Add inline murmurhash32(uint32) function.

The function already existed in tidbitmap.c but more users requiring
fast hashing of 32bit ints are coming up.

Author: Andres Freund
Discussion: https://postgr.es/m/20170914061207.zxotvyopetm7lrrp@alap3.anarazel.de

src/backend/nodes/tidbitmap.c

@@ -45,6 +45,7 @@
 #include "nodes/tidbitmap.h"
 #include "storage/lwlock.h"
 #include "utils/dsa.h"
+#include "utils/hashutils.h"
 
 /*
  * The maximum number of tuples per page is not large (typically 256 with
@@ -237,30 +238,13 @@ static int	tbm_comparator(const void *left, const void *right);
 static int tbm_shared_comparator(const void *left, const void *right,
 					  void *arg);
 
-/*
- * Simple inline murmur hash implementation for the exact width required, for
- * performance.
- */
-static inline uint32
-hash_blockno(BlockNumber b)
-{
-	uint32		h = b;
-
-	h ^= h >> 16;
-	h *= 0x85ebca6b;
-	h ^= h >> 13;
-	h *= 0xc2b2ae35;
-	h ^= h >> 16;
-	return h;
-}
-
 /* define hashtable mapping block numbers to PagetableEntry's */
 #define SH_USE_NONDEFAULT_ALLOCATOR
 #define SH_PREFIX pagetable
 #define SH_ELEMENT_TYPE PagetableEntry
 #define SH_KEY_TYPE BlockNumber
 #define SH_KEY blockno
-#define SH_HASH_KEY(tb, key) hash_blockno(key)
+#define SH_HASH_KEY(tb, key) murmurhash32(key)
 #define SH_EQUAL(tb, a, b) a == b
 #define SH_SCOPE static inline
 #define SH_DEFINE

src/include/utils/hashutils.h

@@ -20,4 +20,22 @@ hash_combine(uint32 a, uint32 b)
 	return a;
 }
 
+
+/*
+ * Simple inline murmur hash implementation hashing a 32 bit ingeger, for
+ * performance.
+ */
+static inline uint32
+murmurhash32(uint32 data)
+{
+	uint32		h = data;
+
+	h ^= h >> 16;
+	h *= 0x85ebca6b;
+	h ^= h >> 13;
+	h *= 0xc2b2ae35;
+	h ^= h >> 16;
+	return h;
+}
+
 #endif							/* HASHUTILS_H */