From mboxrd@z Thu Jan 1 00:00:00 1970 From: Alexey Dobriyan Subject: [PATCH v2] xfrm: branchless addr4_match() on 64-bit Date: Sat, 25 Mar 2017 19:41:17 +0300 Message-ID: <20170325164117.GB4950@avx2> References: <20170323233247.GE31372@avx2> <063D6719AE5E284EB5DD2968C1650D6DCFFB93A8@AcuExch.aculab.com> <20170325163712.GA4950@avx2> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: "herbert@gondor.apana.org.au" , "davem@davemloft.net" , "netdev@vger.kernel.org" , David.Laight@ACULAB.COM To: steffen.klassert@secunet.com Return-path: Received: from mail-lf0-f67.google.com ([209.85.215.67]:33470 "EHLO mail-lf0-f67.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751395AbdCYQlW (ORCPT ); Sat, 25 Mar 2017 12:41:22 -0400 Received: by mail-lf0-f67.google.com with SMTP id r36so2187603lfi.0 for ; Sat, 25 Mar 2017 09:41:21 -0700 (PDT) Content-Disposition: inline In-Reply-To: <20170325163712.GA4950@avx2> Sender: netdev-owner@vger.kernel.org List-ID: 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 --- 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__