even if every CPU of interest can generate a consistent result
for such a case, it's not a given that two different CPU's -- even
different steppings of "the same" CPU -- will yield the same answer.

     Also, let's rewrite your function just a little:

	int foo(int *a, int *b) {
	    return *a == *++b;
	}

What "should" the result be when a == b?  "Should" *a yield the
original or the incremented value, and why?  Or how about

	int foo(int *a, int *b, int *c) {
	    return *a * *++b + *a * *++c;
	}

... when a==b and a==c?  May the compiler apply the distributive
law and do common sub-expression elimination?  Why, or why not,
and what are the implications for optimizers?

-- 
Eric Sosman
esosman@ieee-dot-org.invalid

Back to comp.lang.c | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread

Thread

Pre And Post Increment Operator Output Yogesh Yadav Pacheria <yogeshpacheria@gmail.com> - 2012-06-21 12:56 -0700
  Re: Pre And Post Increment Operator Output Paul N <gw7rib@aol.com> - 2012-06-21 13:08 -0700
    Re: Pre And Post Increment Operator Output Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-06-21 16:52 -0400
  Re: Pre And Post Increment Operator Output John Gordon <gordon@panix.com> - 2012-06-21 20:11 +0000
  Re: Pre And Post Increment Operator Output Andrew Cooper <amc96@cam.ac.uk> - 2012-06-21 21:29 +0100
  Re: Pre And Post Increment Operator Output "BartC" <bc@freeuk.com> - 2012-06-21 22:08 +0100
    Re: Pre And Post Increment Operator Output Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-06-21 17:32 -0400
      Re: Pre And Post Increment Operator Output "BartC" <bc@freeuk.com> - 2012-06-22 00:00 +0100
        Re: Pre And Post Increment Operator Output Keith Thompson <kst-u@mib.org> - 2012-06-21 16:51 -0700
          Re: Pre And Post Increment Operator Output "BartC" <bc@freeuk.com> - 2012-06-22 11:09 +0100
            Re: Pre And Post Increment Operator Output nick_keighley_nospam@hotmail.com - 2012-06-22 05:36 -0700
            Re: Pre And Post Increment Operator Output Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-06-22 14:34 +0100
              Re: Pre And Post Increment Operator Output Les Cargill <lcargill99@comcast.com> - 2012-06-22 12:45 -0500
              Re: Pre And Post Increment Operator Output nick_keighley_nospam@hotmail.com - 2012-06-25 00:27 -0700
                Re: Pre And Post Increment Operator Output Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-06-25 14:32 +0100
                Re: Pre And Post Increment Operator Output Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-06-25 09:42 -0400
                Re: Pre And Post Increment Operator Output Kenneth Brody <kenbrody@spamcop.net> - 2012-06-25 11:33 -0400
              Re: Pre And Post Increment Operator Output Richard Damon <news.x.richarddamon@xoxy.net> - 2012-07-04 16:27 -0400
                Re: Pre And Post Increment Operator Output Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-07-04 17:46 -0400
                Re: Pre And Post Increment Operator Output Richard Damon <news.x.richarddamon@xoxy.net> - 2012-07-05 09:08 -0400
                Re: Pre And Post Increment Operator Output "io_x" <a@b.c.invalid> - 2012-07-06 17:34 +0200
            Re: Pre And Post Increment Operator Output Keith Thompson <kst-u@mib.org> - 2012-06-22 12:31 -0700
              Re: Pre And Post Increment Operator Output Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-06-22 16:21 -0400
        Re: Pre And Post Increment Operator Output Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-06-21 21:06 -0400
          Re: Pre And Post Increment Operator Output Noob <root@127.0.0.1> - 2012-06-22 13:30 +0200
            Re: BartC (was: Pre And Post Increment Operator Output) "BartC" <bc@freeuk.com> - 2012-06-22 13:06 +0100
            Re: Pre And Post Increment Operator Output Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-06-22 09:26 -0400
              Re: Pre And Post Increment Operator Output "Bartc" <bc@freeuk.com> - 2012-07-01 16:33 +0100
                Re: Pre And Post Increment Operator Output Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-07-01 17:15 +0100
                Re: Pre And Post Increment Operator Output "Bartc" <bc@freeuk.com> - 2012-07-01 18:45 +0100
                Re: Pre And Post Increment Operator Output Keith Thompson <kst-u@mib.org> - 2012-07-01 13:03 -0700
                Re: Pre And Post Increment Operator Output Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-07-02 00:18 +0100
        Re: Pre And Post Increment Operator Output Phil Carmody <thefatphil_demunged@yahoo.co.uk> - 2012-06-24 23:45 +0300
      Re: Pre And Post Increment Operator Output Tim Rentsch <txr@alumni.caltech.edu> - 2012-06-24 09:15 -0700
        Re: Pre And Post Increment Operator Output Phil Carmody <thefatphil_demunged@yahoo.co.uk> - 2012-06-24 23:54 +0300
          Re: Pre And Post Increment Operator Output Tim Rentsch <txr@alumni.caltech.edu> - 2012-06-24 15:12 -0700
            Re: Pre And Post Increment Operator Output "BartC" <bc@freeuk.com> - 2012-06-25 11:00 +0100
              Re: Pre And Post Increment Operator Output Tim Rentsch <txr@alumni.caltech.edu> - 2012-06-25 13:53 -0700
  Re: Pre And Post Increment Operator Output Keith Thompson <kst-u@mib.org> - 2012-06-21 15:17 -0700
  Re: Pre And Post Increment Operator Output Yogesh Yadav Pacheria <yogeshpacheria@gmail.com> - 2012-06-21 21:03 -0700
    Re: Pre And Post Increment Operator Output nick_keighley_nospam@hotmail.com - 2012-06-22 00:06 -0700
    Re: Pre And Post Increment Operator Output Mark Bluemel <mark_bluemel@pobox.com> - 2012-06-22 08:30 +0100
    Re: Pre And Post Increment Operator Output John Bode <jfbode1029@gmail.com> - 2012-06-22 09:18 -0700
      Re: Pre And Post Increment Operator Output Keith Thompson <kst-u@mib.org> - 2012-06-22 12:36 -0700
      Re: Pre And Post Increment Operator Output lawrence.jones@siemens.com - 2012-06-22 14:25 -0400
  Re: Pre And Post Increment Operator Output "Heinrich Wolf" <invalid@invalid.invalid> - 2012-06-22 10:27 +0200
    Re: Pre And Post Increment Operator Output Kenneth Brody <kenbrody@spamcop.net> - 2012-06-22 13:46 -0400
      Re: Pre And Post Increment Operator Output Michael Press <rubrum@pacbell.net> - 2012-07-13 02:09 -0700
  Re: Pre And Post Increment Operator Output Ian Collins <ian-news@hotmail.com> - 2012-06-22 23:11 +1200
    Re: Pre And Post Increment Operator Output Mark Bluemel <mark_bluemel@pobox.com> - 2012-06-22 12:18 +0100

csiph-web