linux-kernel - RE: [PATCH v2] net: sctp: Don't use 64 kilobyte lookup table for four elements

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Message-ID: <063D6719AE5E284EB5DD2968C1650D6D1CBA431C@AcuExch.aculab.com>
Date:	Mon, 28 Sep 2015 15:32:44 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Eric Dumazet' <eric.dumazet@...il.com>
CC:	'Neil Horman' <nhorman@...driver.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	"David S. Miller" <davem@...emloft.net>,
	Vlad Yasevich <vyasevich@...il.com>,
	Marcelo Ricardo Leitner <marcelo.leitner@...il.com>,
	"linux-sctp@...r.kernel.org" <linux-sctp@...r.kernel.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH v2] net: sctp: Don't use 64 kilobyte lookup table for
 four elements

From: Eric Dumazet
> Sent: 28 September 2015 15:27
> On Mon, 2015-09-28 at 14:12 +0000, David Laight wrote:
> > From: Neil Horman
> > > Sent: 28 September 2015 14:51
> > > On Mon, Sep 28, 2015 at 02:34:04PM +0200, Denys Vlasenko wrote:
> > > > Seemingly innocuous sctp_trans_state_to_prio_map[] array
> > > > is way bigger than it looks, since
> > > > "[SCTP_UNKNOWN] = 2" expands into "[0xffff] = 2" !
> > > >
> > > > This patch replaces it with switch() statement.
> >
> > What about just adding 1 (and masking) before indexing the array?
> > That might require a static inline function with a local static array.
> >
> > Or define the array as (say) [16] and just mask the state before using
> > it as an index?
> 
> Just let the compiler do its job, instead of obfuscating source.
> 
> Compilers can transform a switch into an (optimal) table if it is really
> a gain.

The compiler can choose between a jump table and nested ifs for a switch
statement. I've never seen it convert one into a data array index.

	David