hash: make 64-bit even on 32-bit

[fio.git] / hash.h

diff --git a/hash.h b/hash.h
index 0c3cdda77be642842337f6fe949da7c24581e661..d227b938a246881229ab5d5e304427da8f03cff3 100644 (file)
--- a/hash.h
+++ b/hash.h
@@ -28,13 +28,31 @@
 #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(uint64_t val)
 {
-       unsigned long hash = val;
+       uint64_t 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;
+       uint64_t n = hash;
        n <<= 18;
        hash -= n;
        n <<= 33;
@@ -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)
 {