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 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 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 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