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