1/* $OpenBSD: ffs.S,v 1.3 2009/10/28 06:49:54 deraadt Exp $ */ 2/* $NetBSD: ffs.S,v 1.5 2003/04/05 23:08:52 bjh21 Exp $ */ 3/* 4 * Copyright (c) 2001 Christopher Gilbert 5 * All rights reserved. 6 * 7 * Redistribution and use in source and binary forms, with or without 8 * modification, are permitted provided that the following conditions 9 * are met: 10 * 1. Redistributions of source code must retain the above copyright 11 * notice, this list of conditions and the following disclaimer. 12 * 2. Redistributions in binary form must reproduce the above copyright 13 * notice, this list of conditions and the following disclaimer in the 14 * documentation and/or other materials provided with the distribution. 15 * 3. The name of the company nor the name of the author may be used to 16 * endorse or promote products derived from this software without specific 17 * prior written permission. 18 * 19 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR 20 * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES 21 * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. 22 * IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, 23 * INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES 24 * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR 25 * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 26 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 27 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 28 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 29 * SUCH DAMAGE. 30 */ 31 32#include <machine/asm.h> 33 34/* 35 * ffs - find first set bit, this algorithm isolates the first set 36 * bit, then multiplies the number by 0x0450fbaf which leaves the top 37 * 6 bits as an index into the table. This algorithm should be a win 38 * over the checking each bit in turn as per the C compiled version. 39 * 40 * under ARMv5 there's an instruction called CLZ (count leading Zero's) that 41 * could be used 42 * 43 * This is the ffs algorithm devised by d.seal and posted to comp.sys.arm on 44 * 16 Feb 1994. 45 */ 46 47ENTRY(ffs) 48 /* Standard trick to isolate bottom bit in r0 or 0 if r0 = 0 on entry */ 49 rsb r1, r0, #0 50 ands r0, r0, r1 51 /* 52 * now r0 has at most one set bit, call this X 53 * if X = 0, all further instructions are skipped 54 */ 55 adrne r2, .L_ffs_table 56 orrne r0, r0, r0, lsl #4 /* r0 = X * 0x11 */ 57 orrne r0, r0, r0, lsl #6 /* r0 = X * 0x451 */ 58 rsbne r0, r0, r0, lsl #16 /* r0 = X * 0x0450fbaf */ 59 60 /* now lookup in table indexed on top 6 bits of r0 */ 61 ldrneb r0, [ r2, r0, lsr #26 ] 62 63 mov pc, lr 64.text; 65.type .L_ffs_table, _ASM_TYPE_OBJECT; 66.L_ffs_table: 67/* 0 1 2 3 4 5 6 7 */ 68 .byte 0, 1, 2, 13, 3, 7, 0, 14 /* 0- 7 */ 69 .byte 4, 0, 8, 0, 0, 0, 0, 15 /* 8-15 */ 70 .byte 11, 5, 0, 0, 9, 0, 0, 26 /* 16-23 */ 71 .byte 0, 0, 0, 0, 0, 22, 28, 16 /* 24-31 */ 72 .byte 32, 12, 6, 0, 0, 0, 0, 0 /* 32-39 */ 73 .byte 10, 0, 0, 25, 0, 0, 21, 27 /* 40-47 */ 74 .byte 31, 0, 0, 0, 0, 24, 0, 20 /* 48-55 */ 75 .byte 30, 0, 23, 19, 29, 18, 17, 0 /* 56-63 */ 76