ffs.S revision 1.5
1 /* $NetBSD: ffs.S,v 1.5 2013/09/30 12:31:27 skrll Exp $ */ 2 /* 3 * Copyright (c) 2001 Christopher Gilbert 4 * All rights reserved. 5 * 6 * Redistribution and use in source and binary forms, with or without 7 * modification, are permitted provided that the following conditions 8 * are met: 9 * 1. Redistributions of source code must retain the above copyright 10 * notice, this list of conditions and the following disclaimer. 11 * 2. Redistributions in binary form must reproduce the above copyright 12 * notice, this list of conditions and the following disclaimer in the 13 * documentation and/or other materials provided with the distribution. 14 * 3. The name of the company nor the name of the author may be used to 15 * endorse or promote products derived from this software without specific 16 * prior written permission. 17 * 18 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR 19 * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES 20 * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. 21 * IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, 22 * INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES 23 * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR 24 * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 25 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 26 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 27 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 28 * SUCH DAMAGE. 29 */ 30 31 #include <machine/asm.h> 32 33 RCSID("$NetBSD: ffs.S,v 1.5 2013/09/30 12:31:27 skrll Exp $") 34 35 /* 36 * ffs - find first set bit, this algorithm isolates the first set 37 * bit, then multiplies the number by 0x0450fbaf which leaves the top 38 * 6 bits as an index into the table. This algorithm should be a win 39 * over the checking each bit in turn as per the C compiled version. 40 * 41 * On ARMv5 we use CLZ (count leading Zero's) and then subtract the result 42 * from 32. 43 * 44 * This is the ffs algorithm devised by d.seal and posted to comp.sys.arm on 45 * 16 Feb 1994. 46 */ 47 WEAK_ALIAS(__ffssi2,ffs) 48 #if (defined(_ARM_ARCH_5) && !defined(__thumb__)) || defined(_ARM_ARCH_T2) 49 #if defined(_ARM_ARCH_T2) 50 ENTRY(ffs) 51 #else 52 ARM_ENTRY(ffs) 53 #endif 54 /* (X & -X) gives LSB or zero. */ 55 neg r1, r0 56 and r0, r0, r1 57 clz r0, r0 58 rsb r0, r0, #32 59 RET 60 END(ffs) 61 #else 62 ARM_ENTRY(ffs) 63 /* Standard trick to isolate bottom bit in r0 or 0 if r0 = 0 on entry */ 64 neg r1, r0 65 ands r0, r0, r1 66 /* 67 * now r0 has at most one set bit, call this X 68 * if X = 0, all further instructions are skipped 69 */ 70 adrne r2, .L_ffs_table 71 orrne r0, r0, r0, lsl #4 /* r0 = X * 0x11 */ 72 orrne r0, r0, r0, lsl #6 /* r0 = X * 0x451 */ 73 rsbne r0, r0, r0, lsl #16 /* r0 = X * 0x0450fbaf */ 74 75 /* now lookup in table indexed on top 6 bits of r0 */ 76 ldrbne r0, [r2, r0, lsr #26 ] 77 78 RET 79 .text; 80 .type .L_ffs_table, _ASM_TYPE_OBJECT; 81 .L_ffs_table: 82 /* 0 1 2 3 4 5 6 7 */ 83 .byte 0, 1, 2, 13, 3, 7, 0, 14 /* 0- 7 */ 84 .byte 4, 0, 8, 0, 0, 0, 0, 15 /* 8-15 */ 85 .byte 11, 5, 0, 0, 9, 0, 0, 26 /* 16-23 */ 86 .byte 0, 0, 0, 0, 0, 22, 28, 16 /* 24-31 */ 87 .byte 32, 12, 6, 0, 0, 0, 0, 0 /* 32-39 */ 88 .byte 10, 0, 0, 25, 0, 0, 21, 27 /* 40-47 */ 89 .byte 31, 0, 0, 0, 0, 24, 0, 20 /* 48-55 */ 90 .byte 30, 0, 23, 19, 29, 18, 17, 0 /* 56-63 */ 91 END(ffs) 92 #endif 93
Indexes created Tue Sep 30 17:09:57 GMT 2025