From: Or Gerlitz <ogerlitz@mellanox•com>
To: Tom Herbert <therbert@google•com>
Cc: Eric Dumazet <eric.dumazet@gmail•com>,
Jerry Chu <hkchu@google•com>,
"Eric Dumazet" <edumazet@google•com>,
Herbert Xu <herbert@gondor•apana.org.au>,
Linux Netdev List <netdev@vger•kernel.org>,
Yan Burman <yanb@mellanox•com>,
Shlomo Pongratz <shlomop@mellanox•com>
Subject: Re: [PATCH net-next V2 3/3] net: Add GRO support for vxlan traffic
Date: Wed, 8 Jan 2014 11:45:34 +0200 [thread overview]
Message-ID: <52CD1E3E.3090806@mellanox.com> (raw)
In-Reply-To: <CA+mtBx_Zctape7mAA=d-bvrFJbzn8f6tW5jBnPUcL5Cfhfai-g@mail.gmail.com>
On 07/01/2014 23:09, Tom Herbert wrote:
> On Tue, Jan 7, 2014 at 12:12 PM, Or Gerlitz <or.gerlitz@gmail•com> wrote:
>> On Tue, Jan 7, 2014 at 10:02 PM, Eric Dumazet <eric.dumazet@gmail•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@google•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.
OK, I will follow that.
Or.
next prev parent reply other threads:[~2014-01-08 9:46 UTC|newest]
Thread overview: 36+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-01-07 15:29 [PATCH net-next V2 0/3] net: Add GRO support for UDP encapsulating protocols Or Gerlitz
2014-01-07 15:29 ` [PATCH net-next V2 1/3] " Or Gerlitz
2014-01-07 16:33 ` Eric Dumazet
2014-01-07 20:19 ` Or Gerlitz
2014-01-07 20:32 ` Eric Dumazet
2014-01-07 20:37 ` Or Gerlitz
2014-01-07 21:38 ` Eric Dumazet
2014-01-08 8:04 ` Or Gerlitz
[not found] ` <1389182291.26646.79.camel@edumazet-glaptop2.roam.corp.google.com>
2014-01-08 12:15 ` Or Gerlitz
2014-01-07 22:11 ` Jerry Chu
2014-01-08 8:02 ` Or Gerlitz
2014-01-09 3:12 ` Jerry Chu
2014-01-09 6:35 ` Or Gerlitz
2014-01-09 7:19 ` Or Gerlitz
2014-01-07 18:44 ` Tom Herbert
2014-01-07 20:21 ` Or Gerlitz
2014-01-07 23:04 ` Tom Herbert
2014-01-08 16:11 ` Or Gerlitz
2014-01-08 16:29 ` Tom Herbert
2014-01-08 16:31 ` Or Gerlitz
2014-01-07 21:19 ` David Miller
2014-01-07 21:40 ` Tom Herbert
2014-01-07 15:29 ` [PATCH net-next V2 2/3] net: Export gro_find_by_type helpers Or Gerlitz
2014-01-07 15:29 ` [PATCH net-next V2 3/3] net: Add GRO support for vxlan traffic Or Gerlitz
2014-01-07 16:34 ` Eric Dumazet
2014-01-07 19:43 ` Or Gerlitz
2014-01-07 20:04 ` Eric Dumazet
2014-01-07 20:10 ` Or Gerlitz
2014-01-07 18:08 ` Tom Herbert
2014-01-07 19:43 ` Or Gerlitz
2014-01-07 20:02 ` Eric Dumazet
2014-01-07 20:12 ` Or Gerlitz
2014-01-07 21:09 ` Tom Herbert
2014-01-08 9:45 ` Or Gerlitz [this message]
[not found] ` <CAJZOPZLsMvmHwmMjhsuKb__2HncMXMm=p6UFnT4XX5d8hZnGxw@mail.gmail.com>
2014-01-07 19:52 ` Tom Herbert
2014-01-07 16:45 ` [PATCH net-next V2 0/3] net: Add GRO support for UDP encapsulating protocols Eric Dumazet
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