netdev - Re: [PATCH] net: code cleanups

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Date:	Thu, 30 Sep 2010 14:09:19 +0800
From:	Changli Gao <xiaosuo@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	"David S. Miller" <davem@...emloft.net>,
	Alexey Kuznetsov <kuznet@....inr.ac.ru>,
	"Pekka Savola (ipv6)" <pekkas@...core.fi>,
	James Morris <jmorris@...ei.org>,
	Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
	Patrick McHardy <kaber@...sh.net>, netdev@...r.kernel.org
Subject: Re: [PATCH] net: code cleanups

On Thu, Sep 30, 2010 at 1:16 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> Le jeudi 30 septembre 2010 à 10:24 +0800, Changli Gao a écrit :
>> Compare operations are more readable, and compilers generate the same code
>> for the both.
>>
>
> You have a buggy compiler then.

gcc version 4.4.3 (Gentoo 4.4.3-r2 p1.2)

             rth = rcu_dereference(rth->dst.rt_next)) {
                if ((((__force u32)rth->fl.fl4_dst ^ (__force u32)daddr) |
                     ((__force u32)rth->fl.fl4_src ^ (__force u32)saddr) |
                     (rth->fl.iif ^ iif) |
    2f12:       44 3b 80 dc 00 00 00    cmp    0xdc(%rax),%r8d
    2f19:       0f 85 a2 00 00 00       jne    2fc1 <ip_route_input_common+0x145
>
                     rth->fl.oif |
    2f1f:       83 b8 d8 00 00 00 00    cmpl   $0x0,0xd8(%rax)
    2f26:       0f 85 95 00 00 00       jne    2fc1 <ip_route_input_common+0x145
>
        tos &= IPTOS_RT_MASK;
        hash = rt_hash(daddr, saddr, iif, rt_genid(net));

        for (rth = rcu_dereference(rt_hash_table[hash].chain); rth;
             rth = rcu_dereference(rth->dst.rt_next)) {
                if ((((__force u32)rth->fl.fl4_dst ^ (__force u32)daddr) |
    2f2c:       44 3b b8 e4 00 00 00    cmp    0xe4(%rax),%r15d
    2f33:       0f 85 88 00 00 00       jne    2fc1
<ip_route_input_common+0x145>
                     ((__force u32)rth->fl.fl4_src ^ (__force u32)saddr) |
    2f39:       44 3b b0 e8 00 00 00    cmp    0xe8(%rax),%r14d
    2f40:       75 7f                   jne    2fc1
<ip_route_input_common+0x145>


>
> I know this code is ugly, but please keep it as is, dont add conditional
> branches on hot paths.
>

If the compiler doesn't generate conditional branches, we have to
touch every necessary field of all the cache entries in one hash
bucket. Is it better than condition branch? I think the compiler
developers know it better.

And the compiler reorders the conditional branches, is it expected?

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