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

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Date:   Sat, 9 Dec 2017 01:23:30 +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 12:22 AM, David Laight <David.Laight@...lab.com> wrote:
> From: Xin Long
>> Sent: 08 December 2017 16:18
>>
> ...
>> >> Alternatively you could preform the dereference in two steps (i.e. declare an si
>> >> pointer on the stack and set it equal to asoc->stream.si, then deref
>> >> si->make_datafrag at call time.  That will at least give the compiler an
>> >> opportunity to preload the first pointer.
>
> You want to save the function pointer itself.
>
> ...
>> Another small difference:
>>   as you can see, comparing to (X), (Y) is using 0x28(%rsp) in the loop,
>>   instead of %r13.
>>
>> So that's what I can see from the related generated code.
>> If 0x848(%r13) is not worse than 0x28(%rsp) for cpu, I think
>> asoc->stream.si->make_datafrag() is even better. No ?
>
> That code must have far too many life local variables.
> Otherwise there's be a caller saved register available.
>
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.

For the codes style, comparing to the if-else, I think this one is
more readable. (ignore extendible stuff first, as probably no more
new type of data chunk).