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>

diff --git a/net/core/filter.c b/net/core/filter.c
index c0b68f7c8036b0a2903826304014912c76bf8c50..23e0a2a9a4dec09de358839ed6c396e74cd802fe 100644 (file)
--- 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,
@@ -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: