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" [e-mail]
To: "ReactOS Development List" [e-mail]
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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