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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