From: David Laight <David.Laight@ACULAB.COM>
To: 'Alexey Dobriyan' <adobriyan@gmail.com>
Cc: "davem@davemloft.net" <davem@davemloft.net>,
"netdev@vger.kernel.org" <netdev@vger.kernel.org>,
"xemul@openvz.org" <xemul@openvz.org>
Subject: RE: [PATCH 3/3] netns: fix net_generic() "id - 1" bloat
Date: Wed, 14 Dec 2016 14:41:51 +0000 [thread overview]
Message-ID: <063D6719AE5E284EB5DD2968C1650D6DB023F798@AcuExch.aculab.com> (raw)
In-Reply-To: <CACVxJT9HAMex1Pa+VbTVJwDPuY_dnfgfQgAZFSjMuUmgCiZD1w@mail.gmail.com>
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
prev parent reply other threads:[~2016-12-14 14:42 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-12-02 1:21 [PATCH 3/3] netns: fix net_generic() "id - 1" bloat Alexey Dobriyan
2016-12-05 12:49 ` David Laight
2016-12-05 14:48 ` Alexey Dobriyan
2016-12-07 10:49 ` David Laight
2016-12-13 14:23 ` Alexey Dobriyan
2016-12-13 14:42 ` David Laight
2016-12-14 13:19 ` Alexey Dobriyan
2016-12-14 14:41 ` David Laight [this message]
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