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

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Date:   Fri, 8 Dec 2017 16:22:05 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Xin Long' <lucien.xin@...il.com>,
        Marcelo Ricardo Leitner <marcelo.leitner@...il.com>
CC:     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

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.

	David