netdev - Re: [PATCH v2] net: sctp: Don't use 64 kilobyte lookup table for four elements

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Message-ID: <1443450427.2531.2.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Mon, 28 Sep 2015 07:27:07 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Laight <David.Laight@...LAB.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

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.


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