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

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

[Eric Dumazet]

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;

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

[Ben Hutchings]

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.

> 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;
> 
> 
> --
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-- 
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.

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

[Eric Dumazet]

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(-)

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;

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

[David Miller]

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.