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

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Date:   Tue, 13 Dec 2016 14:42:53 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Alexey Dobriyan' <adobriyan@...il.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

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.

	David