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From: "Aslan Kral" <aslanski2002@yahoo.com>
Newsgroups: comp.lang.ada,comp.realtime
Subject: Re: 10 rules for benchmarking (was Re: Teaching new tricks to an old
 dog (C++ -->Ada))
Date: Sat, 12 Mar 2005 15:29:24 +0200
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Date: 2005-03-12T15:29:24+02:00
List-Id: <comp.lang.ada>


"Jim Rogers" <jimmaureenrogers@att.net>, haber iletisinde �unlar�
yazd�:DYBYd.373707$w62.141894@bgtnsc05-news.ops.worldnet.att.net...
> "Aslan Kral" <aslanski2002@yahoo.com> wrote in
> news:39g6jbF62g4qjU1@individual.net:
>
> >
> > "Aslan Kral" <aslanski2002@yahoo.com>, haber iletisinde �unlar�
> > yazd�:39fr2iF5uq3mvU1@individual.net...
> >> > IIRC, the C codes that were posted to find the highest bit set
> >> > thread were about 1/10th of that number. Just to double check, I
> >> > wrote and measured some C code. On my 3GHz Xeon running with cygwin
> >> > on WinXP, when compiled with gcc3.3.3 using -mpcu=i686 -O0, I got a
> >> > total run-time of 2.3s, for about 69 cycles/iteration, and with -O2
> >> > I get 0.94s for about 28 cycles/iteration.
> >> >
> >>
> >
> > Oops! I failed to read the -O2 part. It makes sense now. And your code
> > doesn't use any lookup table that is why it is slower than Willem's.
> > Anyway the version with lookup table is quite close to "bsr" version.
> > So yours also can get faster with the addition of lookup table. It
> > would be interesting to see how much Ada can get close to "bsr"
> > version below! (By adding a lookup table, I mean.)
> >
> > __inline unsigned FindHighestBit(unsigned n)
> > {
> >    __asm
> >    {
> >       bsr eax, n
> >    }
> > }
> > .
> >
>
> I sems to have lost the C version of the lookup table.
> Could you please repost that or send me the version.
>
> Thanks.
>
> Jim Rogers
>
OK.

static unsigned highBitSmall[256] =
{
  0,0,1,1,2,2,2,2,3,3,3,3,3,3,3,3,4,4,4,4,4,4,4,4,4,4,4,4,4,4,4,4,
  5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,
  6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,
  6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,
  7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,
  7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,
  7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,
  7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7
};

__inline unsigned FindHighestBit( unsigned n )
{
    unsigned bits = 0;
    if (n >> 16) {
    n = n >> 16;
    bits = 16;
    }
    if (n >> 8) {
    n = n >> 8;
    bits += 8;
    }
    return bits + highBitSmall[n];
}