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

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Message-Id: <20100129020128.716af8fb.akpm@linux-foundation.org>
Date:	Fri, 29 Jan 2010 02:01:28 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	mingo@...hat.com, hpa@...or.com, eranian@...gle.com,
	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	a.p.zijlstra@...llo.nl, torvalds@...ux-foundation.org,
	tglx@...utronix.de, mingo@...e.hu
Cc:	tip-bot for Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	linux-tip-commits@...r.kernel.org, linux-kernel@...r.kernel.org,
	eranian@...gle.com, hpa@...or.com, mingo@...hat.com,
	torvalds@...ux-foundation.org, tglx@...utronix.de, mingo@...e.hu
Subject: Re: [tip:perf/core] bitops: Provide compile time
 HWEIGHT{8,16,32,64}

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.
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