netdev - Re: [PATCH net-next v4 2/8] 6lowpan: add uncompress header size function

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Date:	Wed, 26 Feb 2014 19:32:11 +0100
From:	Alexander Aring <alex.aring@...il.com>
To:	Joe Perches <joe@...ches.com>
Cc:	David Laight <David.Laight@...LAB.COM>,
	"alex.bluesman.smirnov@...il.com" <alex.bluesman.smirnov@...il.com>,
	"dbaryshkov@...il.com" <dbaryshkov@...il.com>,
	"davem@...emloft.net" <davem@...emloft.net>,
	"linux-zigbee-devel@...ts.sourceforge.net" 
	<linux-zigbee-devel@...ts.sourceforge.net>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"martin.townsend@...lon.com" <martin.townsend@...lon.com>
Subject: Re: [PATCH net-next v4 2/8] 6lowpan: add uncompress header size
 function

On Wed, Feb 26, 2014 at 10:24:45AM -0800, Joe Perches wrote:
> On Wed, 2014-02-26 at 16:10 +0000, David Laight wrote:
> > From: Alexander Aring
> []
> > > diff --git a/net/ieee802154/6lowpan.h b/net/ieee802154/6lowpan.h
> []
> > > +static inline u8 lowpan_addr_mode_size(const u8 addr_mode)
> > > +{
> > > +	switch (addr_mode) {
> > > +	case LOWPAN_IPHC_ADDR_00:
> > > +		return 16;
> > > +	case LOWPAN_IPHC_ADDR_01:
> > > +		return 8;
> > > +	case LOWPAN_IPHC_ADDR_02:
> > > +		return 2;
> > > +	default:
> > > +		return 0;
> > > +	}
> > > +}
> > 
> > The compiler will generate much better code if you index an array instead
> > of using a switch statement.
> 
> Are you sure of that?
> 
> Perhaps the compiler would inline the assignment
> anyway if addr_mode is __builtin_constant_p
> 
> gcc 4.8 here does the same thing with:
> 
> static inline unsigned char f1(unsigned char a)
> {
> 	switch (a) {
> 	case 0: return 16;
> 	case 1: return 8;
> 	case 2: return 2;
> 	default: return 0;
> 	}
> }
> 
> static inline unsigned char f2(unsigned char a)
> {
> 	static const unsigned char rtns[] = { 16, 8, 2, 0 };
> 
> 	return rtns[a & 3];
> }

to make a mask with & 3 is already done, so we don't need that.
I am not sure what I should do now, change or not change? :-)

- Alex
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