| Raghu Gandham | 405b802 | 2012-07-25 18:16:42 -0700 | [diff] [blame] | 1 | /* $NetBSD: ffs.S,v 1.5 2003/04/05 23:08:52 bjh21 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 | /* |
| 32 | * ffs - find first set bit, this algorithm isolates the first set |
| 33 | * bit, then multiplies the number by 0x0450fbaf which leaves the top |
| 34 | * 6 bits as an index into the table. This algorithm should be a win |
| 35 | * over the checking each bit in turn as per the C compiled version. |
| 36 | * |
| 37 | * This is the ffs algorithm devised by d.seal and posted to comp.sys.arm on |
| 38 | * 16 Feb 1994. |
| 39 | */ |
| 40 | |
| 41 | .text |
| 42 | |
| 43 | .ent ffs |
| 44 | .type ffs, @function |
| 45 | .global ffs |
| 46 | .align 4 |
| 47 | .set noreorder |
| 48 | ffs: |
| 49 | #if (__mips==32) |
| 50 | subu $t0,$0,$a0 |
| 51 | and $a0,$t0 |
| 52 | clz $t0,$a0 |
| 53 | li $v0,32 |
| 54 | j $ra |
| 55 | subu $v0,$t0 |
| 56 | #else |
| 57 | /* Size of the ffs routine in bytes |
| 58 | * This is used to index .L_ffs_table from $t9 |
| 59 | * FIXME: there must be a better way to do this |
| 60 | */ |
| 61 | #define FFSSIZE 12*4 |
| 62 | subu $t0,$0,$a0 |
| 63 | and $a0,$t0 |
| 64 | /* |
| 65 | * now a0 has at most one set bit, call this X |
| 66 | * if X = 0, all further instructions are skipped |
| 67 | */ |
| 68 | sll $t0,$a0,4 /* t0 = X * 0x00000010 */ |
| 69 | or $a0,$t0 /* a0 = X * 0x00000011 */ |
| 70 | sll $t0,$a0,6 /* t0 = X * 0x00000440 */ |
| 71 | or $a0,$t0 /* a0 = X * 0x00000451 */ |
| 72 | sll $t0,$a0,16 /* t0 = X * 0x04510000 */ |
| 73 | subu $a0,$t0,$a0 /* a0 = X * 0x0450fbaf */ |
| 74 | |
| 75 | /* now lookup in table indexed on top 6 bits of a0 */ |
| 76 | srl $a0,25 |
| 77 | addu $t9,$a0 |
| 78 | j $ra |
| 79 | lbu $v0,FFSSIZE($t9) |
| 80 | .L_ffs_table: |
| 81 | /* 0 1 2 3 4 5 6 7 */ |
| 82 | .byte 0, 1, 2, 13, 3, 7, 0, 14 /* 0- 7 */ |
| 83 | .byte 4, 0, 8, 0, 0, 0, 0, 15 /* 8-15 */ |
| 84 | .byte 11, 5, 0, 0, 9, 0, 0, 26 /* 16-23 */ |
| 85 | .byte 0, 0, 0, 0, 0, 22, 28, 16 /* 24-31 */ |
| 86 | .byte 32, 12, 6, 0, 0, 0, 0, 0 /* 32-39 */ |
| 87 | .byte 10, 0, 0, 25, 0, 0, 21, 27 /* 40-47 */ |
| 88 | .byte 31, 0, 0, 0, 0, 24, 0, 20 /* 48-55 */ |
| 89 | .byte 30, 0, 23, 19, 29, 18, 17, 0 /* 56-63 */ |
| 90 | |
| 91 | #endif |
| 92 | .end ffs |