From: nikita <nikita>
Date: Thu, 23 Nov 2006 18:05:40 +0000 (+0000)
Subject: liblustre: add hash_long() from linux/hash.c.
X-Git-Tag: v1_8_0_110~486^2~85
X-Git-Url: https://git.whamcloud.com/?a=commitdiff_plain;h=9051d52e938259769a97b1e8aa2734dc2953f8d3;p=fs%2Flustre-release.git

liblustre: add hash_long() from linux/hash.c.
---

diff --git a/lustre/include/liblustre.h b/lustre/include/liblustre.h
index 96fafa8..d7380c7 100644
--- a/lustre/include/liblustre.h
+++ b/lustre/include/liblustre.h
@@ -957,4 +957,60 @@ static inline void set_fs(mm_segment_t seg)
 #include <lustre_export.h>
 #include <lustre_net.h>
 
+/* Fast hashing routine for a long.
+   (C) 2002 William Lee Irwin III, IBM */
+
+/*
+ * Knuth recommends primes in approximately golden ratio to the maximum
+ * integer representable by a machine word for multiplicative hashing.
+ * Chuck Lever verified the effectiveness of this technique:
+ * http://www.citi.umich.edu/techreports/reports/citi-tr-00-1.pdf
+ *
+ * These primes are chosen to be bit-sparse, that is operations on
+ * them can use shifts and additions instead of multiplications for
+ * machines where multiplications are slow.
+ */
+#if BITS_PER_LONG == 32
+/* 2^31 + 2^29 - 2^25 + 2^22 - 2^19 - 2^16 + 1 */
+#define GOLDEN_RATIO_PRIME 0x9e370001UL
+#elif BITS_PER_LONG == 64
+/*  2^63 + 2^61 - 2^57 + 2^54 - 2^51 - 2^18 + 1 */
+#define GOLDEN_RATIO_PRIME 0x9e37fffffffc0001UL
+#else
+#error Define GOLDEN_RATIO_PRIME for your wordsize.
+#endif
+
+static inline unsigned long hash_long(unsigned long val, unsigned int bits)
+{
+	unsigned long hash = val;
+
+#if BITS_PER_LONG == 64
+	/*  Sigh, gcc can't optimise this alone like it does for 32 bits. */
+	unsigned long n = hash;
+	n <<= 18;
+	hash -= n;
+	n <<= 33;
+	hash -= n;
+	n <<= 3;
+	hash += n;
+	n <<= 3;
+	hash -= n;
+	n <<= 4;
+	hash += n;
+	n <<= 2;
+	hash += n;
+#else
+	/* On some cpus multiply is faster, on others gcc will do shifts */
+	hash *= GOLDEN_RATIO_PRIME;
+#endif
+
+	/* High bits are more random, so use them. */
+	return hash >> (BITS_PER_LONG - bits);
+}
+	
+static inline unsigned long hash_ptr(void *ptr, unsigned int bits)
+{
+	return hash_long((unsigned long)ptr, bits);
+}
+
 #endif