netdev - Re: [PATCH net-next] net: sched: use no more than one page in struct fw

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Message-ID: <20140317135101.GA8956@casper.infradead.org>
Date:	Mon, 17 Mar 2014 13:51:01 +0000
From:	Thomas Graf <tgraf@...g.ch>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	David Miller <davem@...emloft.net>,
	John Fastabend <john.fastabend@...il.com>,
	netdev@...r.kernel.org
Subject: Re: [PATCH net-next] net: sched: use no more than one page in struct
 fw_head

On 03/16/14 at 09:06am, Eric Dumazet wrote:
> From: Eric Dumazet <edumazet@...gle.com>
> 
> In commit b4e9b520ca5d ("[NET_SCHED]: Add mask support to fwmark
> classifier") Patrick added an u32 field in fw_head, making it slightly
> bigger than one page.
> 
> Change the layout of this structure and let compiler emit a reciprocal
> divide for fw_hash(), as this makes the core more readable and
> more efficient those days.

I think you  need to educate me a bit on this. objdump
spits out the following:

static u32 fw_hash(u32 handle)
{
        return handle % HTSIZE;
  1d:   bf ff 01 00 00          mov    edi,0x1ff
  22:   89 f0                   mov    eax,esi
  24:   31 d2                   xor    edx,edx
  26:   f7 f7                   div    edi

Doesn't look like a reciprocal div to me. Where did I
screw up or why doesn't gcc optimize it properly?
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