Re: 答复: [PATCH][nf-next] netfilter: replace modulo operation with bitwise AND

[Florian Westphal <fw@xxxxxxxxx>]

Li,Rongqing <lirongqing@xxxxxxxxx> wrote:
> 
> 
> > -----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

& 1023.

With -O2 gcc emits same code regardless of % 1024 or & 1023.

> 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

But in that case the value isn't known at compile time.

gcc can't know that '= reg1 % reg2' can be rewritten as 'reg1 & (reg2 - 1)'

But it can do it if reg2 is a known constant (and a power of 2).