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/* $OpenBSD: ffs.S,v 1.2 2004/02/01 05:40:52 drahn Exp $ */
/* $NetBSD: ffs.S,v 1.5 2003/04/05 23:08:52 bjh21 Exp $ */
/*
* Copyright (c) 2001 Christopher Gilbert
* All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
* 1. Redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer.
* 2. Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in the
* documentation and/or other materials provided with the distribution.
* 3. The name of the company nor the name of the author may be used to
* endorse or promote products derived from this software without specific
* prior written permission.
*
* THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
* IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
* OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
* IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT,
* INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
* (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
* SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
* HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
* LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
* OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
* SUCH DAMAGE.
*/
#include <machine/asm.h>
RCSID("$NetBSD: ffs.S,v 1.5 2003/04/05 23:08:52 bjh21 Exp $")
/*
* ffs - find first set bit, this algorithm isolates the first set
* bit, then multiplies the number by 0x0450fbaf which leaves the top
* 6 bits as an index into the table. This algorithm should be a win
* over the checking each bit in turn as per the C compiled version.
*
* under ARMv5 there's an instruction called CLZ (count leading Zero's) that
* could be used
*
* This is the ffs algorithm devised by d.seal and posted to comp.sys.arm on
* 16 Feb 1994.
*/
ENTRY(ffs)
/* Standard trick to isolate bottom bit in r0 or 0 if r0 = 0 on entry */
rsb r1, r0, #0
ands r0, r0, r1
/*
* now r0 has at most one set bit, call this X
* if X = 0, all further instructions are skipped
*/
adrne r2, .L_ffs_table
orrne r0, r0, r0, lsl #4 /* r0 = X * 0x11 */
orrne r0, r0, r0, lsl #6 /* r0 = X * 0x451 */
rsbne r0, r0, r0, lsl #16 /* r0 = X * 0x0450fbaf */
/* now lookup in table indexed on top 6 bits of r0 */
ldrneb r0, [ r2, r0, lsr #26 ]
mov pc, lr
.text;
.type .L_ffs_table, _ASM_TYPE_OBJECT;
.L_ffs_table:
/* 0 1 2 3 4 5 6 7 */
.byte 0, 1, 2, 13, 3, 7, 0, 14 /* 0- 7 */
.byte 4, 0, 8, 0, 0, 0, 0, 15 /* 8-15 */
.byte 11, 5, 0, 0, 9, 0, 0, 26 /* 16-23 */
.byte 0, 0, 0, 0, 0, 22, 28, 16 /* 24-31 */
.byte 32, 12, 6, 0, 0, 0, 0, 0 /* 32-39 */
.byte 10, 0, 0, 25, 0, 0, 21, 27 /* 40-47 */
.byte 31, 0, 0, 0, 0, 24, 0, 20 /* 48-55 */
.byte 30, 0, 23, 19, 29, 18, 17, 0 /* 56-63 */
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