From: YOSHIFUJI Hideaki / 吉藤英明 <yoshfuji@linux-ipv6.org>
Date: Sat, 27 Mar 2010 01:24:16 +0000 (+0000)
Subject: ipv6 fib: Use "Sweezle" to optimize addr_bit_test().
X-Git-Url: https://git.stricted.de/?a=commitdiff_plain;h=02cdce53f3d0d3eee8188944c96150ee8c97100d;p=GitHub%2FLineageOS%2Fandroid_kernel_samsung_universal7580.git
ipv6 fib: Use "Sweezle" to optimize addr_bit_test().
addr_bit_test() is used in various places in IPv6 routing table
subsystem. It checks if the given fn_bit is set,
where fn_bit counts bits from MSB in words in network-order.
fn_bit : 0 .... 31 32 .... 64 65 .... 95 96 ....127
fn_bit >> 5 gives offset of word, and (~fn_bit & 0x1f) gives
count from LSB in the network-endian word in question.
fn_bit >> 5 : 0 1 2 3
~fn_bit & 0x1f: 31 .... 0 31 .... 0 31 .... 0 31 .... 0
Thus, the mask was generated as htonl(1 << (~fn_bit & 0x1f)).
This can be optimized by "sweezle" (See include/asm-generic/bitops/le.h).
In little-endian,
htonl(1 << bit) = 1 << (bit ^ BITOP_BE32_SWIZZLE)
where
BITOP_BE32_SWIZZLE is (0x1f & ~7)
So,
htonl(1 << (~fn_bit & 0x1f)) = 1 << ((~fn_bit & 0x1f) ^ (0x1f & ~7))
= 1 << ((~fn_bit ^ ~7) & 0x1f)
= 1 << ((~fn_bit ^ BITOP_BE32_SWIZZLE) & 0x1f)
In big-endian, BITOP_BE32_SWIZZLE is equal to 0.
1 << ((~fn_bit ^ BITOP_BE32_SWIZZLE) & 0x1f)
= 1 << ((~fn_bit) & 0x1f)
= htonl(1 << (~fn_bit & 0x1f))
Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
---
diff --git a/net/ipv6/ip6_fib.c b/net/ipv6/ip6_fib.c
index 2f9847924fa..68119ef6286 100644
--- a/net/ipv6/ip6_fib.c
+++ b/net/ipv6/ip6_fib.c
@@ -127,12 +127,23 @@ static __inline__ u32 fib6_new_sernum(void)
/*
* test bit
*/
+#if defined(__LITTLE_ENDIAN)
+# define BITOP_BE32_SWIZZLE (0x1F & ~7)
+#else
+# define BITOP_BE32_SWIZZLE 0
+#endif
static __inline__ __be32 addr_bit_set(void *token, int fn_bit)
{
__be32 *addr = token;
-
- return htonl(1 << ((~fn_bit)&0x1F)) & addr[fn_bit>>5];
+ /*
+ * Here,
+ * 1 << ((~fn_bit ^ BITOP_BE32_SWIZZLE) & 0x1f)
+ * is optimized version of
+ * htonl(1 << ((~fn_bit)&0x1F))
+ * See include/asm-generic/bitops/le.h.
+ */
+ return (1 << ((~fn_bit ^ BITOP_BE32_SWIZZLE) & 0x1f)) & addr[fn_bit >> 5];
}
static __inline__ struct fib6_node * node_alloc(void)