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

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Date:	Wed, 26 Feb 2014 10:24:45 -0800
From:	Joe Perches <joe@...ches.com>
To:	David Laight <David.Laight@...LAB.COM>
Cc:	'Alexander Aring' <alex.aring@...il.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, 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];
}


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