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Date: Wed, 26 May 2004 23:04:00 +0200
From: Mikael Pettersson <mikpe@csd.uu.se>
To: Paul Jackson <pj@sgi.com>
Cc: akiyama.nobuyuk@jp.fujitsu.com, akpm@osdl.org,
       linux-kernel@vger.kernel.org
Subject: Re: [PATCH] NMI trigger switch support for debugging
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Paul Jackson writes:
 > Mikael Pettersson, replying to AKIYAMA Nobuyuki:
 > >  > +	if (!old_state == !unknown_nmi_panic)
 > >  > +		return 0;
 > > 
 > > This conditional looks terribly obscure.
 > 
 > Would the following variant seem clearer:
 > 
 > 	if (!!unknown_nmi_panic == !!old_state)
 > 		return 0;
 > 
 > Odd, I know.  For those of us familiar with the '!!' idiom, which
 > converts any value to its binary logical equivalent 0 (if zero) or
 > 1 (otherwise), this reads as:
 > 
 > 	if (the logical value of unknown_nmi_panic is unchanged)
 > 		return 0;

The !! idiom has the advantage of making it crystal clear
that the author is comparing boolean-normalised values.
The code I commented on was unusual enough that I couldn't
ignore the possibility of a bug.

In this case, I'd prefer the !! idiom, or moving the test into
the two state-changing code snippets below it.

/Mikael