Re: [ros-dev] ping Alex regarding log2() for scheduler

[Magnus Olsen]

Hi I have not test run this code. But i think this form maby 
are faster that v3 of  Royce  Mitchell III code 
it leave to the compiler to optimze it more or I complete wrong 

 int highest_bit ( unsigned int i )
 {
     int ret = 0;
     int n ;
     int t;
     int x;

    for (t = 0; t>3;t++)
   {
      x = (16 << t);
      n = i >> x;
     if ( n )  i = n, ret  += x ;
   }
      return ret + (i>>1);
}


----- Original Message ----- 
From: "Royce Mitchell III" &lt;royce3(a)ev1.net&gt;
To: "ReactOS Development List" &lt;ros-dev(a)reactos.com&gt;
Sent: Wednesday, March 23, 2005 10:08 PM
Subject: Re: [ros-dev] ping Alex regarding log2() for scheduler


>   Royce Mitchell III wrote:
>  
> >   Mike Nordell wrote:
> >
> > >   int highest_bit(unsigned int i)
> > >  {
> > >     int temp;
> > >     int ret = 0;
> > >     temp = i > 0xffff; i >>= 16*temp; ret = 16*temp;
> > >     temp = i > 0xff;   i >>=  8*temp; ret += 8*temp;
> > >     temp = i > 0xf;    i >>=  4*temp; ret += 4*temp;
> > >     temp = i > 0x3;    i >>=  2*temp; ret += 2*temp;
> > >     ret += (i>>1);
> > >     return ret;
> > >  }
> > >   
> > >  
> >  This is almost, but not quite, 50% slower in msvc6 w/ "compile for 
> >  speed". :(
> >
> >  Perhaps the reordering could help... 
>  
>  FWIW, I also came up with the following code last night, which was only 
>  a hair slower than the code I sent to Alex, but perhaps might lend 
>  itself to speed-up better?
>  
>  int highest_bit_v3 ( unsigned int i )
>  {
>      int ret = 0;
>      int n = i >> 16;
>      if ( n )
>          i = n, ret = 16;
>      n = i >> 8;
>      if ( n )
>          i = n, ret += 8;
>      n = i >> 4;
>      if ( n )
>          i = n, ret += 4;
>      n = i >> 2;
>      if ( n )
>          i = n, ++ret, ++ret;
>      return ret + (i>>1);
>  }
>  
>  
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