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142 lines
3.9 KiB
142 lines
3.9 KiB
///////////////////////////////////////////////////////////////////////////////
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//
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/// \file fastpos.h
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/// \brief Kind of two-bit version of bit scan reverse
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///
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// Authors: Igor Pavlov
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// Lasse Collin
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//
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// This file has been put into the public domain.
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// You can do whatever you want with this file.
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//
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///////////////////////////////////////////////////////////////////////////////
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#ifndef LZMA_FASTPOS_H
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#define LZMA_FASTPOS_H
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// LZMA encodes match distances by storing the highest two bits using
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// a six-bit value [0, 63], and then the missing lower bits.
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// Dictionary size is also stored using this encoding in the .xz
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// file format header.
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//
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// fastpos.h provides a way to quickly find out the correct six-bit
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// values. The following table gives some examples of this encoding:
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//
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// dist return
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// 0 0
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// 1 1
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// 2 2
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// 3 3
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// 4 4
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// 5 4
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// 6 5
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// 7 5
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// 8 6
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// 11 6
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// 12 7
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// ... ...
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// 15 7
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// 16 8
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// 17 8
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// ... ...
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// 23 8
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// 24 9
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// 25 9
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// ... ...
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//
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//
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// Provided functions or macros
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// ----------------------------
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//
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// get_dist_slot(dist) is the basic version. get_dist_slot_2(dist)
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// assumes that dist >= FULL_DISTANCES, thus the result is at least
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// FULL_DISTANCES_BITS * 2. Using get_dist_slot(dist) instead of
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// get_dist_slot_2(dist) would give the same result, but get_dist_slot_2(dist)
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// should be tiny bit faster due to the assumption being made.
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//
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//
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// Size vs. speed
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// --------------
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//
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// With some CPUs that have fast BSR (bit scan reverse) instruction, the
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// size optimized version is slightly faster than the bigger table based
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// approach. Such CPUs include Intel Pentium Pro, Pentium II, Pentium III
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// and Core 2 (possibly others). AMD K7 seems to have slower BSR, but that
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// would still have speed roughly comparable to the table version. Older
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// x86 CPUs like the original Pentium have very slow BSR; on those systems
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// the table version is a lot faster.
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//
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// On some CPUs, the table version is a lot faster when using position
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// dependent code, but with position independent code the size optimized
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// version is slightly faster. This occurs at least on 32-bit SPARC (no
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// ASM optimizations).
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//
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// I'm making the table version the default, because that has good speed
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// on all systems I have tried. The size optimized version is sometimes
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// slightly faster, but sometimes it is a lot slower.
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#ifdef HAVE_SMALL
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# define get_dist_slot(dist) \
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((dist) <= 4 ? (dist) : get_dist_slot_2(dist))
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static inline uint32_t
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get_dist_slot_2(uint32_t dist)
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{
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const uint32_t i = bsr32(dist);
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return (i + i) + ((dist >> (i - 1)) & 1);
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}
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#else
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#define FASTPOS_BITS 13
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extern const uint8_t lzma_fastpos[1 << FASTPOS_BITS];
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#define fastpos_shift(extra, n) \
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((extra) + (n) * (FASTPOS_BITS - 1))
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#define fastpos_limit(extra, n) \
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(UINT32_C(1) << (FASTPOS_BITS + fastpos_shift(extra, n)))
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#define fastpos_result(dist, extra, n) \
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lzma_fastpos[(dist) >> fastpos_shift(extra, n)] \
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+ 2 * fastpos_shift(extra, n)
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static inline uint32_t
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get_dist_slot(uint32_t dist)
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{
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// If it is small enough, we can pick the result directly from
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// the precalculated table.
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if (dist < fastpos_limit(0, 0))
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return lzma_fastpos[dist];
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if (dist < fastpos_limit(0, 1))
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return fastpos_result(dist, 0, 1);
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return fastpos_result(dist, 0, 2);
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}
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#ifdef FULL_DISTANCES_BITS
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static inline uint32_t
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get_dist_slot_2(uint32_t dist)
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{
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assert(dist >= FULL_DISTANCES);
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if (dist < fastpos_limit(FULL_DISTANCES_BITS - 1, 0))
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return fastpos_result(dist, FULL_DISTANCES_BITS - 1, 0);
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if (dist < fastpos_limit(FULL_DISTANCES_BITS - 1, 1))
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return fastpos_result(dist, FULL_DISTANCES_BITS - 1, 1);
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return fastpos_result(dist, FULL_DISTANCES_BITS - 1, 2);
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}
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#endif
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#endif
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#endif
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