diff options
Diffstat (limited to 'sys/libkern/arm/ffs.S')
| -rw-r--r-- | sys/libkern/arm/ffs.S | 77 |
1 files changed, 77 insertions, 0 deletions
diff --git a/sys/libkern/arm/ffs.S b/sys/libkern/arm/ffs.S new file mode 100644 index 0000000..4e6fe3d --- /dev/null +++ b/sys/libkern/arm/ffs.S @@ -0,0 +1,77 @@ +/* $NetBSD: ffs.S,v 1.3 2003/04/05 23:27:15 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 <machine/asm.h> +__FBSDID("$FreeBSD$"); + + +/* + * 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 +.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 */ + |
