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 82static inline uint32_t 83get_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 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 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