Path: utzoo!attcan!uunet!aplcen!uakari.primate.wisc.edu!zaphod.mps.ohio-state.edu!usc!srhqla!demott!kdq
From: kdq@demott.COM (Kevin D. Quitt)
Newsgroups: comp.lang.c
Subject: Re: count of bits set in a long
Message-ID: <613@demott.COM>
Date: 27 Sep 90 16:56:14 GMT
References: <Mazyneu00WB6EAjWo8@andrew.cmu.edu>
Reply-To: kdq@demott.COM (Kevin D. Quitt)
Organization: DeMott Electronics Co., Van Nuys CA
Lines: 29

In article <Mazyneu00WB6EAjWo8@andrew.cmu.edu> sa1z+@andrew.cmu.edu (Sudheer Apte) writes:
>I was thinking along these lines:
>
>static int num_bits[256] = {0, 1, 1, 2, 1, ... /* etc. */ };
>
>struct four_pack { char a; char b; char c; char d;};
>
>int bit_count(i)
>unsigned long i;
>{
>	struct four_pack f;
>	f = (struct four_pack) i;
>	return num_bits[f.a] + num_bits[f.b] + num_bits[f.c] +
>		num_bits[f.d];
>}
>
>Would this be faster, in reality, than shifts and iteration? 
>

    Massively.  Look at the code produced - it is generally two or three
instructions per byte. 

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Kevin D. Quitt         demott!kdq   kdq@demott.com
DeMott Electronics Co. 14707 Keswick St.   Van Nuys, CA 91405-1266
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