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

[Royce Mitchell III]

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);
}