X-Git-Url: https://git.kernel.dk/?p=fio.git;a=blobdiff_plain;f=hash.h;h=1d7608beb40500dba75541f374b815f88672eadb;hp=0c3cdda77be642842337f6fe949da7c24581e661;hb=4e2d9699d87459cc0aeff80cb135af2db5f3bd84;hpb=fddc6604f91ebf76d9090741f9d4f5a4d33be0c6

diff --git a/hash.h b/hash.h
index 0c3cdda7..1d7608be 100644
--- a/hash.h
+++ b/hash.h
@@ -28,11 +28,29 @@
 #error Define GOLDEN_RATIO_PRIME for your wordsize.
 #endif
 
-static inline unsigned long hash_long(unsigned long val, unsigned int bits)
+/*
+ * 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 unsigned long __hash_long(unsigned long val)
 {
 	unsigned long hash = val;
 
 #if BITS_PER_LONG == 64
+	hash *= GOLDEN_RATIO_64;
+#else
 	/*  Sigh, gcc can't optimise this alone like it does for 32 bits. */
 	unsigned long n = hash;
 	n <<= 18;
@@ -47,25 +65,32 @@ static inline unsigned long hash_long(unsigned long val, unsigned int bits)
 	hash += n;
 	n <<= 2;
 	hash += n;
-#else
-	/* On some cpus multiply is faster, on others gcc will do shifts */
-	hash *= GOLDEN_RATIO_PRIME;
 #endif
 
+	return hash;
+}
+
+static inline unsigned long hash_long(unsigned long val, unsigned int bits)
+{
 	/* High bits are more random, so use them. */
-	return hash >> (BITS_PER_LONG - bits);
+	return __hash_long(val) >> (BITS_PER_LONG - bits);
+}
+
+static inline uint64_t __hash_u64(uint64_t val)
+{
+	return val * GOLDEN_RATIO_64;
 }
 	
 static inline unsigned long hash_ptr(void *ptr, unsigned int bits)
 {
-	return hash_long((unsigned long)ptr, bits);
+	return hash_long((uintptr_t)ptr, bits);
 }
 
 /*
  * Bob Jenkins jhash
  */
 
-#define JHASH_INITVAL	GOLDEN_RATIO_PRIME
+#define JHASH_INITVAL	GOLDEN_RATIO_32
 
 static inline uint32_t rol32(uint32_t word, uint32_t shift)
 {