ffs.S revision 256281
1181834Sroberto/* $NetBSD: ffs.S,v 1.5 2003/04/05 23:08:52 bjh21 Exp $ */ 2181834Sroberto/* 3181834Sroberto * Copyright (c) 2001 Christopher Gilbert 4181834Sroberto * All rights reserved. 5181834Sroberto * 6181834Sroberto * Redistribution and use in source and binary forms, with or without 7181834Sroberto * modification, are permitted provided that the following conditions 8181834Sroberto * are met: 9181834Sroberto * 1. Redistributions of source code must retain the above copyright 10181834Sroberto * notice, this list of conditions and the following disclaimer. 11181834Sroberto * 2. Redistributions in binary form must reproduce the above copyright 12181834Sroberto * notice, this list of conditions and the following disclaimer in the 13181834Sroberto * documentation and/or other materials provided with the distribution. 14181834Sroberto * 3. The name of the company nor the name of the author may be used to 15181834Sroberto * endorse or promote products derived from this software without specific 16181834Sroberto * prior written permission. 17181834Sroberto * 18181834Sroberto * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR 19181834Sroberto * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES 20181834Sroberto * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. 21181834Sroberto * IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, 22181834Sroberto * INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES 23181834Sroberto * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR 24181834Sroberto * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 25181834Sroberto * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 26181834Sroberto * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 27181834Sroberto * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 28181834Sroberto * SUCH DAMAGE. 29181834Sroberto */ 30181834Sroberto 31181834Sroberto#include <machine/asm.h> 32181834Sroberto 33181834Sroberto__FBSDID("$FreeBSD: stable/10/lib/libc/arm/string/ffs.S 172616 2007-10-13 12:06:31Z cognet $"); 34181834Sroberto 35181834Sroberto/* 36181834Sroberto * ffs - find first set bit, this algorithm isolates the first set 37181834Sroberto * bit, then multiplies the number by 0x0450fbaf which leaves the top 38181834Sroberto * 6 bits as an index into the table. This algorithm should be a win 39181834Sroberto * over the checking each bit in turn as per the C compiled version. 40181834Sroberto * 41181834Sroberto * under ARMv5 there's an instruction called CLZ (count leading Zero's) that 42181834Sroberto * could be used 43181834Sroberto * 44181834Sroberto * This is the ffs algorithm devised by d.seal and posted to comp.sys.arm on 45181834Sroberto * 16 Feb 1994. 46181834Sroberto */ 47181834Sroberto 48181834SrobertoENTRY(ffs) 49181834Sroberto /* Standard trick to isolate bottom bit in r0 or 0 if r0 = 0 on entry */ 50181834Sroberto rsb r1, r0, #0 51181834Sroberto ands r0, r0, r1 52181834Sroberto#ifndef _ARM_ARCH_5 53181834Sroberto /* 54181834Sroberto * now r0 has at most one set bit, call this X 55181834Sroberto * if X = 0, all further instructions are skipped 56181834Sroberto */ 57181834Sroberto adrne r2, .L_ffs_table 58181834Sroberto orrne r0, r0, r0, lsl #4 /* r0 = X * 0x11 */ 59181834Sroberto orrne r0, r0, r0, lsl #6 /* r0 = X * 0x451 */ 60181834Sroberto rsbne r0, r0, r0, lsl #16 /* r0 = X * 0x0450fbaf */ 61181834Sroberto 62181834Sroberto /* now lookup in table indexed on top 6 bits of r0 */ 63181834Sroberto ldrneb r0, [ r2, r0, lsr #26 ] 64181834Sroberto 65181834Sroberto RET 66181834Sroberto.text; 67181834Sroberto.type .L_ffs_table, _ASM_TYPE_OBJECT; 68181834Sroberto.L_ffs_table: 69181834Sroberto/* 0 1 2 3 4 5 6 7 */ 70181834Sroberto .byte 0, 1, 2, 13, 3, 7, 0, 14 /* 0- 7 */ 71181834Sroberto .byte 4, 0, 8, 0, 0, 0, 0, 15 /* 8-15 */ 72181834Sroberto .byte 11, 5, 0, 0, 9, 0, 0, 26 /* 16-23 */ 73181834Sroberto .byte 0, 0, 0, 0, 0, 22, 28, 16 /* 24-31 */ 74181834Sroberto .byte 32, 12, 6, 0, 0, 0, 0, 0 /* 32-39 */ 75181834Sroberto .byte 10, 0, 0, 25, 0, 0, 21, 27 /* 40-47 */ 76181834Sroberto .byte 31, 0, 0, 0, 0, 24, 0, 20 /* 48-55 */ 77181834Sroberto .byte 30, 0, 23, 19, 29, 18, 17, 0 /* 56-63 */ 78181834Sroberto#else 79181834Sroberto clzne r0, r0 80181834Sroberto rsbne r0, r0, #32 81181834Sroberto RET 82181834Sroberto#endif 83181834Sroberto