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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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