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Commit b3832117 authored by Alexander Duyck's avatar Alexander Duyck Committed by David S. Miller
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fib_trie: Use index & (~0ul << n->bits) instead of index >> n->bits 



In doing performance testing and analysis of the changes I recently found
that by shifting the index I had created an unnecessary dependency.

I have updated the code so that we instead shift a mask by bits and then
just test against that as that should save us about 2 CPU cycles since we
can generate the mask while the key and pos are being processed.

Signed-off-by: default avatarAlexander Duyck <alexander.h.duyck@redhat.com>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent bc579ae5

net/ipv4/fib_trie.c

+8 −8
Original line number Diff line number Diff line
@@ -961,12 +961,12 @@ static struct tnode *fib_find_node(struct trie *t, u32 key) 
		 * prefix plus zeros for the bits in the cindex. The index

		 * is the difference between the key and this value.  From

		 * this we can actually derive several pieces of data.

		 *   if !(index >> bits)

		 *     we know the value is cindex

		 *   else

		 *   if (index & (~0ul << bits))

		 *     we have a mismatch in skip bits and failed

		 *   else

		 *     we know the value is cindex

		 */

		if (index >> n->bits)

		if (index & (~0ul << n->bits))

			return NULL;



		/* we have found a leaf. Prefixes have already been compared */
@@ -1301,12 +1301,12 @@ int fib_table_lookup(struct fib_table *tb, const struct flowi4 *flp, 
		 * prefix plus zeros for the "bits" in the prefix. The index

		 * is the difference between the key and this value.  From

		 * this we can actually derive several pieces of data.

		 *   if !(index >> bits)

		 *     we know the value is child index

		 *   else

		 *   if (index & (~0ul << bits))

		 *     we have a mismatch in skip bits and failed

		 *   else

		 *     we know the value is cindex

		 */

		if (index >> n->bits)

		if (index & (~0ul << n->bits))

			break;



		/* we have found a leaf. Prefixes have already been compared */