Date: Thu, 11 Jun 2015 10:49:58 -0400 From: Richard Yao <ryao@gentoo.org> To: Erich Dollansky <erichsfreebsdlist@alogt.com>, "freebsd-hackers@freebsd.org" <freebsd-hackers@FreeBSD.org> Subject: Re: allow ffs & co. a binary search Message-ID: <5579A016.4040800@gentoo.org> In-Reply-To: <20150607081315.7c0f09fb@B85M-HD3-0.alogt.com> References: <20150607081315.7c0f09fb@B85M-HD3-0.alogt.com>
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On 06/06/2015 08:13 PM, Erich Dollansky wrote:
> Hi,
>
> I stumbled these days over the functions to find the first or last bit
> set. They are currently written like this:
>
> /*
> * Find First Set bit
> */
> int
> ffs(int mask)
> {
> int bit;
>
> if (mask == 0)
> return (0);
> for (bit = 1; !(mask & 1); bit++)
> mask = (unsigned int)mask >> 1;
> return (bit);
> }
>
> With other words, the program loops until it finds what it searches for.
>
> Why not do it the binary way?
The fastest portable way of calculating highest bit set is to use a
debrujin sequence:
https://graphics.stanford.edu/~seander/bithacks.html#IntegerLogDeBruijn
The binary approach is inferior, especially on Pentium 4 processors.
Just my two cents.
>
> int res;
>
> res = 0;
> IF (Number != 0) THEN
> res = 1;
> IF ((Number & 0xFFFFFFFF00000000) != 0) THEN
> Number = Number & 0xFFFFFFFF00000000;
> res = res + 32;
> END;
> IF ((Number & 0xFFFF0000FFFF0000) != 0) THEN
> Number = Number & 0xFFFF0000FFFF0000;
> res = res + 16;
> END;
> IF ((Number & 0xFF00FF00FF00FF00) != 0) THEN
> Number = Number & 0xFF00FF00FF00FF00;
> res = res + 8;
> END;
> IF ((Number & 0xF0F0F0F0F0F0F0F0) != 0) THEN
> Number = Number & 0xF0F0F0F0F0F0F0F0;
> res = res + 4;
> END;
> IF ((Number & 0xCCCCCCCCCCCCCCCC) != 0) THEN
> Number = Number & 0xCCCCCCCCCCCCCCCC;
> res = res + 2;
> END;
> IF ((Number & 0xAAAAAAAAAAAAAAAA) != 0) THEN
> Number = Number & 0xAAAAAAAAAAAAAAAA;
> res = res + 1;
> END;
> END;
> return res;
>
> If you like the binary way I could give you the sources for the
> complete family following your style to replace the older functions.
>
> Erich
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