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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
****************************************************************************/
.text
.align 2
.global find_first_set_bit
.type find_first_set_bit,@function
find_first_set_bit:
| this is a coldfire version of the ffs algorithm devised by D.Seal
| and posted to comp.sys.arm on 16 Feb 1994.
|
| Output modified to suit rockbox purposes.
| Standard trick to isolate bottom bit in r0 or 0 if r0 = 0 on entry
move.l 4(%sp), %d1 | %d1 = %d1 & -%d1
lea.l L_ffs_table, %a0 | %a0 = table address
move.l %d1, %d0 |
neg.l %d1 |
and.l %d0, %d1 |
| now %d1 has at most one set bit, call this X
move.l #0x0450fbaf, %d0 | %d0 = multiplier
mulu.l %d0, %d1 | %d1 = X * 0x0450fbaf
| now lookup in table indexed on top 6 bits of %d0
moveq.l #26, %d0 | %d0 = final shift count
lsr.l %d0, %d1 |
| the top 24 bits of d0 are 0 so just load the value over it
move.b (%a0, %d1.l), %d0 |
rts |
.size find_first_set_bit, .-find_first_set_bit
.section .irodata
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
.size L_ffs_table, .-L_ffs_table
|
