Re: [PATCH 5/7] lib/random32.c: Make prandom_u32_max efficient for powers of 2

Daniel Borkmann
2014-06-08T12:03:47Z

On 06/07/2014 10:28 AM, George Spelvin wrote:
> The multiply-and-shift is efficient in the general case, but slower than
> a simple bitwise AND if the range is a power of 2. Make the function
> handle this case so callers don't have to worry about micro-optimizing it.
>
> Signed-off-by: George Spelvin <linux@horizon.com>
> ---
> include/linux/random.h | 14 +++++++++++++-
> 1 file changed, 13 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/random.h b/include/linux/random.h
> index 57fbbffd..e1f3ec9a 100644
> --- a/include/linux/random.h
> +++ b/include/linux/random.h
> @@ -47,11 +47,23 @@ void prandom_bytes_state(struct rnd_state *state, void *buf, int nbytes);
> * generator, that is, prandom_u32(). This is useful when requesting a
> * random index of an array containing ep_ro elements, for example.
> *
> + * If ep_ro is a power of 2 known at compile time, a modulo operation
> + * reduces to a simple mask to extract the low order bits. Otherwise,
> + * it uses a multiply and shift, which is faster than a general modulus.
> + *
> * Returns: pseudo-random number in interval [0, ep_ro)
> */
> static inline u32 prandom_u32_max(u32 ep_ro)
> {
> - return (u32)(((u64) prandom_u32() * ep_ro) >> 32);
> + /*
> + * Instead of just __builtin_constant_p(ep_ro), this test is
> + * "is it known at compile time that ep_ro is a power of 2?", and
> + * can in theory handle the case that it's an unknown power of 2.
> + */
> + if (__builtin_constant_p(ep_ro & (ep_ro-1)) && !(ep_ro & (ep_ro-1)))
> + return prandom_u32() & (ep_ro-1);
> + else
> + return (u32)((u64)prandom_u32() * ep_ro >> 32);

Okay, I guess it's fine since we expect a random number here anyway, so
it doesn't matter much how we get to the result; otherwise I'd have
complained as both function don't do the same thing. Please leave some
whitespace, i.e. I mean "ep_ro-1" -> "ep_ro - 1".

Btw, there are many other users which hard code prandom_u32_max() resp.
reciprocal_scale() function in the source tree. If you have some cycles,
feel free to migrate them and let them use these functions.

> }
>
> /*
>