netdev - Re: [PATCHv2 net-next 04/12] sctp: implement make_datafrag for sctp_stream

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Message-ID: <CADvbK_dvEmGr9N_-qGW1gqBXZO7snxv0f8vZM2vZKYcCptqbEQ@mail.gmail.com>
Date:   Sat, 9 Dec 2017 01:37:14 +0800
From:   Xin Long <lucien.xin@...il.com>
To:     David Laight <David.Laight@...lab.com>
Cc:     Marcelo Ricardo Leitner <marcelo.leitner@...il.com>,
        Neil Horman <nhorman@...driver.com>,
        network dev <netdev@...r.kernel.org>,
        "linux-sctp@...r.kernel.org" <linux-sctp@...r.kernel.org>,
        "davem@...emloft.net" <davem@...emloft.net>
Subject: Re: [PATCHv2 net-next 04/12] sctp: implement make_datafrag for sctp_stream_interleave

On Sat, Dec 9, 2017 at 1:29 AM, David Laight <David.Laight@...lab.com> wrote:
> From: Xin Long ...
>> Hi, David, Sorry,  I'm not sure we're worrying about the cpu cost or
>> codes style now ?
>>
>> For cpu cost,  I think 0x848(%r13) operation must be better than the
>> generated code of if-else.
>
> Nope - the call xxx(%ryyy) is likely to be a data cache miss and a complete
> cpu pipeline stall.
>
> The conditional will be a data cache hit and the code (for one branch)
> will be prefetched and speculatively executed.
>
> Some very modern cpu might manage to predict indirect jumps, but for
> most it is a full pipeline stall.
Thanks for the CPU information.

The thing is with if-else can't avoid xxx(%ryyy) in this case, as Marcelo
said above. It seems if-else will just be a extra cost compare to this one.