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| author | Alexey Dobriyan <adobriyan@gmail.com> | 2017-03-25 19:41:17 +0300 |
|---|---|---|
| committer | Steffen Klassert <steffen.klassert@secunet.com> | 2017-03-27 07:04:14 +0200 |
| commit | 6c786bcb29dd684533ec165057f437d5bb34a4b2 (patch) | |
| tree | f185635c44186df8b31a7991ac26366a9fbd0481 | |
| parent | e1b0048e18d4637603cf6f43b9b24345abdeec5c (diff) | |
| download | linux-6c786bcb29dd684533ec165057f437d5bb34a4b2.tar.gz linux-6c786bcb29dd684533ec165057f437d5bb34a4b2.tar.bz2 linux-6c786bcb29dd684533ec165057f437d5bb34a4b2.zip | |
xfrm: branchless addr4_match() on 64-bit
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 <adobriyan@gmail.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
| -rw-r--r-- | include/net/xfrm.h | 4 |
1 files changed, 2 insertions, 2 deletions
diff --git a/include/net/xfrm.h b/include/net/xfrm.h index 43b93d1134ed..9e3dc7b81a4d 100644 --- 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__ |