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>
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e1b0048e
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