netdev - Re: [PATCH 3/3] netns: fix net_generic() "id

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Date:   Wed, 14 Dec 2016 16:19:33 +0300
From:   Alexey Dobriyan <adobriyan@...il.com>
To:     David Laight <David.Laight@...lab.com>
Cc:     "davem@...emloft.net" <davem@...emloft.net>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "xemul@...nvz.org" <xemul@...nvz.org>
Subject: Re: [PATCH 3/3] netns: fix net_generic() "id - 1" bloat

On Tue, Dec 13, 2016 at 5:42 PM, David Laight <David.Laight@...lab.com> wrote:
> From: Alexey Dobriyan
>> Sent: 13 December 2016 14:23
> ...
>> Well, the point of the patch is to save .text, so might as well save
>> as much as possible. Any form other than "ptr[id]" is going
>> to be either bigger or bigger and slower and "ptr" should be the first field.
>
> You've not read and understood the next bit:
>
>> > However if you offset the 'id' values so that only
>> > values 2 up are valid the code becomes:
>> >         return net->gen2->ptr[id - 2];
>> > which will be exactly the same code as:
>> >         return net->gen1->ptr[id];
>> > but it is much more obvious that 'id' values must be >= 2.
>> >
>> > The '2' should be generated from the structure offset, but with my method
>> > is doesn't actually matter if it is wrong.
>
> If you have foo->bar[id - const] then the compiler has to add the
> offset of 'bar' and subtract for 'const'.
> If the numbers match no add or subtract is needed.
>
> It is much cleaner to do this by explicitly removing the offset on the
> accesses than using a union.

Surprisingly, the trick only works if array index is cast to "unsigned long"
before subtracting.

Code becomes

    ...
    ptr = ng->ptr[(unsigned long)id - 3];
    ...

I'll post a patch when net-next reopens.