Re: [tip:perf/core] bitops: Provide compile timeHWEIGHT{8,16,32,64}

[Andrew Morton]

On Fri, 29 Jan 2010 09:28:04 GMT tip-bot for Peter Zijlstra <a.p.zijlstra@xxxxxxxxx> 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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