1129202Scognet/* $NetBSD: ffs.S,v 1.5 2003/04/05 23:08:52 bjh21 Exp $ */ 2129202Scognet/* 3129202Scognet * Copyright (c) 2001 Christopher Gilbert 4129202Scognet * All rights reserved. 5129202Scognet * 6129202Scognet * Redistribution and use in source and binary forms, with or without 7129202Scognet * modification, are permitted provided that the following conditions 8129202Scognet * are met: 9129202Scognet * 1. Redistributions of source code must retain the above copyright 10129202Scognet * notice, this list of conditions and the following disclaimer. 11129202Scognet * 2. Redistributions in binary form must reproduce the above copyright 12129202Scognet * notice, this list of conditions and the following disclaimer in the 13129202Scognet * documentation and/or other materials provided with the distribution. 14129202Scognet * 3. The name of the company nor the name of the author may be used to 15129202Scognet * endorse or promote products derived from this software without specific 16129202Scognet * prior written permission. 17129202Scognet * 18129202Scognet * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR 19129202Scognet * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES 20129202Scognet * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. 21129202Scognet * IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, 22129202Scognet * INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES 23129202Scognet * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR 24129202Scognet * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 25129202Scognet * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 26129202Scognet * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 27129202Scognet * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 28129202Scognet * SUCH DAMAGE. 29129202Scognet */ 30129202Scognet 31129202Scognet#include <machine/asm.h> 32129202Scognet 33129202Scognet__FBSDID("$FreeBSD$"); 34129202Scognet 35129202Scognet/* 36129202Scognet * ffs - find first set bit, this algorithm isolates the first set 37129202Scognet * bit, then multiplies the number by 0x0450fbaf which leaves the top 38129202Scognet * 6 bits as an index into the table. This algorithm should be a win 39129202Scognet * over the checking each bit in turn as per the C compiled version. 40129202Scognet * 41129202Scognet * under ARMv5 there's an instruction called CLZ (count leading Zero's) that 42129202Scognet * could be used 43129202Scognet * 44129202Scognet * This is the ffs algorithm devised by d.seal and posted to comp.sys.arm on 45129202Scognet * 16 Feb 1994. 46129202Scognet */ 47129202Scognet 48129202ScognetENTRY(ffs) 49129202Scognet /* Standard trick to isolate bottom bit in r0 or 0 if r0 = 0 on entry */ 50129202Scognet rsb r1, r0, #0 51129202Scognet ands r0, r0, r1 52172616Scognet#ifndef _ARM_ARCH_5 53129202Scognet /* 54129202Scognet * now r0 has at most one set bit, call this X 55129202Scognet * if X = 0, all further instructions are skipped 56129202Scognet */ 57129202Scognet adrne r2, .L_ffs_table 58129202Scognet orrne r0, r0, r0, lsl #4 /* r0 = X * 0x11 */ 59129202Scognet orrne r0, r0, r0, lsl #6 /* r0 = X * 0x451 */ 60129202Scognet rsbne r0, r0, r0, lsl #16 /* r0 = X * 0x0450fbaf */ 61129202Scognet 62129202Scognet /* now lookup in table indexed on top 6 bits of r0 */ 63129202Scognet ldrneb r0, [ r2, r0, lsr #26 ] 64129202Scognet 65137464Scognet RET 66129202Scognet.text; 67129202Scognet.type .L_ffs_table, _ASM_TYPE_OBJECT; 68129202Scognet.L_ffs_table: 69129202Scognet/* 0 1 2 3 4 5 6 7 */ 70129202Scognet .byte 0, 1, 2, 13, 3, 7, 0, 14 /* 0- 7 */ 71129202Scognet .byte 4, 0, 8, 0, 0, 0, 0, 15 /* 8-15 */ 72129202Scognet .byte 11, 5, 0, 0, 9, 0, 0, 26 /* 16-23 */ 73129202Scognet .byte 0, 0, 0, 0, 0, 22, 28, 16 /* 24-31 */ 74129202Scognet .byte 32, 12, 6, 0, 0, 0, 0, 0 /* 32-39 */ 75129202Scognet .byte 10, 0, 0, 25, 0, 0, 21, 27 /* 40-47 */ 76129202Scognet .byte 31, 0, 0, 0, 0, 24, 0, 20 /* 48-55 */ 77129202Scognet .byte 30, 0, 23, 19, 29, 18, 17, 0 /* 56-63 */ 78137343Scognet#else 79137343Scognet clzne r0, r0 80137343Scognet rsbne r0, r0, #32 81137464Scognet RET 82137343Scognet#endif 83