linux-kernel - Re: [PATCH RFC] rcu: change return type to bool

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Date:	Sun, 24 May 2015 00:41:58 -0700
From:	Joe Perches <joe@...ches.com>
To:	Nicholas Mc Guire <der.herr@...r.at>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Nicholas Mc Guire <hofrat@...dl.org>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Josh Triplett <josh@...htriplett.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC] rcu: change return type to bool

On Sun, 2015-05-24 at 09:27 +0200, Nicholas Mc Guire wrote:
> On Sat, 23 May 2015, Steven Rostedt wrote:
[]
> > > -	return sum;
> > > +	return !!sum;
> > 
> > Hmm I wonder if gcc is smart enough to do the above without the need
> > for !!? That is, will it turn to !! because the return of the function
> > is bool, or does gcc complain about it not being bool without the !!?
> > Not a criticism of the patch, just a curiosity.
> >
> gcc will not complain if you assign a unsigned long to a boolean
> as I understand it it is a macro and is not doing any type 
> checking/promotion at all - so anything can be assigned to a bool
> without warning (including double and pointers).
> The !! will though always make the type compatible with int so it is 
> a well defined type atleast as far as __builtin_types_compatible_p()
> goes, and !! also makes static code checkers happy (that are maybe not
> as smart as gcc) and it does make the intent of sum being treated 
> as boolean here clear.

6.3.1.2 Boolean type

When any scalar value is converted to _Bool, the result is 0 if the
value compares equal to 0; otherwise, the result is 1.


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