netdev - [PATCH net-next-2.6] filter: use reciprocal divide

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Date:	Fri, 19 Nov 2010 09:04:46 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	"David S. Miller" <davem@...emloft.net>
Cc:	Hagen Paul Pfeifer <hagen@...u.net>, netdev@...r.kernel.org,
	Changli Gao <xiaosuo@...il.com>
Subject: [PATCH net-next-2.6] filter: use reciprocal divide

At compile time, we can replace the DIV_K instruction (divide by a
constant value) by a reciprocal divide.

At exec time, the expensive divide is replaced by a multiply, a less
expensive operation on most processors.

Signed-off-by: Eric Dumazet <eric.dumazet@...il.com>
---
 net/core/filter.c |    4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/net/core/filter.c b/net/core/filter.c
index a1edb5d..13853c7 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -37,6 +37,7 @@
 #include <asm/uaccess.h>
 #include <asm/unaligned.h>
 #include <linux/filter.h>
+#include <linux/reciprocal_div.h>
 
 enum {
 	BPF_S_RET_K = 1,
@@ -202,7 +203,7 @@ unsigned int sk_run_filter(struct sk_buff *skb, struct sock_filter *filter, int
 			A /= X;
 			continue;
 		case BPF_S_ALU_DIV_K:
-			A /= f_k;
+			A = reciprocal_divide(A, f_k);
 			continue;
 		case BPF_S_ALU_AND_X:
 			A &= X;
@@ -503,6 +504,7 @@ int sk_chk_filter(struct sock_filter *filter, int flen)
 			/* check for division by zero */
 			if (ftest->k == 0)
 				return -EINVAL;
+			ftest->k = reciprocal_value(ftest->k);
 			break;
 		case BPF_S_LD_MEM:
 		case BPF_S_LDX_MEM:


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