netdev - Re: [PATCH v2] rps: optimize rps_get

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Date:	Thu, 22 Apr 2010 14:10:20 +0800
From:	Changli Gao <xiaosuo@...il.com>
To:	David Miller <davem@...emloft.net>
Cc:	therbert@...gle.com, eric.dumazet@...il.com, netdev@...r.kernel.org
Subject: Re: [PATCH v2] rps: optimize rps_get_cpu()

On Thu, Apr 22, 2010 at 1:40 PM, David Miller <davem@...emloft.net> wrote:
>
> I'll buy you a cookie if you can find a multiply generated by the
> compiler for "x * 4".  It's going to use shifts and those are
> basically free.

On amd64:

                if (pskb_may_pull(skb, (ihl * 4) + 4)) {
    2794:       8d 34 9d 04 00 00 00    lea    0x4(,%rbx,4),%esi
    279b:       4c 89 ef                mov    %r13,%rdi
    279e:       e8 a5 fd ff ff          callq  2548 <pskb_may_pull>
    27a3:       85 c0                   test   %eax,%eax
    27a5:       74 28                   je     27cf <get_rps_cpu+0x169>
                        __be16 *hports = (__be16 *) (skb->data + (ihl * 4));
    27a7:       8d 04 9d 00 00 00 00    lea    0x0(,%rbx,4),%eax

the compiler uses lea instead of multiply, and it should be more
efficient, but i'm not sure. Is there a equivalent of lea on the other
architectures?

>
> Please just change one thing at a time.  It would have helped you
> here.  I was willing to apply the port dereference part of your
> change, but not necessarily the 'ihl' changes.  But because you've
> combined them, I have no choice but to reject everything.
>

Ok. I think the first version is ready to apply, it only has the port
dereference part.

-- 
Regards，
Changli Gao(xiaosuo@...il.com)
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