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

Eric Dumazet
2010-11-19T08:04:51Z

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@gmail.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;