/* $OpenBSD: ffs.S,v 1.4 2015/08/31 02:53:56 guenther Exp $ */ /* $NetBSD: ffs.S,v 1.5 2003/04/05 23:08:52 bjh21 Exp $ */ /* * Copyright (c) 2001 Christopher Gilbert * All rights reserved. * * Redistribution and use in source and binary forms, with or without * modification, are permitted provided that the following conditions * are met: * 1. Redistributions of source code must retain the above copyright * notice, this list of conditions and the following disclaimer. * 2. Redistributions in binary form must reproduce the above copyright * notice, this list of conditions and the following disclaimer in the * documentation and/or other materials provided with the distribution. * 3. The name of the company nor the name of the author may be used to * endorse or promote products derived from this software without specific * prior written permission. * * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. * IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, * INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF * SUCH DAMAGE. */ #include "SYS.h" /* * ffs - find first set bit, this algorithm isolates the first set * bit, then multiplies the number by 0x0450fbaf which leaves the top * 6 bits as an index into the table. This algorithm should be a win * over the checking each bit in turn as per the C compiled version. * * under ARMv5 there's an instruction called CLZ (count leading Zero's) that * could be used * * This is the ffs algorithm devised by d.seal and posted to comp.sys.arm on * 16 Feb 1994. */ ENTRY(ffs) /* Standard trick to isolate bottom bit in r0 or 0 if r0 = 0 on entry */ rsb r1, r0, #0 ands r0, r0, r1 /* * now r0 has at most one set bit, call this X * if X = 0, all further instructions are skipped */ adrne r2, .L_ffs_table orrne r0, r0, r0, lsl #4 /* r0 = X * 0x11 */ orrne r0, r0, r0, lsl #6 /* r0 = X * 0x451 */ rsbne r0, r0, r0, lsl #16 /* r0 = X * 0x0450fbaf */ /* now lookup in table indexed on top 6 bits of r0 */ ldrneb r0, [ r2, r0, lsr #26 ] mov pc, lr END_WEAK(ffs) .text; .type .L_ffs_table, _ASM_TYPE_OBJECT; .L_ffs_table: /* 0 1 2 3 4 5 6 7 */ .byte 0, 1, 2, 13, 3, 7, 0, 14 /* 0- 7 */ .byte 4, 0, 8, 0, 0, 0, 0, 15 /* 8-15 */ .byte 11, 5, 0, 0, 9, 0, 0, 26 /* 16-23 */ .byte 0, 0, 0, 0, 0, 22, 28, 16 /* 24-31 */ .byte 32, 12, 6, 0, 0, 0, 0, 0 /* 32-39 */ .byte 10, 0, 0, 25, 0, 0, 21, 27 /* 40-47 */ .byte 31, 0, 0, 0, 0, 24, 0, 20 /* 48-55 */ .byte 30, 0, 23, 19, 29, 18, 17, 0 /* 56-63 */