[PPPOE]: Improve hashing function in hash_item().

The new code produces the same results as the old version and is
~ 3 to 6 times faster for 4-bit hashes on the CPUs I tested.

Signed-off-by: Florian Zumbiehl <florz@florz.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/drivers/net/pppoe.c b/drivers/net/pppoe.c
index 35a7385..68631a5 100644 (file)
--- a/drivers/net/pppoe.c
+++ b/drivers/net/pppoe.c
@@ -108,19 +108,24 @@ static inline int cmp_addr(struct pppoe_addr *a, unsigned long sid, char *addr)
                (memcmp(a->remote,addr,ETH_ALEN) == 0));
 }
 
-static int hash_item(unsigned long sid, unsigned char *addr)
+#if 8%PPPOE_HASH_BITS
+#error 8 must be a multiple of PPPOE_HASH_BITS
+#endif
+
+static int hash_item(unsigned int sid, unsigned char *addr)
 {
-       char hash = 0;
-       int i, j;
+       unsigned char hash = 0;
+       unsigned int i;
 
-       for (i = 0; i < ETH_ALEN ; ++i) {
-               for (j = 0; j < 8/PPPOE_HASH_BITS ; ++j) {
-                       hash ^= addr[i] >> ( j * PPPOE_HASH_BITS );
-               }
+       for (i = 0 ; i < ETH_ALEN ; i++) {
+               hash ^= addr[i];
+       }
+       for (i = 0 ; i < sizeof(sid_t)*8 ; i += 8 ){
+               hash ^= sid>>i;
+       }
+       for (i = 8 ; (i>>=1) >= PPPOE_HASH_BITS ; ) {
+               hash ^= hash>>i;
        }
-
-       for (i = 0; i < (sizeof(unsigned long)*8) / PPPOE_HASH_BITS ; ++i)
-               hash ^= sid >> (i*PPPOE_HASH_BITS);
 
        return hash & ( PPPOE_HASH_SIZE - 1 );
 }