mm: Export migrate_page_move_mapping and migrate_page_copy

[pandora-kernel.git] / include / linux / hash.h

diff --git a/include/linux/hash.h b/include/linux/hash.h
index b80506b..44a3b95 100644 (file)
--- a/include/linux/hash.h
+++ b/include/linux/hash.h
@@ -31,10 +31,29 @@
 #error Wordsize not 32 or 64
 #endif
 
+/*
+ * The above primes are actively bad for hashing, since they are
+ * too sparse. The 32-bit one is mostly ok, the 64-bit one causes
+ * real problems. Besides, the "prime" part is pointless for the
+ * multiplicative hash.
+ *
+ * Although a random odd number will do, it turns out that the golden
+ * ratio phi = (sqrt(5)-1)/2, or its negative, has particularly nice
+ * properties.
+ *
+ * These are the negative, (1 - phi) = (phi^2) = (3 - sqrt(5))/2.
+ * (See Knuth vol 3, section 6.4, exercise 9.)
+ */
+#define GOLDEN_RATIO_32 0x61C88647
+#define GOLDEN_RATIO_64 0x61C8864680B583EBull
+
 static inline u64 hash_64(u64 val, unsigned int bits)
 {
        u64 hash = val;
 
+#if BITS_PER_LONG == 64
+       hash = hash * GOLDEN_RATIO_64;
+#else
        /*  Sigh, gcc can't optimise this alone like it does for 32 bits. */
        u64 n = hash;
        n <<= 18;
@@ -49,6 +68,7 @@ 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);