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

[Hannes Frederic Sowa]

On Sat, Jun 7, 2014, at 1:28, George Spelvin wrote:
> 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);
>  }

Have you checked assembler output if this helps anything at all? Constant propagation in the compiler should be able to figure that out all by itself. The only places I use __builtin_constant_p today are where I also make use of inline assembler.

Please check this as it makes the code more complicated and I doubt it is worth it.

Btw, IIRC there is a function is_power_of_2 somewhere. ;)

Thanks,

  Hannes
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