netdev - RE: [PATCH net-next v3] bnxt_en: New Broadcom ethernet driver.

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Message-ID: <063D6719AE5E284EB5DD2968C1650D6D1CBBC7E7@AcuExch.aculab.com>
Date:	Tue, 20 Oct 2015 15:47:53 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Stephen Hemminger' <stephen@...workplumber.org>,
	Michael Chan <mchan@...adcom.com>
CC:	"davem@...emloft.net" <davem@...emloft.net>,
	"prashant@...adcom.com" <prashant@...adcom.com>,
	"eddie.wai@...adcom.com" <eddie.wai@...adcom.com>,
	"huangjw@...adcom.com" <huangjw@...adcom.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: RE: [PATCH net-next v3] bnxt_en: New Broadcom ethernet driver.

From: Stephen Hemminger
...
> On Sat, 17 Oct 2015 00:21:44 -0400
> Michael Chan <mchan@...adcom.com> wrote:
> 
> > +static bool bnxt_vf_pciid(int idx)
> > +{
> > +	if (idx == BCM57304_VF || idx == BCM57404_VF)
> > +		return true;
> > +	return false;
> > +}
> > +
> 
> I prefer just returning result of logical operation
> rather than adding unnecessary if statement.
> And never use (signed) int when unsigned is the real
> data type. Also avoid any unnecessary expansion of 16 bit
> value.

Actually 16-bit (and 8-bit) values always have to be extended to 'int'
before any arithmetic operations.
On systems with 16-bit arithmetic instructions (like x86) the
compiler can only use them if it can determine that result would be
the same (as if they were extended).

This can mean a lot of extra instructions if you do arithmetic
on 16-bit values the compiler is holding in registers.

Similarly using 16-bit values as function parameters/results
can require additional masking instructions.

IMHO This really all means that you should use [unsigned] int for
local variables even when the domain of the value is known
to be much smaller.

	David
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