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