Current addr4_match() code has special test for /0 prefixes because of
standard required undefined behaviour. However, it is possible to omit
it on 64-bit because shifting can be done within a 64-bit register and
then truncated to the expected value (which is 0 mask).
Implicit truncation by htonl() fits nicely into R32-within-R64 model
on x86-64.
Space savings: none (coincidence)
Branch savings: 1
Before:
movzx eax,BYTE PTR [rdi+0x2a] # ->prefixlen_d
test al,al
jne xfrm_selector_match + 0x23f
...
movzx eax,BYTE PTR [rbx+0x2b] # ->prefixlen_s
test al,al
je xfrm_selector_match + 0x1c7
After (no branches):
mov r8d,0x20
mov rdx,0xffffffffffffffff
mov esi,DWORD PTR [rsi+0x2c]
mov ecx,r8d
sub cl,BYTE PTR [rdi+0x2a]
xor esi,DWORD PTR [rbx]
mov rdi,rdx
xor eax,eax
shl rdi,cl
bswap edi
Signed-off-by: Alexey Dobriyan <adobri...@gmail.com>
---
include/net/xfrm.h | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
--- a/include/net/xfrm.h
+++ b/include/net/xfrm.h
@@ -845,9 +845,9 @@ static inline bool addr_match(const void *token1, const
void *token2,
static inline bool addr4_match(__be32 a1, __be32 a2, u8 prefixlen)
{
/* C99 6.5.7 (3): u32 << 32 is undefined behaviour */
- if (prefixlen == 0)
+ if (sizeof(long) == 4 && prefixlen == 0)
return true;
- return !((a1 ^ a2) & htonl(0xFFFFFFFFu << (32 - prefixlen)));
+ return !((a1 ^ a2) & htonl(~0UL << (32 - prefixlen)));
}
static __inline__
