linux-kernel - Re: [tip:perf/core] bitops: Provide compile time HWEIGHT{8,16,32,64}

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Message-Id: <20100129021357.4db5823e.akpm@linux-foundation.org>
Date:	Fri, 29 Jan 2010 02:13:57 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	mingo@...hat.com, hpa@...or.com, eranian@...gle.com,
	linux-kernel@...r.kernel.org, a.p.zijlstra@...llo.nl,
	torvalds@...ux-foundation.org, tglx@...utronix.de,
	linux-tip-commits@...r.kernel.org
Subject: Re: [tip:perf/core] bitops: Provide compile time
 HWEIGHT{8,16,32,64}

On Fri, 29 Jan 2010 11:04:31 +0100 Ingo Molnar <mingo@...e.hu> wrote:

> 
> * Andrew Morton <akpm@...ux-foundation.org> wrote:
> 
> > On Fri, 29 Jan 2010 09:28:04 GMT tip-bot for Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:
> > 
> > > +#define HWEIGHT8(w)			\
> > > +      (	(!!((w) & (1ULL << 0))) +	\
> > > +	(!!((w) & (1ULL << 1))) +	\
> > > +	(!!((w) & (1ULL << 2))) +	\
> > > +	(!!((w) & (1ULL << 3))) +	\
> > > +	(!!((w) & (1ULL << 4))) +	\
> > > +	(!!((w) & (1ULL << 5))) +	\
> > > +	(!!((w) & (1ULL << 6))) +	\
> > > +	(!!((w) & (1ULL << 7)))	)
> > > +
> > > +#define HWEIGHT16(w) (HWEIGHT8(w)  + HWEIGHT8(w >> 8))
> > > +#define HWEIGHT32(w) (HWEIGHT16(w) + HWEIGHT16(w >> 16))
> > > +#define HWEIGHT64(w) (HWEIGHT32(w) + HWEIGHT32(w >> 32))
> > 
> > Would be nice if it had a comment explaining why it exists.  If people
> > accidentally use this with non-constant arguments, the generated code
> > will be pretty ghastly.
> > 
> > Or add some barf-if-not-__constant_p() thing, perhaps.
> 
> Yeah, agreed.
> 

Also...

Should we just do

#define HWEIGHT(x) HWEIGHT64(x)

and make HWEIGHT() the sole officially-exported interface?  I mean, all
it does is emit an obfuscated constant - perhaps we can save users from
having to pick which one of the above to use by giving them a "this one
always works" interface.

That might require some casting to suppress "shift out of range"
warnings though.

<wonders if we'd otherwise end up needing an HWEIGHT_LONG()>
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