返回信息流一个64bit的数据，只可能有一位（bit）被置1，如何能快速找到这一位？
例:输入0x8000 0000 0000 0000，输出64
ps：移位的方法就不用说了，有更快的方法吗，最好4步内完成。
谢！
这是一条镜像帖。来源：北邮人论坛 / cpp / #46150同步于 2010/11/12
该镜像源已超过 30 天没有更新，可能在源站已被删除。
CPP机器人发帖
【求助】如何快速找出哪一位被置1
arm
2010/11/12镜像同步18 回复
订阅后，新回复会通过你的通知中心匿名送达。
9 条回复
用bitset吧……
【 在 arm (armstrong) 的大作中提到: 】
: 一个64bit的数据，只可能有一位（bit）被置1，如何能快速找到这一位？
: 例:输入0x8000 0000 0000 0000，输出64
: ps：移位的方法就不用说了，有更快的方法吗，最好4步内完成。
: ...................
【 在 renne 的大作中提到: 】
: 用bitset吧……
: 【 在 arm (armstrong) 的大作中提到: 】
: : 一个64bit的数据，只可能有一位（bit）被置1，如何能快速找到这一位？
: ...................
能说具体点儿吗 不懂
【 在 wks 的大作中提到: 】
: __builtin_ctz 这个怎么样？
: --
: 免费贴代码：http://wksprivate.appspot.com/pastebin/
: ...................
这个能在4步内完成吗 不了解这个
方法1
1 /**
2 * compat.c - Tweaks for Windows compatibility
3 *
4 * Copyright (c) 2002 Richard Russon
5 * Copyright (c) 2002-2004 Anton Altaparmakov
6 *
7 * This program/include file is free software; you can redistribute it and/or
8 * modify it under the terms of the GNU General Public License as published
9 * by the Free Software Foundation; either version 2 of the License, or
10 * (at your option) any later version.
11 *
12 * This program/include file is distributed in the hope that it will be
13 * useful, but WITHOUT ANY WARRANTY; without even the implied warranty
14 * of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
15 * GNU General Public License for more details.
16 *
17 * You should have received a copy of the GNU General Public License
18 * along with this program (in the main directory of the Linux-NTFS
19 * distribution in the file COPYING); if not, write to the Free Software
20 * Foundation,Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
21 */
22
23 #ifdef WINDOWS
24
25 #ifdef HAVE_CONFIG_H
26 #include "config.h"
27 #endif
28
29 #include "compat.h"
30
31 /* TODO: Add check for FFS in the configure script... (AIA) */
32
33 #ifndef HAVE_FFS
34 /**
35 * ffs - Find the first set bit in an int
36 * @x:
37 *
38 * Description...
39 *
40 * Returns:
41 */
42 int ffs(int x)
43 {
44 int r = 1;
45
46 if (!x)
47 return 0;
48 if (!(x & 0xffff)) {
49 x >>= 16;
50 r += 16;
51 }
52 if (!(x & 0xff)) {
53 x >>= 8;
54 r += 8;
55 }
56 if (!(x & 0xf)) {
57 x >>= 4;
58 r += 4;
59 }
60 if (!(x & 3)) {
61 x >>= 2;
62 r += 2;
63 }
64 if (!(x & 1)) {
65 x >>= 1;
66 r += 1;
67 }
68 return r;
69 }
70 #endif /* HAVE_FFS */
71
72 #endif /* WINDOWS */
73
74
方法2
1 /*
2 * CDDL HEADER START
3 *
4 * The contents of this file are subject to the terms of the
5 * Common Development and Distribution License, Version 1.0 only
6 * (the "License"). You may not use this file except in compliance
7 * with the License.
8 *
9 * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
10 * or http://www.opensolaris.org/os/licensing.
11 * See the License for the specific language governing permissions
12 * and limitations under the License.
13 *
14 * When distributing Covered Code, include this CDDL HEADER in each
15 * file and include the License file at usr/src/OPENSOLARIS.LICENSE.
16 * If applicable, add the following below this CDDL HEADER, with the
17 * fields enclosed by brackets "[]" replaced with your own identifying
18 * information: Portions Copyright [yyyy] [name of copyright owner]
19 *
20 * CDDL HEADER END
21 */
22 /*
23 * Copyright 1992 Sun Microsystems, Inc. All rights reserved.
24 * Use is subject to license terms.
25 */
26
27 ! .seg "data"
28 ! .asciz ident "%Z%%M% %I% %E% SMI"
29 .seg ".text"
30
31 .file "ffs.s"
32
33 #include <sun4/asm_linkage.h>
34
35 ENTRY(ffs)
36 tst %o0 ! if zero, done
37 bz 2f
38 clr %o1 ! delay slot, return zero if no bit set
39 1:
40 inc %o1 ! bit that will get checked
41 btst 1, %o0
42 be 1b ! if bit is zero, keep checking
43 srl %o0, 1, %o0 ! shift input right until we hit a 1 bit
44 2:
45 retl
46 mov %o1, %o0 ! return value is in o1
47 SET_SIZE(ffs)
48
这个貌似只适用32bit的数据，如果是64bit数据的话，应该还得在47行和48行之间多增加一个if语句吧。
【 在 ki 的大作中提到: 】
: 方法1
: 1 /**
: 2 * compat.c - Tweaks for Windows compatibility
: ...................