[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);
}