linux-kernel - [PATCH 1/1] core-kernel: use multiply instead of shifts in hash

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Message-Id: <4fe8d847.0fbd0e0a.7895.ffffa29fSMTPIN_ADDED@gmr-mx.google.com>
Date:	Fri, 15 Jun 2012 15:18:26 -0700
From:	Andrew Hunter <ahh@...gle.com>
To:	unlisted-recipients:; (no To-header on input)
Subject: [PATCH 1/1] core-kernel: use multiply instead of shifts in hash_64

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 <ahh@...gle.com>
Reviewed-by: Paul Turner <pjt@...gle.com>
---
 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
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