ffs.S revision 1.6
1/* $OpenBSD: ffs.S,v 1.6 2018/01/18 08:23:44 guenther Exp $ */ 2 3/* 4 * Copyright (c) 1992, 1993 5 * The Regents of the University of California. All rights reserved. 6 * 7 * This software was developed by the Computer Systems Engineering group 8 * at Lawrence Berkeley Laboratory under DARPA contract BG 91-66 and 9 * contributed to Berkeley. 10 * 11 * Redistribution and use in source and binary forms, with or without 12 * modification, are permitted provided that the following conditions 13 * are met: 14 * 1. Redistributions of source code must retain the above copyright 15 * notice, this list of conditions and the following disclaimer. 16 * 2. Redistributions in binary form must reproduce the above copyright 17 * notice, this list of conditions and the following disclaimer in the 18 * documentation and/or other materials provided with the distribution. 19 * 3. Neither the name of the University nor the names of its contributors 20 * may be used to endorse or promote products derived from this software 21 * without specific prior written permission. 22 * 23 * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 24 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 25 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 26 * ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 27 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 28 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 29 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 30 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 31 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 32 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 33 * SUCH DAMAGE. 34 */ 35 36#include "SYS.h" 37/* 38 * ffs returns the number of the rightmost bit set in its argument, 39 * i.e., the lowest value such that (x & (ffs(x) - 1)) is nonzero. 40 * If no bits are set, ffs returns 0. 41 * 42 * We use a table lookup on each byte. 43 * 44 * In each section below, %o1 is the current byte (0, 1, 2, or 3). 45 * The last byte is handled specially: for the first three, 46 * if that byte is nonzero, we return the table value 47 * (plus 0, 8, or 16 for the byte number), but for the last 48 * one, we just return the table value plus 24. This means 49 * that ffstab[0] must be -24 so that ffs(0) will return 0. 50 */ 51ENTRY(ffs) 52#ifdef __PIC__ 53 PICCY_SET(_C_LABEL(__ffstab), %o2, %o3) 54#else 55 set _C_LABEL(__ffstab), %o2 56#endif 57 andcc %o0, 0xff, %o1 ! get low byte 58 be,a 1f ! try again if 0 59 srl %o0, 8, %o0 ! delay slot, get ready for next byte 60 61 retl ! return ffstab[%o1] 62 ldsb [%o2 + %o1], %o0 63 641: 65 andcc %o0, 0xff, %o1 ! byte 1 like byte 0... 66 be,a 2f 67 srl %o0, 8, %o0 ! (use delay to prepare for byte 2) 68 69 ldsb [%o2 + %o1], %o0 70 retl ! return ffstab[%o1] + 8 71 add %o0, 8, %o0 72 732: 74 andcc %o0, 0xff, %o1 75 be,a 3f 76 srl %o0, 8, %o0 ! (prepare for byte 3) 77 78 ldsb [%o2 + %o1], %o0 79 retl ! return ffstab[%o1] + 16 80 add %o0, 16, %o0 81 823: ! just return ffstab[%o0] + 24 83 ldsb [%o2 + %o0], %o0 84 retl 85 add %o0, 24, %o0 86END(ffs) 87.protected ffs 88 89 .section .rodata 90_C_LABEL(__ffstab): 91 .byte -24,1,2,1,3,1,2,1,4,1,2,1,3,1,2,1 /* 00-0f */ 92 .byte 5,1,2,1,3,1,2,1,4,1,2,1,3,1,2,1 /* 10-1f */ 93 .byte 6,1,2,1,3,1,2,1,4,1,2,1,3,1,2,1 /* 20-2f */ 94 .byte 5,1,2,1,3,1,2,1,4,1,2,1,3,1,2,1 /* 30-3f */ 95 .byte 7,1,2,1,3,1,2,1,4,1,2,1,3,1,2,1 /* 40-4f */ 96 .byte 5,1,2,1,3,1,2,1,4,1,2,1,3,1,2,1 /* 50-5f */ 97 .byte 6,1,2,1,3,1,2,1,4,1,2,1,3,1,2,1 /* 60-6f */ 98 .byte 5,1,2,1,3,1,2,1,4,1,2,1,3,1,2,1 /* 70-7f */ 99 .byte 8,1,2,1,3,1,2,1,4,1,2,1,3,1,2,1 /* 80-8f */ 100 .byte 5,1,2,1,3,1,2,1,4,1,2,1,3,1,2,1 /* 10-9f */ 101 .byte 6,1,2,1,3,1,2,1,4,1,2,1,3,1,2,1 /* a0-af */ 102 .byte 5,1,2,1,3,1,2,1,4,1,2,1,3,1,2,1 /* b0-bf */ 103 .byte 7,1,2,1,3,1,2,1,4,1,2,1,3,1,2,1 /* c0-cf */ 104 .byte 5,1,2,1,3,1,2,1,4,1,2,1,3,1,2,1 /* d0-df */ 105 .byte 6,1,2,1,3,1,2,1,4,1,2,1,3,1,2,1 /* e0-ef */ 106 .byte 5,1,2,1,3,1,2,1,4,1,2,1,3,1,2,1 /* f0-ff */ 107