X-Git-Url: https://git.kernel.dk/?p=fio.git;a=blobdiff_plain;f=hash.h;h=66dd3d6916ea5b1386948c2d2a661d8bce59abf9;hp=02b06140146b469abd05772406102fec845747fc;hb=06e49041284eb0698f5b4699409a4d63c82b4b98;hpb=135be493d843d4cae2966a35cbd22a3058ec8e4b

diff --git a/hash.h b/hash.h
index 02b06140..66dd3d69 100644
--- a/hash.h
+++ b/hash.h
@@ -28,15 +28,31 @@
 #error Define GOLDEN_RATIO_PRIME for your wordsize.
 #endif
 
-#define GR_PRIME_64	0x9e37fffffffc0001ULL
+/*
+ * 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)
+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;
@@ -49,9 +65,6 @@ static inline unsigned long __hash_long(unsigned long val)
 	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;
@@ -65,7 +78,7 @@ static inline unsigned long hash_long(unsigned long val, unsigned int bits)
 
 static inline uint64_t __hash_u64(uint64_t val)
 {
-	return val * GR_PRIME_64;
+	return val * GOLDEN_RATIO_64;
 }
 	
 static inline unsigned long hash_ptr(void *ptr, unsigned int bits)
@@ -77,7 +90,7 @@ static inline unsigned long hash_ptr(void *ptr, unsigned int 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)
 {
@@ -128,17 +141,17 @@ static inline uint32_t jhash(const void *key, uint32_t length, uint32_t initval)
 	/* Last block: affect all 32 bits of (c) */
 	/* All the case statements fall through */
 	switch (length) {
-	case 12: c += (uint32_t) k[11] << 24;
-	case 11: c += (uint32_t) k[10] << 16;
-	case 10: c += (uint32_t) k[9] << 8;
-	case 9:  c += k[8];
-	case 8:  b += (uint32_t) k[7] << 24;
-	case 7:  b += (uint32_t) k[6] << 16;
-	case 6:  b += (uint32_t) k[5] << 8;
-	case 5:  b += k[4];
-	case 4:  a += (uint32_t) k[3] << 24;
-	case 3:  a += (uint32_t) k[2] << 16;
-	case 2:  a += (uint32_t) k[1] << 8;
+	case 12: c += (uint32_t) k[11] << 24;	/* fall through */
+	case 11: c += (uint32_t) k[10] << 16;	/* fall through */
+	case 10: c += (uint32_t) k[9] << 8;	/* fall through */
+	case 9:  c += k[8];			/* fall through */
+	case 8:  b += (uint32_t) k[7] << 24;	/* fall through */
+	case 7:  b += (uint32_t) k[6] << 16;	/* fall through */
+	case 6:  b += (uint32_t) k[5] << 8;	/* fall through */
+	case 5:  b += k[4];			/* fall through */
+	case 4:  a += (uint32_t) k[3] << 24;	/* fall through */
+	case 3:  a += (uint32_t) k[2] << 16;	/* fall through */
+	case 2:  a += (uint32_t) k[1] << 8;	/* fall through */
 	case 1:  a += k[0];
 		 __jhash_final(a, b, c);
 	case 0: /* Nothing left to add */