hash_64(val) = val * (a 64-bit constant). It is "optimized" by
replacing the multiply by a bunch of shifts and adds. On modern
machines, this is not an optimization; remove it.
Running this hash function in a independent benchmark, it's about three times
as fast (1ns vs 3ns) with a multiply as with a shift on Westmere. It's also
considerably smaller (and since we inline this function often, that matters.)
Signed-off-by: Andrew Hunter <[email protected]>
Reviewed-by: Paul Turner <[email protected]>
---
include/linux/hash.h | 6 ++++--
1 files changed, 4 insertions(+), 2 deletions(-)
diff --git a/include/linux/hash.h b/include/linux/hash.h
index b80506b..daabc3d 100644
--- a/include/linux/hash.h
+++ b/include/linux/hash.h
@@ -34,7 +34,9 @@
static inline u64 hash_64(u64 val, unsigned int bits)
{
u64 hash = val;
-
+#if BITS_PER_LONG == 64
+ hash *= GOLDEN_RATIO_PRIME_64;
+#else
/* Sigh, gcc can't optimise this alone like it does for 32 bits. */
u64 n = hash;
n <<= 18;
@@ -49,7 +51,7 @@ static inline u64 hash_64(u64 val, unsigned int bits)
hash += n;
n <<= 2;
hash += n;
-
+#endif
/* High bits are more random, so use them. */
return hash >> (64 - bits);
}
--
1.7.7.3