target/linux/generic/patches-3.18/080-19-fib_trie-Use-index-0ul-n-bits-instead-of-index-n-bit.patch

   1 From: Alexander Duyck <alexander.h.duyck@redhat.com>
   2 Date: Thu, 22 Jan 2015 15:51:08 -0800
   3 Subject: [PATCH] fib_trie: Use index & (~0ul << n->bits) instead of index >>
   4  n->bits
   5
   6 In doing performance testing and analysis of the changes I recently found
   7 that by shifting the index I had created an unnecessary dependency.
   8
   9 I have updated the code so that we instead shift a mask by bits and then
  10 just test against that as that should save us about 2 CPU cycles since we
  11 can generate the mask while the key and pos are being processed.
  12
  13 Signed-off-by: Alexander Duyck <alexander.h.duyck@redhat.com>
  14 Signed-off-by: David S. Miller <davem@davemloft.net>
  15 ---
  16
  17 --- a/net/ipv4/fib_trie.c
  18 +++ b/net/ipv4/fib_trie.c
  19 @@ -961,12 +961,12 @@ static struct tnode *fib_find_node(struc
  20                  * prefix plus zeros for the bits in the cindex. The index
  21                  * is the difference between the key and this value.  From
  22                  * this we can actually derive several pieces of data.
  23 -                *   if !(index >> bits)
  24 -                *     we know the value is cindex
  25 -                *   else
  26 +                *   if (index & (~0ul << bits))
  27                  *     we have a mismatch in skip bits and failed
  28 +                *   else
  29 +                *     we know the value is cindex
  30                  */
  31 -               if (index >> n->bits)
  32 +               if (index & (~0ul << n->bits))
  33                         return NULL;
  34
  35                 /* we have found a leaf. Prefixes have already been compared */
  36 @@ -1301,12 +1301,12 @@ int fib_table_lookup(struct fib_table *t
  37                  * prefix plus zeros for the "bits" in the prefix. The index
  38                  * is the difference between the key and this value.  From
  39                  * this we can actually derive several pieces of data.
  40 -                *   if !(index >> bits)
  41 -                *     we know the value is child index
  42 -                *   else
  43 +                *   if (index & (~0ul << bits))
  44                  *     we have a mismatch in skip bits and failed
  45 +                *   else
  46 +                *     we know the value is cindex
  47                  */
  48 -               if (index >> n->bits)
  49 +               if (index & (~0ul << n->bits))
  50                         break;
  51
  52                 /* we have found a leaf. Prefixes have already been compared */