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Diffstat (limited to 'media/libjxl/src/lib/jxl/ans_common.h')
| -rw-r--r-- | media/libjxl/src/lib/jxl/ans_common.h | 143 |
1 files changed, 143 insertions, 0 deletions
diff --git a/media/libjxl/src/lib/jxl/ans_common.h b/media/libjxl/src/lib/jxl/ans_common.h new file mode 100644 index 0000000000..12ce1eff36 --- /dev/null +++ b/media/libjxl/src/lib/jxl/ans_common.h @@ -0,0 +1,143 @@ +// Copyright (c) the JPEG XL Project Authors. All rights reserved. +// +// Use of this source code is governed by a BSD-style +// license that can be found in the LICENSE file. + +#ifndef LIB_JXL_ANS_COMMON_H_ +#define LIB_JXL_ANS_COMMON_H_ + +#include <stdint.h> + +#include <algorithm> +#include <hwy/cache_control.h> // Prefetch +#include <vector> + +#include "lib/jxl/ans_params.h" +#include "lib/jxl/base/byte_order.h" +#include "lib/jxl/base/compiler_specific.h" +#include "lib/jxl/base/status.h" + +namespace jxl { + +// Returns the precision (number of bits) that should be used to store +// a histogram count such that Log2Floor(count) == logcount. +static JXL_INLINE uint32_t GetPopulationCountPrecision(uint32_t logcount, + uint32_t shift) { + int32_t r = std::min<int>( + logcount, int(shift) - int((ANS_LOG_TAB_SIZE - logcount) >> 1)); + if (r < 0) return 0; + return r; +} + +// Returns a histogram where the counts are positive, differ by at most 1, +// and add up to total_count. The bigger counts (if any) are at the beginning +// of the histogram. +std::vector<int> CreateFlatHistogram(int length, int total_count); + +// An alias table implements a mapping from the [0, ANS_TAB_SIZE) range into +// the [0, ANS_MAX_ALPHABET_SIZE) range, satisfying the following conditions: +// - each symbol occurs as many times as specified by any valid distribution +// of frequencies of the symbols. A valid distribution here is an array of +// ANS_MAX_ALPHABET_SIZE that contains numbers in the range [0, ANS_TAB_SIZE], +// and whose sum is ANS_TAB_SIZE. +// - lookups can be done in constant time, and also return how many smaller +// input values map into the same symbol, according to some well-defined order +// of input values. +// - the space used by the alias table is given by a small constant times the +// index of the largest symbol with nonzero probability in the distribution. +// Each of the entries in the table covers a range of `entry_size` values in the +// [0, ANS_TAB_SIZE) range; consecutive entries represent consecutive +// sub-ranges. In the range covered by entry `i`, the first `cutoff` values map +// to symbol `i`, while the others map to symbol `right_value`. +// +// TODO(veluca): consider making the order used for computing offsets easier to +// define - it is currently defined by the algorithm to compute the alias table. +// Beware of breaking the implicit assumption that symbols that come after the +// cutoff value should have an offset at least as big as the cutoff. + +struct AliasTable { + struct Symbol { + size_t value; + size_t offset; + size_t freq; + }; + +// Working set size matters here (~64 tables x 256 entries). +// offsets0 is always zero (beginning of [0] side among the same symbol). +// offsets1 is an offset of (pos >= cutoff) side decremented by cutoff. +#pragma pack(push, 1) + struct Entry { + uint8_t cutoff; // < kEntrySizeMinus1 when used by ANS. + uint8_t right_value; // < alphabet size. + uint16_t freq0; + + // Only used if `greater` (see Lookup) + uint16_t offsets1; // <= ANS_TAB_SIZE + uint16_t freq1_xor_freq0; // for branchless ternary in Lookup + }; +#pragma pack(pop) + + // Dividing `value` by `entry_size` determines `i`, the entry which is + // responsible for the input. If the remainder is below `cutoff`, then the + // mapped symbol is `i`; since `offsets[0]` stores the number of occurrences + // of `i` "before" the start of this entry, the offset of the input will be + // `offsets[0] + remainder`. If the remainder is above cutoff, the mapped + // symbol is `right_value`; since `offsets[1]` stores the number of + // occurrences of `right_value` "before" this entry, minus the `cutoff` value, + // the input offset is then `remainder + offsets[1]`. + static JXL_INLINE Symbol Lookup(const Entry* JXL_RESTRICT table, size_t value, + size_t log_entry_size, + size_t entry_size_minus_1) { + const size_t i = value >> log_entry_size; + const size_t pos = value & entry_size_minus_1; + +#if JXL_BYTE_ORDER_LITTLE + uint64_t entry; + memcpy(&entry, &table[i].cutoff, sizeof(entry)); + const size_t cutoff = entry & 0xFF; // = MOVZX + const size_t right_value = (entry >> 8) & 0xFF; // = MOVZX + const size_t freq0 = (entry >> 16) & 0xFFFF; +#else + // Generates multiple loads with complex addressing. + const size_t cutoff = table[i].cutoff; + const size_t right_value = table[i].right_value; + const size_t freq0 = table[i].freq0; +#endif + + const bool greater = pos >= cutoff; + +#if JXL_BYTE_ORDER_LITTLE + const uint64_t conditional = greater ? entry : 0; // = CMOV + const size_t offsets1_or_0 = (conditional >> 32) & 0xFFFF; + const size_t freq1_xor_freq0_or_0 = conditional >> 48; +#else + const size_t offsets1_or_0 = greater ? table[i].offsets1 : 0; + const size_t freq1_xor_freq0_or_0 = greater ? table[i].freq1_xor_freq0 : 0; +#endif + + // WARNING: moving this code may interfere with CMOV heuristics. + Symbol s; + s.value = greater ? right_value : i; + s.offset = offsets1_or_0 + pos; + s.freq = freq0 ^ freq1_xor_freq0_or_0; // = greater ? freq1 : freq0 + // XOR avoids implementation-defined conversion from unsigned to signed. + // Alternatives considered: BEXTR is 2 cycles on HSW, SET+shift causes + // spills, simple ternary has a long dependency chain. + + return s; + } + + static HWY_INLINE void Prefetch(const Entry* JXL_RESTRICT table, size_t value, + size_t log_entry_size) { + const size_t i = value >> log_entry_size; + hwy::Prefetch(table + i); + } +}; + +// Computes an alias table for a given distribution. +void InitAliasTable(std::vector<int> distribution, uint32_t range, + size_t log_alpha_size, AliasTable::Entry* JXL_RESTRICT a); + +} // namespace jxl + +#endif // LIB_JXL_ANS_COMMON_H_ |