1 files changed, 58 insertions, 0 deletions

diff --git a/include/linux/hash.h b/include/linux/hash.h
new file mode 100644
index 000000000000..acf17bb8e7f9
--- /dev/null
+++ b/include/linux/hash.h
@@ -0,0 +1,58 @@
+#ifndef _LINUX_HASH_H
+#define _LINUX_HASH_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 /* _LINUX_HASH_H */