1 // SPDX-License-Identifier: LGPL-2.1-or-later
2 /*
3 * Provides fixed-point logarithm operations.
4 *
5 * Copyright (C) 2006 Christoph Pfister (christophpfister@gmail.com)
6 */
7
8 #include <linux/bitops.h>
9 #include <linux/export.h>
10 #include <linux/int_log.h>
11 #include <linux/kernel.h>
12 #include <linux/types.h>
13
14 #include <asm/bug.h>
15
16 static const unsigned short logtable[256] = {
17 0x0000, 0x0171, 0x02e0, 0x044e, 0x05ba, 0x0725, 0x088e, 0x09f7,
18 0x0b5d, 0x0cc3, 0x0e27, 0x0f8a, 0x10eb, 0x124b, 0x13aa, 0x1508,
19 0x1664, 0x17bf, 0x1919, 0x1a71, 0x1bc8, 0x1d1e, 0x1e73, 0x1fc6,
20 0x2119, 0x226a, 0x23ba, 0x2508, 0x2656, 0x27a2, 0x28ed, 0x2a37,
21 0x2b80, 0x2cc8, 0x2e0f, 0x2f54, 0x3098, 0x31dc, 0x331e, 0x345f,
22 0x359f, 0x36de, 0x381b, 0x3958, 0x3a94, 0x3bce, 0x3d08, 0x3e41,
23 0x3f78, 0x40af, 0x41e4, 0x4319, 0x444c, 0x457f, 0x46b0, 0x47e1,
24 0x4910, 0x4a3f, 0x4b6c, 0x4c99, 0x4dc5, 0x4eef, 0x5019, 0x5142,
25 0x526a, 0x5391, 0x54b7, 0x55dc, 0x5700, 0x5824, 0x5946, 0x5a68,
26 0x5b89, 0x5ca8, 0x5dc7, 0x5ee5, 0x6003, 0x611f, 0x623a, 0x6355,
27 0x646f, 0x6588, 0x66a0, 0x67b7, 0x68ce, 0x69e4, 0x6af8, 0x6c0c,
28 0x6d20, 0x6e32, 0x6f44, 0x7055, 0x7165, 0x7274, 0x7383, 0x7490,
29 0x759d, 0x76aa, 0x77b5, 0x78c0, 0x79ca, 0x7ad3, 0x7bdb, 0x7ce3,
30 0x7dea, 0x7ef0, 0x7ff6, 0x80fb, 0x81ff, 0x8302, 0x8405, 0x8507,
31 0x8608, 0x8709, 0x8809, 0x8908, 0x8a06, 0x8b04, 0x8c01, 0x8cfe,
32 0x8dfa, 0x8ef5, 0x8fef, 0x90e9, 0x91e2, 0x92db, 0x93d2, 0x94ca,
33 0x95c0, 0x96b6, 0x97ab, 0x98a0, 0x9994, 0x9a87, 0x9b7a, 0x9c6c,
34 0x9d5e, 0x9e4f, 0x9f3f, 0xa02e, 0xa11e, 0xa20c, 0xa2fa, 0xa3e7,
35 0xa4d4, 0xa5c0, 0xa6ab, 0xa796, 0xa881, 0xa96a, 0xaa53, 0xab3c,
36 0xac24, 0xad0c, 0xadf2, 0xaed9, 0xafbe, 0xb0a4, 0xb188, 0xb26c,
37 0xb350, 0xb433, 0xb515, 0xb5f7, 0xb6d9, 0xb7ba, 0xb89a, 0xb97a,
38 0xba59, 0xbb38, 0xbc16, 0xbcf4, 0xbdd1, 0xbead, 0xbf8a, 0xc065,
39 0xc140, 0xc21b, 0xc2f5, 0xc3cf, 0xc4a8, 0xc580, 0xc658, 0xc730,
40 0xc807, 0xc8de, 0xc9b4, 0xca8a, 0xcb5f, 0xcc34, 0xcd08, 0xcddc,
41 0xceaf, 0xcf82, 0xd054, 0xd126, 0xd1f7, 0xd2c8, 0xd399, 0xd469,
42 0xd538, 0xd607, 0xd6d6, 0xd7a4, 0xd872, 0xd93f, 0xda0c, 0xdad9,
43 0xdba5, 0xdc70, 0xdd3b, 0xde06, 0xded0, 0xdf9a, 0xe063, 0xe12c,
44 0xe1f5, 0xe2bd, 0xe385, 0xe44c, 0xe513, 0xe5d9, 0xe69f, 0xe765,
45 0xe82a, 0xe8ef, 0xe9b3, 0xea77, 0xeb3b, 0xebfe, 0xecc1, 0xed83,
46 0xee45, 0xef06, 0xefc8, 0xf088, 0xf149, 0xf209, 0xf2c8, 0xf387,
47 0xf446, 0xf505, 0xf5c3, 0xf680, 0xf73e, 0xf7fb, 0xf8b7, 0xf973,
48 0xfa2f, 0xfaea, 0xfba5, 0xfc60, 0xfd1a, 0xfdd4, 0xfe8e, 0xff47,
49 };
50
51 unsigned int intlog2(u32 value)
52 {
53 /**
54 * returns: log2(value) * 2^24
55 * wrong result if value = 0 (log2(0) is undefined)
56 */
57 unsigned int msb;
58 unsigned int logentry;
59 unsigned int significand;
60 unsigned int interpolation;
61
62 if (unlikely(value == 0)) {
63 WARN_ON(1);
64 return 0;
65 }
66
67 /* first detect the msb (count begins at 0) */
68 msb = fls(value) - 1;
69
70 /**
71 * now we use a logtable after the following method:
72 *
73 * log2(2^x * y) * 2^24 = x * 2^24 + log2(y) * 2^24
74 * where x = msb and therefore 1 <= y < 2
75 * first y is determined by shifting the value left
76 * so that msb is bit 31
77 * 0x00231f56 -> 0x8C7D5800
78 * the result is y * 2^31 -> "significand"
79 * then the highest 9 bits are used for a table lookup
80 * the highest bit is discarded because it's always set
81 * the highest nine bits in our example are 100011000
82 * so we would use the entry 0x18
83 */
84 significand = value << (31 - msb);
85 logentry = (significand >> 23) % ARRAY_SIZE(logtable);
86
87 /**
88 * last step we do is interpolation because of the
89 * limitations of the log table the error is that part of
90 * the significand which isn't used for lookup then we
91 * compute the ratio between the error and the next table entry
92 * and interpolate it between the log table entry used and the
93 * next one the biggest error possible is 0x7fffff
94 * (in our example it's 0x7D5800)
95 * needed value for next table entry is 0x800000
96 * so the interpolation is
97 * (error / 0x800000) * (logtable_next - logtable_current)
98 * in the implementation the division is moved to the end for
99 * better accuracy there is also an overflow correction if
100 * logtable_next is 256
101 */
102 interpolation = ((significand & 0x7fffff) *
103 ((logtable[(logentry + 1) % ARRAY_SIZE(logtable)] -
104 logtable[logentry]) & 0xffff)) >> 15;
105
106 /* now we return the result */
107 return ((msb << 24) + (logtable[logentry] << 8) + interpolation);
108 }
109 EXPORT_SYMBOL(intlog2);
110
111 unsigned int intlog10(u32 value)
112 {
113 /**
114 * returns: log10(value) * 2^24
115 * wrong result if value = 0 (log10(0) is undefined)
116 */
117 u64 log;
118
119 if (unlikely(value == 0)) {
120 WARN_ON(1);
121 return 0;
122 }
123
124 log = intlog2(value);
125
126 /**
127 * we use the following method:
128 * log10(x) = log2(x) * log10(2)
129 */
130
131 return (log * 646456993) >> 31;
132 }
133 EXPORT_SYMBOL(intlog10);
134
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