netdev - Re: [net-next 5/8] net/mlx5e: Calc vlan_tag

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Date:   Sun, 5 Feb 2017 13:22:08 +0200
From:   Saeed Mahameed <saeedm@....mellanox.co.il>
To:     David Laight <David.Laight@...lab.com>
Cc:     Saeed Mahameed <saeedm@...lanox.com>,
        "David S. Miller" <davem@...emloft.net>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [net-next 5/8] net/mlx5e: Calc vlan_tag_present only once on xmit

On Wed, Feb 1, 2017 at 1:20 PM, David Laight <David.Laight@...lab.com> wrote:
> From: Saeed Mahameed
>> Sent: 31 January 2017 20:59
>> Cache skb_vlan_tag_present(skb) and pass it wherever needed in xmit
>> routines.
> ...
>
> Does this actually generate better code?

Only in case skb pointer is kept in memory (we will save up to 3
skb->vlan_tci dereferences in that case).

> It is quite likely that your 'vlan_present' variable ends up being on stack.
> Whereas the 'skb' is likely to be in a register.

can i assume this to be likely true on all archs ?

> In which case the two loads are likely to be must the same and your
> change has added a write to the stack.
>
>         David
>
>