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/***************************************************************************
* __________ __ ___.
* Open \______ \ ____ ____ | | _\_ |__ _______ ___
* Source | _// _ \_/ ___\| |/ /| __ \ / _ \ \/ /
* Jukebox | | ( <_> ) \___| < | \_\ ( <_> > < <
* Firmware |____|_ /\____/ \___ >__|_ \|___ /\____/__/\_ \
* \/ \/ \/ \/ \/
* $Id$
*
* Copyright (C) 2008 by Michael Sevakis
*
* This program is free software; you can redistribute it and/or
* modify it under the terms of the GNU General Public License
* as published by the Free Software Foundation; either version 2
* of the License, or (at your option) any later version.
*
* This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY
* KIND, either express or implied.
*
****************************************************************************/
#include "config.h"
/****************************************************************************
* int find_first_set_bit(uint32_t val);
*
* Find the index of the least significant set bit in the 32-bit word.
*
* return values:
* 0 - bit 0 is set
* 1 - bit 1 is set
* ...
* 31 - bit 31 is set
* 32 - no bits set
****************************************************************************/
.align 2
.global find_first_set_bit
.type find_first_set_bit,%function
find_first_set_bit:
@ Standard trick to isolate bottom bit in r0 or 0 if r0 = 0 on entry
rsb r2, r0, #0 @ r1 = r0 & -r0
ands r1, r0, r2 @
@ now r1 has at most one set bit, call this X
#if ARM_ARCH >= 5
clz r0, r1 @ Get lead 0's count
rsbne r0, r0, #31 @ lead 0's -> bit index
bx lr @
#else
@ this is the ffs algorithm devised by D.Seal and posted to
@ comp.sys.arm on 16 Feb 1994.
@
@ Output modified to suit Rockbox purposes.
adr r2, L_ffs_table
orrne r1, r1, r1, lsl #4 @ r1 = X * 0x11
orrne r1, r1, r1, lsl #6 @ r1 = X * 0x451
rsbne r1, r1, r1, lsl #16 @ r1 = X * 0x0450fbaf
@ now lookup in table indexed on top 6 bits of r1
ldrb r0, [ r2, r1, lsr #26 ] @
bx lr @
L_ffs_table:
@ 0 1 2 3 4 5 6 7
@----------------------------------------------
.byte 32, 0, 1, 12, 2, 6, 0, 13 @ 0- 7
.byte 3, 0, 7, 0, 0, 0, 0, 14 @ 8-15
.byte 10, 4, 0, 0, 8, 0, 0, 25 @ 16-23
.byte 0, 0, 0, 0, 0, 21, 27, 15 @ 24-31
.byte 31, 11, 5, 0, 0, 0, 0, 0 @ 32-39
.byte 9, 0, 0, 24, 0, 0, 20, 26 @ 40-47
.byte 30, 0, 0, 0, 0, 23, 0, 19 @ 48-55
.byte 29, 0, 22, 18, 28, 17, 16, 0 @ 56-63
#endif
.size find_first_set_bit, .-find_first_set_bit
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