release/src-rt-6.x.4708/tools/misc/xz/src/liblzma/lzma/fastpos.h

   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