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

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Date:	Tue, 7 Jan 2014 13:09:50 -0800
From:	Tom Herbert <therbert@...gle.com>
To:	Or Gerlitz <or.gerlitz@...il.com>
Cc:	Eric Dumazet <eric.dumazet@...il.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 12:12 PM, Or Gerlitz <or.gerlitz@...il.com> wrote:
> 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?

Both. Compiling a simple program and comparing alternatives: gcc
produced the identical code for the single conditional (^ vs !=)
using the cmp instruction. Testing the two conditional case like Eric
provided; the second method (using ^) resulted in 4 more instructions,
but only one branch as opposed to two in the first method (!=). Method
#1 has the advantage of short circuiting when the first condition is
true, so organizing the conditionals to maximize the probability of
short circuit could be beneficial.


>>
>>
>>
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