netdev - Re: [PATCH net-next V2 3/3] net: Add GRO support for vxlan traffic

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Date:	Tue, 7 Jan 2014 22:12:06 +0200
From:	Or Gerlitz <or.gerlitz@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Tom Herbert <therbert@...gle.com>,
	Or Gerlitz <ogerlitz@...lanox.com>,
	Jerry Chu <hkchu@...gle.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	Linux Netdev List <netdev@...r.kernel.org>,
	David Miller <davem@...emloft.net>,
	Yan Burman <yanb@...lanox.com>,
	Shlomo Pongratz <shlomop@...lanox.com>
Subject: Re: [PATCH net-next V2 3/3] net: Add GRO support for vxlan traffic

On Tue, Jan 7, 2014 at 10:02 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Tue, 2014-01-07 at 21:43 +0200, Or Gerlitz wrote:
>> On Tue, Jan 7, 2014 at 8:08 PM, Tom Herbert <therbert@...gle.com> wrote:
>
>> > Why ^ instead of != ?
>>
>> The XOR approach is very popular in the GRO stack, e.g see the IPv4 chain
>> of inet_gro_receive() && tcp_gro_receive(), I guess this might relates
>> to more efficient assembly code for ^ vs. != and/or the fast/elegant
>> transitive nature of that operator
>
> This trick is only needed/used when many compares are folded into a
> single conditional :
>
> if (a->f1 != b->f1 || a->f2 != b->f2)
>
> ->
>
> if (((a->f1 ^ b->f1) | (a->f2 ^ b->f2)) != 0)
>
> Please do not use XOR for a single compare.

OK, but just out of curiosity -- what's the reasoning? clarity or
efficiency or both?
>
>
>
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