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

From: Eric Dumazet <edumazet@google.com>

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

Signed-off-by: Eric Dumazet <edumazet@google.com>
---
 net/ipv4/fib_trie.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/ipv4/fib_trie.c b/net/ipv4/fib_trie.c
index f0cdb30..0bb20c4 100644
--- a/net/ipv4/fib_trie.c
+++ b/net/ipv4/fib_trie.c

@@ -1550,7 +1550,7 @@ int fib_table_lookup(struct fib_table *tb, const struct flowi4 *flp,
 		 * state.directly.
 		 */
 		if (pref_mismatch) {
-			int mp = KEYLENGTH - fls(pref_mismatch);
+			int mp = KEYLENGTH - __fls(pref_mismatch);
 
 			if (tkey_extract_bits(cn->key, mp, cn->pos - mp) != 0)
 				goto backtrace;

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.

Ben.

-- 
Ben Hutchings, Staff Engineer, Solarflare
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.

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:

quoted

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 <redacted>
---
 net/ipv4/fib_trie.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

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;

From: Eric Dumazet <redacted>
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 <redacted>

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.