> -----邮件原件-----
> 发件人: Florian Westphal [mailto:[email protected]]
> 发送时间: 2019年2月25日 19:27
> 收件人: Li,Rongqing <[email protected]>
> 抄送: [email protected]
> 主题: Re: [PATCH][nf-next] netfilter: replace modulo operation with bitwise
> AND
>
> Li RongQing <[email protected]> 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 <[email protected]>
Date: Mon Oct 31 20:43:17 2016 +0000
nfp: use AND instead of modulo to get ring indexes
thanks
-RongQing
