> -----邮件原件----- > 发件人: Florian Westphal [mailto:fw@xxxxxxxxx] > 发送时间: 2019年2月25日 19:27 > 收件人: Li,Rongqing <lirongqing@xxxxxxxxx> > 抄送: netfilter-devel@xxxxxxxxxxxxxxx > 主题: Re: [PATCH][nf-next] netfilter: replace modulo operation with bitwise > AND > > Li RongQing <lirongqing@xxxxxxxxx> wrote: > > CONNTRACK_LOCKS is 1024 and power of 2, so modulo operations can be > > replaced with AND (CONNTRACK_LOCKS - 1) > > > > and bitwise AND operation is quicker than module operation > > Uh. What kind of compiler doesn't figure that out?! > > I would prefer to keep it as-is and let compiler do the optimization. gcc version 7.3.0 (GCC) main() { int i=1000000000; i= i % 1024; return i; } 00000000004004a7 <main>: 4004a7: 55 push %rbp 4004a8: 48 89 e5 mov %rsp,%rbp 4004ab: c7 45 fc 00 ca 9a 3b movl $0x3b9aca00,-0x4(%rbp) 4004b2: 8b 45 fc mov -0x4(%rbp),%eax 4004b5: 99 cltd 4004b6: c1 ea 16 shr $0x16,%edx 4004b9: 01 d0 add %edx,%eax 4004bb: 25 ff 03 00 00 and $0x3ff,%eax 4004c0: 29 d0 sub %edx,%eax 4004c2: 89 45 fc mov %eax,-0x4(%rbp) 4004c5: 8b 45 fc mov -0x4(%rbp),%eax 4004c8: 5d pop %rbp 4004c9: c3 retq 4004ca: 66 0f 1f 44 00 00 nopw 0x0(%rax,%rax,1) ================================= main() { int i=1000000000; i= i & (1024-1); return i; } 00000000004004a7 <main>: 4004a7: 55 push %rbp 4004a8: 48 89 e5 mov %rsp,%rbp 4004ab: c7 45 fc 00 ca 9a 3b movl $0x3b9aca00,-0x4(%rbp) 4004b2: 81 65 fc ff 03 00 00 andl $0x3ff,-0x4(%rbp) 4004b9: 8b 45 fc mov -0x4(%rbp),%eax 4004bc: 5d pop %rbp 4004bd: c3 retq 4004be: 66 90 xchg %ax,%ax Similar patch: commit 1a1d74d378b13ad3f93e8975a0ade0980a49d28b Author: Jakub Kicinski <jakub.kicinski@xxxxxxxxxxxxx> Date: Mon Oct 31 20:43:17 2016 +0000 nfp: use AND instead of modulo to get ring indexes thanks -RongQing
