Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752969AbaFGI2I (ORCPT ); Sat, 7 Jun 2014 04:28:08 -0400 Received: from ns.horizon.com ([71.41.210.147]:53791 "HELO ns.horizon.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with SMTP id S1752805AbaFGI2G (ORCPT ); Sat, 7 Jun 2014 04:28:06 -0400 Date: 7 Jun 2014 04:28:05 -0400 Message-ID: <20140607082805.10120.qmail@ns.horizon.com> From: "George Spelvin" To: davem@davemloft.net, dborkman@redhat.com, linux@horizon.com, shemminger@osdl.org, tytso@mit.edu Subject: [PATCH 5/7] lib/random32.c: Make prandom_u32_max efficient for powers of 2 Cc: linux-kernel@vger.kernel.org In-Reply-To: <20140607081828.9294.qmail@ns.horizon.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 --- 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 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/