1 files changed, 70 insertions, 0 deletions
diff --git a/include/linux/hash.h b/include/linux/hash.h
new file mode 100644
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--- /dev/null
+++ b/include/linux/hash.h
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+#ifndef _LINUX_HASH_H
+#define _LINUX_HASH_H
+/* Fast hashing routine for ints,  longs and pointers.
+   (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.
+ */
+
+#include <asm/types.h>
+
+/* 2^31 + 2^29 - 2^25 + 2^22 - 2^19 - 2^16 + 1 */
+#define GOLDEN_RATIO_PRIME_32 0x9e370001UL
+/*  2^63 + 2^61 - 2^57 + 2^54 - 2^51 - 2^18 + 1 */
+#define GOLDEN_RATIO_PRIME_64 0x9e37fffffffc0001UL
+
+#if BITS_PER_LONG == 32
+#define GOLDEN_RATIO_PRIME GOLDEN_RATIO_PRIME_32
+#define hash_long(val, bits) hash_32(val, bits)
+#elif BITS_PER_LONG == 64
+#define hash_long(val, bits) hash_64(val, bits)
+#define GOLDEN_RATIO_PRIME GOLDEN_RATIO_PRIME_64
+#else
+#error Wordsize not 32 or 64
+#endif
+
+static inline u64 hash_64(u64 val, unsigned int bits)
+{
+	u64 hash = val;
+
+	/*  Sigh, gcc can't optimise this alone like it does for 32 bits. */
+	u64 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;
+
+	/* High bits are more random, so use them. */
+	return hash >> (64 - bits);
+}
+
+static inline u32 hash_32(u32 val, unsigned int bits)
+{
+	/* On some cpus multiply is faster, on others gcc will do shifts */
+	u32 hash = val * GOLDEN_RATIO_PRIME_32;
+
+	/* High bits are more random, so use them. */
+	return hash >> (32 - bits);
+}
+
+static inline unsigned long hash_ptr(const void *ptr, unsigned int bits)
+{
+	return hash_long((unsigned long)ptr, bits);
+}
+#endif /* _LINUX_HASH_H */