Messages in this thread | | | Date | Fri, 29 Jan 2010 11:04:31 +0100 | From | Ingo Molnar <> | Subject | Re: [tip:perf/core] bitops: Provide compile time HWEIGHT{8,16,32,64} |
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* Andrew Morton <akpm@linux-foundation.org> wrote:
> 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.
Yeah, agreed.
Ingo
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