1 /* $NetBSD: ffs.S,v 1.3 2003/04/05 23:27:15 bjh21 Exp $ */
3 * Copyright (c) 2001 Christopher Gilbert
6 * Redistribution and use in source and binary forms, with or without
7 * modification, are permitted provided that the following conditions
9 * 1. Redistributions of source code must retain the above copyright
10 * notice, this list of conditions and the following disclaimer.
11 * 2. Redistributions in binary form must reproduce the above copyright
12 * notice, this list of conditions and the following disclaimer in the
13 * documentation and/or other materials provided with the distribution.
14 * 3. The name of the company nor the name of the author may be used to
15 * endorse or promote products derived from this software without specific
16 * prior written permission.
18 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
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20 * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
21 * IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT,
22 * INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
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25 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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27 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
31 #include <machine/asm.h>
32 __FBSDID("$FreeBSD$");
37 * ffs - find first set bit, this algorithm isolates the first set
38 * bit, then multiplies the number by 0x0450fbaf which leaves the top
39 * 6 bits as an index into the table. This algorithm should be a win
40 * over the checking each bit in turn as per the C compiled version.
42 * under ARMv5 there's an instruction called CLZ (count leading Zero's) that
45 * This is the ffs algorithm devised by d.seal and posted to comp.sys.arm on
51 /* Standard trick to isolate bottom bit in r0 or 0 if r0 = 0 on entry */