1207753Smm/////////////////////////////////////////////////////////////////////////////// 2207753Smm// 3207753Smm/// \file fastpos.h 4207753Smm/// \brief Kind of two-bit version of bit scan reverse 5207753Smm/// 6207753Smm// Authors: Igor Pavlov 7207753Smm// Lasse Collin 8207753Smm// 9207753Smm// This file has been put into the public domain. 10207753Smm// You can do whatever you want with this file. 11207753Smm// 12207753Smm/////////////////////////////////////////////////////////////////////////////// 13207753Smm 14207753Smm#ifndef LZMA_FASTPOS_H 15207753Smm#define LZMA_FASTPOS_H 16207753Smm 17207753Smm// LZMA encodes match distances (positions) by storing the highest two 18207753Smm// bits using a six-bit value [0, 63], and then the missing lower bits. 19207753Smm// Dictionary size is also stored using this encoding in the new .lzma 20207753Smm// file format header. 21207753Smm// 22207753Smm// fastpos.h provides a way to quickly find out the correct six-bit 23207753Smm// values. The following table gives some examples of this encoding: 24207753Smm// 25207753Smm// pos return 26207753Smm// 0 0 27207753Smm// 1 1 28207753Smm// 2 2 29207753Smm// 3 3 30207753Smm// 4 4 31207753Smm// 5 4 32207753Smm// 6 5 33207753Smm// 7 5 34207753Smm// 8 6 35207753Smm// 11 6 36207753Smm// 12 7 37207753Smm// ... ... 38207753Smm// 15 7 39207753Smm// 16 8 40207753Smm// 17 8 41207753Smm// ... ... 42207753Smm// 23 8 43207753Smm// 24 9 44207753Smm// 25 9 45207753Smm// ... ... 46207753Smm// 47207753Smm// 48207753Smm// Provided functions or macros 49207753Smm// ---------------------------- 50207753Smm// 51207753Smm// get_pos_slot(pos) is the basic version. get_pos_slot_2(pos) 52207753Smm// assumes that pos >= FULL_DISTANCES, thus the result is at least 53207753Smm// FULL_DISTANCES_BITS * 2. Using get_pos_slot(pos) instead of 54207753Smm// get_pos_slot_2(pos) would give the same result, but get_pos_slot_2(pos) 55207753Smm// should be tiny bit faster due to the assumption being made. 56207753Smm// 57207753Smm// 58207753Smm// Size vs. speed 59207753Smm// -------------- 60207753Smm// 61207753Smm// With some CPUs that have fast BSR (bit scan reverse) instruction, the 62207753Smm// size optimized version is slightly faster than the bigger table based 63207753Smm// approach. Such CPUs include Intel Pentium Pro, Pentium II, Pentium III 64207753Smm// and Core 2 (possibly others). AMD K7 seems to have slower BSR, but that 65207753Smm// would still have speed roughly comparable to the table version. Older 66207753Smm// x86 CPUs like the original Pentium have very slow BSR; on those systems 67207753Smm// the table version is a lot faster. 68207753Smm// 69207753Smm// On some CPUs, the table version is a lot faster when using position 70207753Smm// dependent code, but with position independent code the size optimized 71207753Smm// version is slightly faster. This occurs at least on 32-bit SPARC (no 72207753Smm// ASM optimizations). 73207753Smm// 74207753Smm// I'm making the table version the default, because that has good speed 75207753Smm// on all systems I have tried. The size optimized version is sometimes 76207753Smm// slightly faster, but sometimes it is a lot slower. 77207753Smm 78207753Smm#ifdef HAVE_SMALL 79207753Smm# define get_pos_slot(pos) ((pos) <= 4 ? (pos) : get_pos_slot_2(pos)) 80207753Smm 81207753Smmstatic inline uint32_t 82207753Smmget_pos_slot_2(uint32_t pos) 83207753Smm{ 84207753Smm const uint32_t i = bsr32(pos); 85207753Smm return (i + i) + ((pos >> (i - 1)) & 1); 86207753Smm} 87207753Smm 88207753Smm 89207753Smm#else 90207753Smm 91207753Smm#define FASTPOS_BITS 13 92207753Smm 93207753Smmextern const uint8_t lzma_fastpos[1 << FASTPOS_BITS]; 94207753Smm 95207753Smm 96207753Smm#define fastpos_shift(extra, n) \ 97207753Smm ((extra) + (n) * (FASTPOS_BITS - 1)) 98207753Smm 99207753Smm#define fastpos_limit(extra, n) \ 100207753Smm (UINT32_C(1) << (FASTPOS_BITS + fastpos_shift(extra, n))) 101207753Smm 102207753Smm#define fastpos_result(pos, extra, n) \ 103207753Smm lzma_fastpos[(pos) >> fastpos_shift(extra, n)] \ 104207753Smm + 2 * fastpos_shift(extra, n) 105207753Smm 106207753Smm 107207753Smmstatic inline uint32_t 108207753Smmget_pos_slot(uint32_t pos) 109207753Smm{ 110207753Smm // If it is small enough, we can pick the result directly from 111207753Smm // the precalculated table. 112207753Smm if (pos < fastpos_limit(0, 0)) 113207753Smm return lzma_fastpos[pos]; 114207753Smm 115207753Smm if (pos < fastpos_limit(0, 1)) 116207753Smm return fastpos_result(pos, 0, 1); 117207753Smm 118207753Smm return fastpos_result(pos, 0, 2); 119207753Smm} 120207753Smm 121207753Smm 122207753Smm#ifdef FULL_DISTANCES_BITS 123207753Smmstatic inline uint32_t 124207753Smmget_pos_slot_2(uint32_t pos) 125207753Smm{ 126207753Smm assert(pos >= FULL_DISTANCES); 127207753Smm 128207753Smm if (pos < fastpos_limit(FULL_DISTANCES_BITS - 1, 0)) 129207753Smm return fastpos_result(pos, FULL_DISTANCES_BITS - 1, 0); 130207753Smm 131207753Smm if (pos < fastpos_limit(FULL_DISTANCES_BITS - 1, 1)) 132207753Smm return fastpos_result(pos, FULL_DISTANCES_BITS - 1, 1); 133207753Smm 134207753Smm return fastpos_result(pos, FULL_DISTANCES_BITS - 1, 2); 135207753Smm} 136207753Smm#endif 137207753Smm 138207753Smm#endif 139207753Smm 140207753Smm#endif 141