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 in 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 | 5 +++++ 1 file changed, 5 insertions(+) --- a/include/net/xfrm.h +++ b/include/net/xfrm.h @@ -844,10 +844,15 @@ static inline bool addr_match(const void *token1, const void *token2, static inline bool addr4_match(__be32 a1, __be32 a2, u8 prefixlen) { +#ifdef CONFIG_64BIT + /* L in UL is not a typo. */ + return !((a1 ^ a2) & htonl(~0UL << (32 - prefixlen))); +#else /* C99 6.5.7 (3): u32 << 32 is undefined behaviour */ if (prefixlen == 0) return true; return !((a1 ^ a2) & htonl(0xFFFFFFFFu << (32 - prefixlen))); +#endif } static __inline__