linux-kernel - Re: [PATCH 6/9] drivers/net/wireless/iwlwifi/iwl-4965.c: Correct use of ! and &

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Message-ID: <47CE8E68.5060701@knosof.co.uk>
Date:	Wed, 05 Mar 2008 12:13:28 +0000
From:	Derek M Jones <derek@...sof.co.uk>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Harvey Harrison <harvey.harrison@...il.com>,
	Christopher Li <sparse@...isli.org>,
	Julia Lawall <julia@...u.dk>, yi.zhu@...el.com,
	linux-wireless@...r.kernel.org,
	ipw3945-devel@...ts.sourceforge.net, linux-kernel@...r.kernel.org,
	kernel-janitors@...r.kernel.org,
	Alexander Viro <viro@....linux.org.uk>,
	linux-sparse@...r.kernel.org, Josh Triplett <josh@...edesktop.org>
Subject: Re: [PATCH 6/9] drivers/net/wireless/iwlwifi/iwl-4965.c: Correct
 use of ! and &

All,

>>> i think there might be similar patterns: "x & !y", "!x | y", "x | !y" ?
>>>
>> Well, (!x & y) and (!x | y) are probably the two that might have been 
>> intended otherwise.  (x & !y), (x | !y) are probably ok.
> 
> i think the proper intention in the latter cases is (x & ~y) and
> (x | ~y).
> 
> My strong bet is that in 99% of the cases they are real bugs and && or 
> || was intended.

Developer knowledge of operator precedence and the issue of what
they intended to write are interesting topics.  Some experimental
work is described in (binary operators only I'm afraid):

www.knosof.co.uk/cbook/accu06a.pdf
www.knosof.co.uk/cbook/accu07a.pdf

The ACCU 2006 experiment provides evidence that developer knowledge
is proportional to the number of occurrences of a construct in
source code, it also shows a stunningly high percentage of incorrect
answers.

The ACCU 2007 experiment provides evidence that the names of the
operands has a significant impact on operator precedence choice.

I wonder what kind of names are used as the operand of unary
operators?

I would expect the ~ operator to have a bitwise name, but the
! operator might have an arithmetic or bitwise name.

-- 
Derek M. Jones                              tel: +44 (0) 1252 520 667
Knowledge Software Ltd                      mailto:derek@...sof.co.uk
Applications Standards Conformance Testing    http://www.knosof.co.uk
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