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Diffstat (limited to 'firmware/target/arm/ffs-arm.S')
-rw-r--r-- | firmware/target/arm/ffs-arm.S | 74 |
1 files changed, 74 insertions, 0 deletions
diff --git a/firmware/target/arm/ffs-arm.S b/firmware/target/arm/ffs-arm.S new file mode 100644 index 0000000000..bb888ab558 --- /dev/null +++ b/firmware/target/arm/ffs-arm.S | |||
@@ -0,0 +1,74 @@ | |||
1 | /*************************************************************************** | ||
2 | * __________ __ ___. | ||
3 | * Open \______ \ ____ ____ | | _\_ |__ _______ ___ | ||
4 | * Source | _// _ \_/ ___\| |/ /| __ \ / _ \ \/ / | ||
5 | * Jukebox | | ( <_> ) \___| < | \_\ ( <_> > < < | ||
6 | * Firmware |____|_ /\____/ \___ >__|_ \|___ /\____/__/\_ \ | ||
7 | * \/ \/ \/ \/ \/ | ||
8 | * $Id$ | ||
9 | * | ||
10 | * Copyright (C) 2008 by Michael Sevakis | ||
11 | * | ||
12 | * All files in this archive are subject to the GNU General Public License. | ||
13 | * See the file COPYING in the source tree root for full license agreement. | ||
14 | * | ||
15 | * This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY | ||
16 | * KIND, either express or implied. | ||
17 | * | ||
18 | ****************************************************************************/ | ||
19 | #include "config.h" | ||
20 | |||
21 | /**************************************************************************** | ||
22 | * int find_first_set_bit(uint32_t val); | ||
23 | * | ||
24 | * Find the index of the least significant set bit in the 32-bit word. | ||
25 | * | ||
26 | * return values: | ||
27 | * 0 - bit 0 is set | ||
28 | * 1 - bit 1 is set | ||
29 | * ... | ||
30 | * 31 - bit 31 is set | ||
31 | * 32 - no bits set | ||
32 | ****************************************************************************/ | ||
33 | .align 2 | ||
34 | .global find_first_set_bit | ||
35 | .type find_first_set_bit,%function | ||
36 | find_first_set_bit: | ||
37 | @ Standard trick to isolate bottom bit in r0 or 0 if r0 = 0 on entry | ||
38 | rsb r2, r0, #0 @ r1 = r0 & -r0 | ||
39 | ands r1, r0, r2 @ | ||
40 | |||
41 | @ now r1 has at most one set bit, call this X | ||
42 | |||
43 | #if ARM_ARCH >= 5 | ||
44 | clz r0, r1 @ Get lead 0's count | ||
45 | rsbne r0, r0, #31 @ lead 0's -> bit index | ||
46 | bx lr @ | ||
47 | #else | ||
48 | @ this is the ffs algorithm devised by D.Seal and posted to | ||
49 | @ comp.sys.arm on 16 Feb 1994. | ||
50 | @ | ||
51 | @ Output modified to suit Rockbox purposes. | ||
52 | |||
53 | adr r2, L_ffs_table | ||
54 | orrne r1, r1, r1, lsl #4 @ r1 = X * 0x11 | ||
55 | orrne r1, r1, r1, lsl #6 @ r1 = X * 0x451 | ||
56 | rsbne r1, r1, r1, lsl #16 @ r1 = X * 0x0450fbaf | ||
57 | |||
58 | @ now lookup in table indexed on top 6 bits of r1 | ||
59 | ldrb r0, [ r2, r1, lsr #26 ] @ | ||
60 | bx lr @ | ||
61 | |||
62 | L_ffs_table: | ||
63 | @ 0 1 2 3 4 5 6 7 | ||
64 | @---------------------------------------------- | ||
65 | .byte 32, 0, 1, 12, 2, 6, 0, 13 @ 0- 7 | ||
66 | .byte 3, 0, 7, 0, 0, 0, 0, 14 @ 8-15 | ||
67 | .byte 10, 4, 0, 0, 8, 0, 0, 25 @ 16-23 | ||
68 | .byte 0, 0, 0, 0, 0, 21, 27, 15 @ 24-31 | ||
69 | .byte 31, 11, 5, 0, 0, 0, 0, 0 @ 32-39 | ||
70 | .byte 9, 0, 0, 24, 0, 0, 20, 26 @ 40-47 | ||
71 | .byte 30, 0, 0, 0, 0, 23, 0, 19 @ 48-55 | ||
72 | .byte 29, 0, 22, 18, 28, 17, 16, 0 @ 56-63 | ||
73 | #endif | ||
74 | .size find_first_set_bit, .-find_first_set_bit | ||