From: Nathan Sidwell <nathan@acm.org>
To: Wei Qin <wqin@EE.Princeton.EDU>
Cc: kelley.r.cook@gm.com, Preeti Aira <ms_preeti@hotmail.com>,
gcc <gcc@gcc.gnu.org>, gcc-help <gcc-help@gcc.gnu.org>
Subject: Re: Number of 1's in 64 bit number...[don't read if not interested in algos/math...]
Date: Thu, 25 Apr 2002 10:36:00 -0000 [thread overview]
Message-ID: <3CC83D87.506F1499@acm.org> (raw)
In-Reply-To: <Pine.SOL.4.43.0204251302420.27320-100000@ivy.ee.princeton.edu>
Wei Qin wrote:
>
> First, I see only 32 bits being counted. The top 32 bits seems missing.
> Second, this is a very slow implementation. A faster one can be
> implemented through table lookup. It takes no masking, no branching.
>
I suspect something like...
int bitson (unsigned long x)
{
x = (x & 0x55555555) + ((x >> 1) & 0x55555555);
x = (x & 0x33333333) + ((x >> 2) & 0x33333333);
x = (x & 0x0F0F0F0F) + ((x >> 4) & 0x0F0F0F0F);
x = (x & 0x00FF00FF) + ((x >> 8) & 0x00FF00FF);
x = (x & 0x0000FFFF) + ((x >> 16) & 0x0000FFFF);
return x;
}
might be better (you can extend that to long long quite simply), and
some of those later masks will be unnecessary as you can prove carry
won't happen
nathan
--
Dr Nathan Sidwell :: Computer Science Department :: Bristol University
The voices in my head told me to say this
nathan@acm.org http://www.cs.bris.ac.uk/~nathan/ nathan@cs.bris.ac.uk
next prev parent reply other threads:[~2002-04-25 17:33 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2002-04-25 9:46 kelley.r.cook
2002-04-25 10:13 ` Wei Qin
2002-04-25 10:36 ` Nathan Sidwell [this message]
2002-04-25 11:24 ` Gabriel Paubert
2002-04-26 2:41 ` [OT] Re: Number of 1's in 64 bit number...[don't read if not intereste d " Bernd Jendrissek
2002-04-26 4:23 ` Gabriel Paubert
2002-04-25 12:15 ` Number of 1's in 64 bit number...[don't read if not interested " Wei Qin
2002-04-25 15:15 ` Richard Henderson
2002-04-25 15:47 ` Wei Qin
2002-04-25 17:55 ` Daniel Berlin
-- strict thread matches above, loose matches on Subject: below --
2002-04-27 12:44 David Rasmussen
2002-04-24 21:55 Preeti Aira
2002-04-24 22:35 ` Alan Modra
2002-04-24 23:08 ` Vivek Srivastava
2002-04-24 23:51 ` Jaswant
2002-04-25 0:39 ` Gerald Pfeifer
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