1 ///////////////////////////////////////////////////////////////////////////////
2 //
3 /// \file fastpos.h
4 /// \brief Kind of two-bit version of bit scan reverse
5 ///
6 // Authors: Igor Pavlov
7 // Lasse Collin
8 //
9 // This file has been put into the public domain.
10 // You can do whatever you want with this file.
11 //
12 ///////////////////////////////////////////////////////////////////////////////
13
14 #ifndef LZMA_FASTPOS_H
15 #define LZMA_FASTPOS_H
16
17 // LZMA encodes match distances by storing the highest two bits using
18 // a six-bit value [0, 63], and then the missing lower bits.
19 // Dictionary size is also stored using this encoding in the .xz
20 // file format header.
21 //
22 // fastpos.h provides a way to quickly find out the correct six-bit
23 // values. The following table gives some examples of this encoding:
24 //
25 // dist return
26 // 0 0
27 // 1 1
28 // 2 2
29 // 3 3
30 // 4 4
31 // 5 4
32 // 6 5
33 // 7 5
34 // 8 6
35 // 11 6
36 // 12 7
37 // ... ...
38 // 15 7
39 // 16 8
40 // 17 8
41 // ... ...
42 // 23 8
43 // 24 9
44 // 25 9
45 // ... ...
46 //
47 //
48 // Provided functions or macros
49 // ----------------------------
50 //
51 // get_dist_slot(dist) is the basic version. get_dist_slot_2(dist)
52 // assumes that dist >= FULL_DISTANCES, thus the result is at least
53 // FULL_DISTANCES_BITS * 2. Using get_dist_slot(dist) instead of
54 // get_dist_slot_2(dist) would give the same result, but get_dist_slot_2(dist)
55 // should be tiny bit faster due to the assumption being made.
56 //
57 //
58 // Size vs. speed
59 // --------------
60 //
61 // With some CPUs that have fast BSR (bit scan reverse) instruction, the
62 // size optimized version is slightly faster than the bigger table based
63 // approach. Such CPUs include Intel Pentium Pro, Pentium II, Pentium III
64 // and Core 2 (possibly others). AMD K7 seems to have slower BSR, but that
65 // would still have speed roughly comparable to the table version. Older
66 // x86 CPUs like the original Pentium have very slow BSR; on those systems
67 // the table version is a lot faster.
68 //
69 // On some CPUs, the table version is a lot faster when using position
70 // dependent code, but with position independent code the size optimized
71 // version is slightly faster. This occurs at least on 32-bit SPARC (no
72 // ASM optimizations).
73 //
74 // I'm making the table version the default, because that has good speed
75 // on all systems I have tried. The size optimized version is sometimes
76 // slightly faster, but sometimes it is a lot slower.
77
78 #ifdef HAVE_SMALL
79 # define get_dist_slot(dist) \
80 ((dist) <= 4 ? (dist) : get_dist_slot_2(dist))
81
82 static inline uint32_t
get_dist_slot_2(uint32_t dist)83 get_dist_slot_2(uint32_t dist)
84 {
85 const uint32_t i = bsr32(dist);
86 return (i + i) + ((dist >> (i - 1)) & 1);
87 }
88
89
90 #else
91
92 #define FASTPOS_BITS 13
93
94 extern const uint8_t lzma_fastpos[1 << FASTPOS_BITS];
95
96
97 #define fastpos_shift(extra, n) \
98 ((extra) + (n) * (FASTPOS_BITS - 1))
99
100 #define fastpos_limit(extra, n) \
101 (UINT32_C(1) << (FASTPOS_BITS + fastpos_shift(extra, n)))
102
103 #define fastpos_result(dist, extra, n) \
104 (uint32_t)(lzma_fastpos[(dist) >> fastpos_shift(extra, n)]) \
105 + 2 * fastpos_shift(extra, n)
106
107
108 static inline uint32_t
get_dist_slot(uint32_t dist)109 get_dist_slot(uint32_t dist)
110 {
111 // If it is small enough, we can pick the result directly from
112 // the precalculated table.
113 if (dist < fastpos_limit(0, 0))
114 return lzma_fastpos[dist];
115
116 if (dist < fastpos_limit(0, 1))
117 return fastpos_result(dist, 0, 1);
118
119 return fastpos_result(dist, 0, 2);
120 }
121
122
123 #ifdef FULL_DISTANCES_BITS
124 static inline uint32_t
get_dist_slot_2(uint32_t dist)125 get_dist_slot_2(uint32_t dist)
126 {
127 assert(dist >= FULL_DISTANCES);
128
129 if (dist < fastpos_limit(FULL_DISTANCES_BITS - 1, 0))
130 return fastpos_result(dist, FULL_DISTANCES_BITS - 1, 0);
131
132 if (dist < fastpos_limit(FULL_DISTANCES_BITS - 1, 1))
133 return fastpos_result(dist, FULL_DISTANCES_BITS - 1, 1);
134
135 return fastpos_result(dist, FULL_DISTANCES_BITS - 1, 2);
136 }
137 #endif
138
139 #endif
140
141 #endif
142