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- From: Alexander Duyck <alexander.h.duyck@redhat.com>
- Date: Thu, 22 Jan 2015 15:51:08 -0800
- Subject: [PATCH] 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: Alexander Duyck <alexander.h.duyck@redhat.com>
- Signed-off-by: David S. Miller <davem@davemloft.net>
- ---
- --- a/net/ipv4/fib_trie.c
- +++ b/net/ipv4/fib_trie.c
- @@ -961,12 +961,12 @@ static struct tnode *fib_find_node(struc
- * 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 *t
- * 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 */
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