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

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Message-ID: <063D6719AE5E284EB5DD2968C1650D6DB023F798@AcuExch.aculab.com>
Date:   Wed, 14 Dec 2016 14:41:51 +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: 14 December 2016 13:20
...
> > 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];
>     ...

The compiler may also be able to optimise it away if 'id' is 'int'
rather than 'unsigned int'.

Oh, if you need casts like that use an accessor function.

	David