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fib_trie: Update last spot w/ idx >> n->bits code and explanation

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This change updates the fib_table_lookup function so that it is in sync
with the fib_find_node function in terms of the explanation for the index
check based on the bits value.

I have also updated it from doing a mask to just doing a compare as I have
found that seems to provide more options to the compiler as I have seen it
turn this into a shift of the value and test under some circumstances.

In addition I addressed one minor issue in which we kept computing the key
^ n->key when checking the fib aliases.  I pulled the xor out of the loop
in order to reduce the number of memory reads in the lookup.  As a result
we should save a couple cycles since the xor is only done once much earlier
in the lookup.

Signed-off-by: Alexander Duyck <alexander.h.duyck@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Alexander Duyck 2015-03-04 15:04:03 -08:00 committed by David S. Miller
parent a7e5353123
commit 71e8b67d0f
1 changed files with 13 additions and 5 deletions
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18
net/ipv4/fib_trie.c
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@ -1201,6 +1201,7 @@ int fib_table_lookup(struct fib_table *tb, const struct flowi4 *flp,
const t_key key = ntohl(flp->daddr);
struct tnode *n, *pn;
struct fib_alias *fa;
unsigned long index;
t_key cindex;
n = rcu_dereference(t->trie);
@ -1216,19 +1217,23 @@ int fib_table_lookup(struct fib_table *tb, const struct flowi4 *flp,
/* Step 1: Travel to the longest prefix match in the trie */
for (;;) {
unsigned long index = get_index(key, n);
index = get_index(key, n);
/* This bit of code is a bit tricky but it combines multiple
* checks into a single check. The prefix consists of the
* 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 & (~0ul << bits))
* if (index >= (1ul << bits))
* we have a mismatch in skip bits and failed
* else
* we know the value is cindex
*
* This check is safe even if bits == KEYLENGTH due to the
* fact that we can only allocate a node with 32 bits if a
* long is greater than 32 bits.
*/
if (index & (~0ul << n->bits))
if (index >= (1ul << n->bits))
break;
/* we have found a leaf. Prefixes have already been compared */
@ -1302,14 +1307,17 @@ int fib_table_lookup(struct fib_table *tb, const struct flowi4 *flp,
}
found:
/* this line carries forward the xor from earlier in the function */
index = key ^ n->key;
/* Step 3: Process the leaf, if that fails fall back to backtracing */
hlist_for_each_entry_rcu(fa, &n->leaf, fa_list) {
struct fib_info *fi = fa->fa_info;
int nhsel, err;
if (((key ^ n->key) >= (1ul << fa->fa_slen)) &&
if ((index >= (1ul << fa->fa_slen)) &&
((BITS_PER_LONG > KEYLENGTH) || (fa->fa_slen != KEYLENGTH)))
continue;
continue;
if (fa->fa_tos && fa->fa_tos != flp->flowi4_tos)
continue;
if (fi->fib_dead)