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 <[email protected]> --- 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); } /* -- 2.0.0 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
