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From: ohk@edeber.tfdt-o.nta.no (Ole-Hjalmar Kristensen FOU.TD/DELAB)
Subject: Re: CRC in Ada?
Date: 1997/03/10
Message-ID: <OHK.97Mar10125714@edeber.tfdt-o.nta.no>#1/1
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References: <1997Mar2.220652@nova.wright.edu> <oslo83b70l.fsf@ted.vigra.com>
Organization: Telenor Online Public Access
Newsgroups: comp.lang.ada
Date: 1997-03-10T00:00:00+00:00
List-Id: <comp.lang.ada>


In article <dewar.857750112@merv> dewar@merv.cs.nyu.edu (Robert Dewar) writes:


   Fergus said

   <<It is somewhat surprising that the difference between the user CPU
   times is so large, but I'm more surprised by the speed of the second
   version than by the slowness of the first.>>

   Well it does not surprise me, and I don't really understand why it
   surprises you. I would expect a large difference here, and indeed
   it is what we see!

   Just shows that measurements can be a useful substitute for guesswork,
   though measurements have the weakness of being system specific, so
   nothing really substitutes for a lot of experience on a lot of difrerent
   machiens!

He didn't say it, I did. 

So tell me, why would you expect a large difference?
I would expect the optimizer to recognize that both (&_iob[0])->_cnt
and (&_iob[0])->_ptr are constant expressions, so that the two loops
differ mainly in using indirect or direct addressing.
Bu I suppose Im'm expecting too much from optimizers.

Here is a version of the first program after it has passed through
cpp:

main() {
   setvbuf((&_iob[0]),0,0000,4096);
   int c,total=0;
   while ((c = (--((&_iob[0]))->_cnt < 0 ? __filbuf((&_iob[0])) : (int)*((&_iob[0]))->_ptr++)) != (-1)) total++;
   printf("total %d\n",total);
}

And the second:

main() {
   int c,n,total=0;
   while ((n = read(0,buf,sizeof(buf))) > 0) {
      int i;
      for (i = 0; i < n; i++) {
         c = buf[i];
         total++;
      }
   }
   printf("total %d\n",total);
}

Ole-Hj. Kristensen