fs/dcache.c: Save one 32-bit multiply in dcache lookup

Noe that we're mixing in the parent pointer earlier, we
don't need to use hash_32() to mix its bits.  Instead, we can
just take the msbits of the hash value directly.

For those applications which use the partial_name_hash(),
move the multiply to end_name_hash.

Signed-off-by: George Spelvin <linux@sciencehorizons.net>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/dcache.c

@@ -106,7 +106,7 @@ static struct hlist_bl_head *dentry_hashtable __read_mostly;
 
 static inline struct hlist_bl_head *d_hash(unsigned int hash)
 {
-	return dentry_hashtable + hash_32(hash, d_hash_shift);
+	return dentry_hashtable + (hash >> (32 - d_hash_shift));
 }
 
 #define IN_LOOKUP_SHIFT 10

include/linux/stringhash.h

@@ -3,6 +3,7 @@
 
 #include <linux/compiler.h>	/* For __pure */
 #include <linux/types.h>	/* For u32, u64 */
+#include <linux/hash.h>
 
 /*
  * Routines for hashing strings of bytes to a 32-bit hash value.
@@ -45,11 +46,12 @@ partial_name_hash(unsigned long c, unsigned long prevhash)
 
 /*
  * Finally: cut down the number of bits to a int value (and try to avoid
- * losing bits)
+ * losing bits).  This also has the property (wanted by the dcache)
+ * that the msbits make a good hash table index.
  */
 static inline unsigned long end_name_hash(unsigned long hash)
 {
-	return (unsigned int)hash;
+	return __hash_32((unsigned int)hash);
 }
 
 /*