diff mbox

[net-next] fib: use __fls() on non null argument

Message ID 1344372347.28967.154.camel@edumazet-glaptop
State Accepted, archived
Delegated to: David Miller
Headers show

Commit Message

Eric Dumazet Aug. 7, 2012, 8:45 p.m. UTC 
From: Eric Dumazet <edumazet@google.com>

On Tue, 2012-08-07 at 21:05 +0100, Ben Hutchings wrote:
> On Tue, 2012-08-07 at 15:30 +0200, Eric Dumazet wrote:
> > From: Eric Dumazet <edumazet@google.com>
> > 
> > __fls(x) is a bit faster than fls(x), granted we know x is non null.
> 
> And it doesn't have the +1 bias, so this change is not correct.

Good catch, I wonder why my routing was still working...

[PATCH v2 net-next] fib: use __fls() on non null argument

__fls(x) is a bit faster than fls(x), granted we know x is non null.

As Ben Hutchings pointed out, fls(x) = __fls(x) + 1

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Ben Hutchings <bhutchings@solarflare.com>
---
 net/ipv4/fib_trie.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)



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Comments

David Miller Aug. 7, 2012, 11:27 p.m. UTC | #1 
From: Eric Dumazet <eric.dumazet@gmail.com>
Date: Tue, 07 Aug 2012 22:45:47 +0200

> [PATCH v2 net-next] fib: use __fls() on non null argument
> 
> __fls(x) is a bit faster than fls(x), granted we know x is non null.
> 
> As Ben Hutchings pointed out, fls(x) = __fls(x) + 1
> 
> Signed-off-by: Eric Dumazet <edumazet@google.com>
> Cc: Ben Hutchings <bhutchings@solarflare.com>

Applied.

This is, btw, the most expensive part of fib_trie on sparc64 since we
really don't have a universal way to do this in a hardware instruction
and therefore we end up with the branch-heavy software implementation :-/

So if anyone can come up with a way to eliminate this fls() entirely,
you will be my hero.
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diff mbox

Patch

diff --git a/net/ipv4/fib_trie.c b/net/ipv4/fib_trie.c
index f0cdb30..f84a0e9 100644
--- a/net/ipv4/fib_trie.c
+++ b/net/ipv4/fib_trie.c
@@ -1550,7 +1550,8 @@  int fib_table_lookup(struct fib_table *tb, const struct flowi4 *flp,
 		 * state.directly.
 		 */
 		if (pref_mismatch) {
-			int mp = KEYLENGTH - fls(pref_mismatch);
+			/* fls(x) = __fls(x) + 1 */
+			int mp = KEYLENGTH - __fls(pref_mismatch) - 1;
 
 			if (tkey_extract_bits(cn->key, mp, cn->pos - mp) != 0)
 				goto backtrace;