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

George Spelvin
2014-06-07T08:28:08Z

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);
}

/*
--
2.0.0