1// SPDX-License-Identifier: 0BSD 2 3/////////////////////////////////////////////////////////////////////////////// 4// 5/// \file fastpos.h 6/// \brief Kind of two-bit version of bit scan reverse 7/// 8// Authors: Igor Pavlov 9// Lasse Collin 10// 11/////////////////////////////////////////////////////////////////////////////// 12 13#ifndef LZMA_FASTPOS_H 14#define LZMA_FASTPOS_H 15 16// LZMA encodes match distances by storing the highest two bits using 17// a six-bit value [0, 63], and then the missing lower bits. 18// Dictionary size is also stored using this encoding in the .xz 19// file format header. 20// 21// fastpos.h provides a way to quickly find out the correct six-bit 22// values. The following table gives some examples of this encoding: 23// 24// dist return 25// 0 0 26// 1 1 27// 2 2 28// 3 3 29// 4 4 30// 5 4 31// 6 5 32// 7 5 33// 8 6 34// 11 6 35// 12 7 36// ... ... 37// 15 7 38// 16 8 39// 17 8 40// ... ... 41// 23 8 42// 24 9 43// 25 9 44// ... ... 45// 46// 47// Provided functions or macros 48// ---------------------------- 49// 50// get_dist_slot(dist) is the basic version. get_dist_slot_2(dist) 51// assumes that dist >= FULL_DISTANCES, thus the result is at least 52// FULL_DISTANCES_BITS * 2. Using get_dist_slot(dist) instead of 53// get_dist_slot_2(dist) would give the same result, but get_dist_slot_2(dist) 54// should be tiny bit faster due to the assumption being made. 55// 56// 57// Size vs. speed 58// -------------- 59// 60// With some CPUs that have fast BSR (bit scan reverse) instruction, the 61// size optimized version is slightly faster than the bigger table based 62// approach. Such CPUs include Intel Pentium Pro, Pentium II, Pentium III 63// and Core 2 (possibly others). AMD K7 seems to have slower BSR, but that 64// would still have speed roughly comparable to the table version. Older 65// x86 CPUs like the original Pentium have very slow BSR; on those systems 66// the table version is a lot faster. 67// 68// On some CPUs, the table version is a lot faster when using position 69// dependent code, but with position independent code the size optimized 70// version is slightly faster. This occurs at least on 32-bit SPARC (no 71// ASM optimizations). 72// 73// I'm making the table version the default, because that has good speed 74// on all systems I have tried. The size optimized version is sometimes 75// slightly faster, but sometimes it is a lot slower. 76 77#ifdef HAVE_SMALL 78# define get_dist_slot(dist) \ 79 ((dist) <= 4 ? (dist) : get_dist_slot_2(dist)) 80 81static inline uint32_t 82get_dist_slot_2(uint32_t dist) 83{ 84 const uint32_t i = bsr32(dist); 85 return (i + i) + ((dist >> (i - 1)) & 1); 86} 87 88 89#else 90 91#define FASTPOS_BITS 13 92 93lzma_attr_visibility_hidden 94extern 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 108static inline uint32_t 109get_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 124static inline uint32_t 125get_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