Re: [PATCH] (1/7) entropy, take 2 - log2

From: Abramo Bagnara (abramo.bagnara@libero.it)
Date: Sat Aug 24 2002 - 04:22:10 EST

Oliver Xymoron wrote:
>
>> +static inline __u32 int_log2_16bits(__u32 word)
> {
> /* Smear msbit right to make an n-bit mask */
> word |= word >> 8;
> word |= word >> 4;
> word |= word >> 2;
> word |= word >> 1;
> - /* Remove one bit to make this a logarithm */
> - word >>= 1;
> - /* Count the bits set in the word */
> - word -= (word >> 1) & 0x555;
> - word = (word & 0x333) + ((word >> 2) & 0x333);
> - word += (word >> 4);
> - word += (word >> 8);
> - return word & 15;
> +
> + return hweight16(word)-1;
> }
> -#endif

I suggest you to use a more efficient version like that below:

static inline int ld2_16(__u16 v)
{
        unsigned r = 0;

        if (v >= 0x100) {
                v >>= 8;
                r += 8;
        }
        if (v >= 0x10) {
                v >>= 4;
                r += 4;
        }
        if (v >= 4) {
                v >>= 2;
                r += 2;
        }
        if (v >= 2)
                r++;
        return r;
} 

-- 
Abramo Bagnara                       mailto:abramo.bagnara@libero.it

Opera Unica                          Phone: +39.546.656023
Via Emilia Interna, 140
48014 Castel Bolognese (RA) - Italy
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/

This archive was generated by hypermail 2b29 : Sat Aug 31 2002 - 22:00:13 EST