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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@gmail.com>
Acked-by: Changli Gao <xiaosuo@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Eric Dumazet 2010-11-18 22:04:46 +00:00 committed by David S. Miller
parent 8c1592d68b
commit c26aed40f4
1 changed files with 3 additions and 1 deletions
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4
net/core/filter.c
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@ -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,
@ -205,7 +206,7 @@ unsigned int sk_run_filter(struct sk_buff *skb, const struct sock_filter *fentry
A /= X;
continue;
case BPF_S_ALU_DIV_K:
A /= K;
A = reciprocal_divide(A, K);
continue;
case BPF_S_ALU_AND_X:
A &= X;
@ -506,6 +507,7 @@ 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: