ffs.S revision 302408
1/* $NetBSD: ffs.S,v 1.3 2003/04/05 23:27:15 bjh21 Exp $ */ 2/*- 3 * Copyright (c) 2001 Christopher Gilbert 4 * All rights reserved. 5 * 6 * Redistribution and use in source and binary forms, with or without 7 * modification, are permitted provided that the following conditions 8 * are met: 9 * 1. Redistributions of source code must retain the above copyright 10 * notice, this list of conditions and the following disclaimer. 11 * 2. Redistributions in binary form must reproduce the above copyright 12 * notice, this list of conditions and the following disclaimer in the 13 * documentation and/or other materials provided with the distribution. 14 * 3. The name of the company nor the name of the author may be used to 15 * endorse or promote products derived from this software without specific 16 * prior written permission. 17 * 18 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR 19 * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES 20 * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. 21 * IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, 22 * INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES 23 * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR 24 * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 25 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 26 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 27 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 28 * SUCH DAMAGE. 29 */ 30 31#include <machine/asm.h> 32__FBSDID("$FreeBSD: stable/11/sys/libkern/arm/ffs.S 275520 2014-12-05 19:08:36Z andrew $"); 33 34 .syntax unified 35 36/* 37 * ffs - find first set bit, this algorithm isolates the first set 38 * bit, then multiplies the number by 0x0450fbaf which leaves the top 39 * 6 bits as an index into the table. This algorithm should be a win 40 * over the checking each bit in turn as per the C compiled version. 41 * 42 * under ARMv5 there's an instruction called CLZ (count leading Zero's) that 43 * could be used 44 * 45 * This is the ffs algorithm devised by d.seal and posted to comp.sys.arm on 46 * 16 Feb 1994. 47 */ 48 49ENTRY(ffs) 50 51 /* Standard trick to isolate bottom bit in r0 or 0 if r0 = 0 on entry */ 52 rsb r1, r0, #0 53 ands r0, r0, r1 54#ifndef _ARM_ARCH_5 55 56 /* 57 * now r0 has at most one set bit, call this X 58 * if X = 0, all further instructions are skipped 59 */ 60 adrne r2, .L_ffs_table 61 orrne r0, r0, r0, lsl #4 /* r0 = X * 0x11 */ 62 orrne r0, r0, r0, lsl #6 /* r0 = X * 0x451 */ 63 rsbne r0, r0, r0, lsl #16 /* r0 = X * 0x0450fbaf */ 64 65 /* now lookup in table indexed on top 6 bits of r0 */ 66 ldrbne r0, [ r2, r0, lsr #26 ] 67 68 RET 69.text; 70.type .L_ffs_table, _ASM_TYPE_OBJECT; 71.L_ffs_table: 72/* 0 1 2 3 4 5 6 7 */ 73 .byte 0, 1, 2, 13, 3, 7, 0, 14 /* 0- 7 */ 74 .byte 4, 0, 8, 0, 0, 0, 0, 15 /* 8-15 */ 75 .byte 11, 5, 0, 0, 9, 0, 0, 26 /* 16-23 */ 76 .byte 0, 0, 0, 0, 0, 22, 28, 16 /* 24-31 */ 77 .byte 32, 12, 6, 0, 0, 0, 0, 0 /* 32-39 */ 78 .byte 10, 0, 0, 25, 0, 0, 21, 27 /* 40-47 */ 79 .byte 31, 0, 0, 0, 0, 24, 0, 20 /* 48-55 */ 80 .byte 30, 0, 23, 19, 29, 18, 17, 0 /* 56-63 */ 81#else 82 clzne r0, r0 83 rsbne r0, r0, #32 84 RET 85#endif 86END(ffs) 87 88