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 (positions) by storing the highest two 18 // bits using a six-bit value [0, 63], and then the missing lower bits. 19 // Dictionary size is also stored using this encoding in the new .lzma 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 // pos 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_pos_slot(pos) is the basic version. get_pos_slot_2(pos) 52 // assumes that pos >= FULL_DISTANCES, thus the result is at least 53 // FULL_DISTANCES_BITS * 2. Using get_pos_slot(pos) instead of 54 // get_pos_slot_2(pos) would give the same result, but get_pos_slot_2(pos) 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_pos_slot(pos) ((pos) <= 4 ? (pos) : get_pos_slot_2(pos)) 80 81 static inline uint32_t 82 get_pos_slot_2(uint32_t pos) 83 { 84 const uint32_t i = bsr32(pos); 85 return (i + i) + ((pos >> (i - 1)) & 1); 86 } 87 88 89 #else 90 91 #define FASTPOS_BITS 13 92 93 extern const uint8_t lzma_fastpos[1 << FASTPOS_BITS]; 94 95 96 #define fastpos_shift(extra, n) \ 97 ((extra) + (n) * (FASTPOS_BITS - 1)) 98 99 #define fastpos_limit(extra, n) \ 100 (UINT32_C(1) << (FASTPOS_BITS + fastpos_shift(extra, n))) 101 102 #define fastpos_result(pos, extra, n) \ 103 lzma_fastpos[(pos) >> fastpos_shift(extra, n)] \ 104 + 2 * fastpos_shift(extra, n) 105 106 107 static inline uint32_t 108 get_pos_slot(uint32_t pos) 109 { 110 // If it is small enough, we can pick the result directly from 111 // the precalculated table. 112 if (pos < fastpos_limit(0, 0)) 113 return lzma_fastpos[pos]; 114 115 if (pos < fastpos_limit(0, 1)) 116 return fastpos_result(pos, 0, 1); 117 118 return fastpos_result(pos, 0, 2); 119 } 120 121 122 #ifdef FULL_DISTANCES_BITS 123 static inline uint32_t 124 get_pos_slot_2(uint32_t pos) 125 { 126 assert(pos >= FULL_DISTANCES); 127 128 if (pos < fastpos_limit(FULL_DISTANCES_BITS - 1, 0)) 129 return fastpos_result(pos, FULL_DISTANCES_BITS - 1, 0); 130 131 if (pos < fastpos_limit(FULL_DISTANCES_BITS - 1, 1)) 132 return fastpos_result(pos, FULL_DISTANCES_BITS - 1, 1); 133 134 return fastpos_result(pos, FULL_DISTANCES_BITS - 1, 2); 135 } 136 #endif 137 138 #endif 139 140 #endif 141