X-Git-Url: https://git.openpandora.org/cgi-bin/gitweb.cgi?a=blobdiff_plain;f=include%2Flinux%2Fhash.h;h=44a3b95f16c18a08fbc078de4718592fa7690538;hb=b2d5074d5d465930612c26de2ecabaf50024583d;hp=b80506bdd733ee181f202ddb6322529d42299d1b;hpb=b2409fb6a49d1f633a8fc488e48043da7d3fd6a7;p=pandora-kernel.git

diff --git a/include/linux/hash.h b/include/linux/hash.h
index b80506bdd733..44a3b95f16c1 100644
--- 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);