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    SubjectRe: [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@chello.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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