*/
/*
- * Copyright (C) 2012, 2013, 2014, 2015 Eric Biggers
+ * Copyright (C) 2012-2016 Eric Biggers
*
* This file is free software; you can redistribute it and/or modify it under
* the terms of the GNU Lesser General Public License as published by the Free
#endif
/*
- * Start a new LZX block (with new Huffman codes) after this many bytes.
- *
- * Note: actual block sizes may slightly exceed this value.
- *
- * TODO: recursive splitting and cost evaluation might be good for an extremely
- * high compression mode, but otherwise it is almost always far too slow for how
- * much it helps. Perhaps some sort of heuristic would be useful?
+ * The compressor always chooses a block of at least MIN_BLOCK_SIZE bytes,
+ * except if the last block has to be shorter.
*/
-#define LZX_DIV_BLOCK_SIZE 32768
+#define MIN_BLOCK_SIZE 6500
/*
- * LZX_CACHE_PER_POS is the number of lz_match structures to reserve in the
- * match cache for each byte position. This value should be high enough so that
- * nearly the time, all matches found in a given block can fit in the match
- * cache. However, fallback behavior (immediately terminating the block) on
- * cache overflow is still required.
+ * The compressor attempts to end blocks after SOFT_MAX_BLOCK_SIZE bytes, but
+ * the final size might be larger due to matches extending beyond the end of the
+ * block. Specifically:
+ *
+ * - The greedy parser may choose an arbitrarily long match starting at the
+ * SOFT_MAX_BLOCK_SIZE'th byte.
+ *
+ * - The lazy parser may choose a sequence of literals starting at the
+ * SOFT_MAX_BLOCK_SIZE'th byte when it sees a sequence of increasing good
+ * matches. The final match may be of arbitrary length. The length of the
+ * literal sequence is approximately limited by the "nice match length"
+ * parameter.
*/
-#define LZX_CACHE_PER_POS 7
+#define SOFT_MAX_BLOCK_SIZE 100000
/*
* LZX_CACHE_LENGTH is the number of lz_match structures in the match cache,
- * excluding the extra "overflow" entries. The per-position multiplier is '1 +
- * LZX_CACHE_PER_POS' instead of 'LZX_CACHE_PER_POS' because there is an
- * overhead of one lz_match per position, used to hold the match count at that
- * position.
+ * excluding the extra "overflow" entries. This value should be high enough so
+ * that nearly the time, all matches found in a given block can fit in the match
+ * cache. However, fallback behavior (immediately terminating the block) on
+ * cache overflow is still required.
*/
-#define LZX_CACHE_LENGTH (LZX_DIV_BLOCK_SIZE * (1 + LZX_CACHE_PER_POS))
+#define LZX_CACHE_LENGTH (SOFT_MAX_BLOCK_SIZE * 5)
/*
* LZX_MAX_MATCHES_PER_POS is an upper bound on the number of matches that can
u32 aligned[LZX_ALIGNEDCODE_NUM_SYMBOLS];
};
+/* Block split statistics. See "Block splitting algorithm" below. */
+#define NUM_LITERAL_OBSERVATION_TYPES 8
+#define NUM_MATCH_OBSERVATION_TYPES 2
+#define NUM_OBSERVATION_TYPES (NUM_LITERAL_OBSERVATION_TYPES + NUM_MATCH_OBSERVATION_TYPES)
+struct block_split_stats {
+ u32 new_observations[NUM_OBSERVATION_TYPES];
+ u32 observations[NUM_OBSERVATION_TYPES];
+ u32 num_new_observations;
+ u32 num_observations;
+};
+
/*
* Represents a run of literals followed by a match or end-of-block. This
* struct is needed to temporarily store items chosen by the parser, since items
u16 adjusted_length;
/* If bit 31 is clear, then this field contains the match header in bits
- * 0-8 and the match offset minus LZX_OFFSET_ADJUSTMENT in bits 9-30.
- * Otherwise, this sequence's literal run was the last literal run in
- * the block, so there is no match that follows it. */
+ * 0-8, and either the match offset plus LZX_OFFSET_ADJUSTMENT or a
+ * recent offset code in bits 9-30. Otherwise (if bit 31 is set), this
+ * sequence's literal run was the last literal run in the block, so
+ * there is no match that follows it. */
u32 adjusted_offset_and_match_hdr;
};
};
}
-/* Pop a match offset off the front (most recently used) end of the queue. */
-static inline u32
-lzx_lru_queue_pop(struct lzx_lru_queue *queue_p)
-{
- u32 offset = queue_p->R & LZX_QUEUE64_OFFSET_MASK;
- queue_p->R >>= LZX_QUEUE64_OFFSET_SHIFT;
- return offset;
-}
-
/* Swap a match offset to the front of the queue. */
static inline struct lzx_lru_queue
lzx_lru_queue_swap(struct lzx_lru_queue queue, unsigned idx)
/* The Huffman symbol frequency counters for the current block. */
struct lzx_freqs freqs;
+ /* Block split statistics. */
+ struct block_split_stats split_stats;
+
/* The Huffman codes for the current and previous blocks. The one with
* index 'codes_index' is for the current block, and the other one is
* for the previous block. */
/* The matches and literals that the parser has chosen for the current
* block. The required length of this array is limited by the maximum
- * number of matches that can ever be chosen for a single block. */
- struct lzx_sequence chosen_sequences[DIV_ROUND_UP(LZX_DIV_BLOCK_SIZE, LZX_MIN_MATCH_LEN)];
+ * number of matches that can ever be chosen for a single block, plus
+ * one for the special entry at the end. */
+ struct lzx_sequence chosen_sequences[
+ DIV_ROUND_UP(SOFT_MAX_BLOCK_SIZE, LZX_MIN_MATCH_LEN) + 1];
/* Tables for mapping adjusted offsets to offset slots */
/* Data for near-optimal parsing */
struct {
/*
- * The graph nodes for the current block.
+ * Array of nodes, one per position, for running the
+ * minimum-cost path algorithm.
*
- * We need at least 'LZX_DIV_BLOCK_SIZE +
- * LZX_MAX_MATCH_LEN - 1' nodes because that is the
- * maximum block size that may be used. Add 1 because
- * we need a node to represent end-of-block.
- *
- * It is possible that nodes past end-of-block are
- * accessed during match consideration, but this can
- * only occur if the block was truncated at
- * LZX_DIV_BLOCK_SIZE. So the same bound still applies.
- * Note that since nodes past the end of the block will
- * never actually have an effect on the items that are
- * chosen for the block, it makes no difference what
- * their costs are initialized to (if anything).
+ * This array must be large enough to accommodate the
+ * worst-case number of nodes, which occurs if we find a
+ * match of length LZX_MAX_MATCH_LEN at position
+ * SOFT_MAX_BLOCK_SIZE - 1, producing a block of length
+ * SOFT_MAX_BLOCK_SIZE - 1 + LZX_MAX_MATCH_LEN. Add one
+ * for the end-of-block node.
*/
- struct lzx_optimum_node optimum_nodes[LZX_DIV_BLOCK_SIZE +
- LZX_MAX_MATCH_LEN - 1 + 1];
+ struct lzx_optimum_node optimum_nodes[SOFT_MAX_BLOCK_SIZE - 1 +
+ LZX_MAX_MATCH_LEN + 1];
/* The cost model for the current block */
struct lzx_costs costs;
/* Can the specified number of bits always be added to 'bitbuf' after any
* pending 16-bit coding units have been flushed? */
-#define CAN_BUFFER(n) ((n) <= (8 * sizeof(machine_word_t)) - 16)
+#define CAN_BUFFER(n) ((n) <= (8 * sizeof(machine_word_t)) - 15)
/*
* Initialize the output bitstream.
static inline void
lzx_flush_bits(struct lzx_output_bitstream *os, unsigned max_num_bits)
{
+ /* Masking the number of bits to shift is only needed to avoid undefined
+ * behavior; we don't actually care about the results of bad shifts. On
+ * x86, the explicit masking generates no extra code. */
+ const u32 shift_mask = 8 * sizeof(os->bitbuf) - 1;
+
if (os->end - os->next < 6)
return;
- put_unaligned_u16_le(os->bitbuf >> (os->bitcount - 16), os->next + 0);
+ put_unaligned_le16(os->bitbuf >> ((os->bitcount - 16) &
+ shift_mask), os->next + 0);
if (max_num_bits > 16)
- put_unaligned_u16_le(os->bitbuf >> (os->bitcount - 32), os->next + 2);
+ put_unaligned_le16(os->bitbuf >> ((os->bitcount - 32) &
+ shift_mask), os->next + 2);
if (max_num_bits > 32)
- put_unaligned_u16_le(os->bitbuf >> (os->bitcount - 48), os->next + 4);
+ put_unaligned_le16(os->bitbuf >> ((os->bitcount - 48) &
+ shift_mask), os->next + 4);
os->next += (os->bitcount >> 4) << 1;
os->bitcount &= 15;
}
return 0;
if (os->bitcount != 0) {
- put_unaligned_u16_le(os->bitbuf << (16 - os->bitcount), os->next);
+ put_unaligned_le16(os->bitbuf << (16 - os->bitcount), os->next);
os->next += 2;
}
STATIC_ASSERT(MAIN_CODEWORD_LIMIT >= 9 &&
MAIN_CODEWORD_LIMIT <= LZX_MAX_MAIN_CODEWORD_LEN);
- STATIC_ASSERT(LENGTH_CODEWORD_LIMIT >= 9 &&
+ STATIC_ASSERT(LENGTH_CODEWORD_LIMIT >= 8 &&
LENGTH_CODEWORD_LIMIT <= LZX_MAX_LEN_CODEWORD_LEN);
STATIC_ASSERT(ALIGNED_CODEWORD_LIMIT >= LZX_NUM_ALIGNED_OFFSET_BITS &&
ALIGNED_CODEWORD_LIMIT <= LZX_MAX_ALIGNED_CODEWORD_LEN);
unsigned lit1 = block_data[1];
unsigned lit2 = block_data[2];
unsigned lit3 = block_data[3];
- lzx_add_bits(os, codes->codewords.main[lit0], codes->lens.main[lit0]);
- lzx_add_bits(os, codes->codewords.main[lit1], codes->lens.main[lit1]);
- lzx_add_bits(os, codes->codewords.main[lit2], codes->lens.main[lit2]);
- lzx_add_bits(os, codes->codewords.main[lit3], codes->lens.main[lit3]);
+ lzx_add_bits(os, codes->codewords.main[lit0],
+ codes->lens.main[lit0]);
+ lzx_add_bits(os, codes->codewords.main[lit1],
+ codes->lens.main[lit1]);
+ lzx_add_bits(os, codes->codewords.main[lit2],
+ codes->lens.main[lit2]);
+ lzx_add_bits(os, codes->codewords.main[lit3],
+ codes->lens.main[lit3]);
lzx_flush_bits(os, 4 * MAIN_CODEWORD_LIMIT);
block_data += 4;
litrunlen -= 4;
}
if (litrunlen--) {
unsigned lit = *block_data++;
- lzx_add_bits(os, codes->codewords.main[lit], codes->lens.main[lit]);
+ lzx_add_bits(os, codes->codewords.main[lit],
+ codes->lens.main[lit]);
if (litrunlen--) {
unsigned lit = *block_data++;
- lzx_add_bits(os, codes->codewords.main[lit], codes->lens.main[lit]);
+ lzx_add_bits(os, codes->codewords.main[lit],
+ codes->lens.main[lit]);
if (litrunlen--) {
unsigned lit = *block_data++;
- lzx_add_bits(os, codes->codewords.main[lit], codes->lens.main[lit]);
+ lzx_add_bits(os, codes->codewords.main[lit],
+ codes->lens.main[lit]);
lzx_flush_bits(os, 3 * MAIN_CODEWORD_LIMIT);
} else {
lzx_flush_bits(os, 2 * MAIN_CODEWORD_LIMIT);
/* 32-bit: write 1 literal at a time. */
do {
unsigned lit = *block_data++;
- lzx_add_bits(os, codes->codewords.main[lit], codes->lens.main[lit]);
+ lzx_add_bits(os, codes->codewords.main[lit],
+ codes->lens.main[lit]);
lzx_flush_bits(os, MAIN_CODEWORD_LIMIT);
} while (--litrunlen);
}
/* If needed, output the length symbol for the match. */
if (adjusted_length >= LZX_NUM_PRIMARY_LENS) {
- lzx_add_bits(os, codes->codewords.len[adjusted_length - LZX_NUM_PRIMARY_LENS],
- codes->lens.len[adjusted_length - LZX_NUM_PRIMARY_LENS]);
+ lzx_add_bits(os, codes->codewords.len[adjusted_length -
+ LZX_NUM_PRIMARY_LENS],
+ codes->lens.len[adjusted_length -
+ LZX_NUM_PRIMARY_LENS]);
if (!CAN_BUFFER(MAX_MATCH_BITS))
lzx_flush_bits(os, LENGTH_CODEWORD_LIMIT);
}
if (!CAN_BUFFER(MAX_MATCH_BITS))
lzx_flush_bits(os, 14);
- lzx_add_bits(os, codes->codewords.aligned[adjusted_offset & LZX_ALIGNED_OFFSET_BITMASK],
- codes->lens.aligned[adjusted_offset & LZX_ALIGNED_OFFSET_BITMASK]);
+ lzx_add_bits(os, codes->codewords.aligned[adjusted_offset &
+ LZX_ALIGNED_OFFSET_BITMASK],
+ codes->lens.aligned[adjusted_offset &
+ LZX_ALIGNED_OFFSET_BITMASK]);
if (!CAN_BUFFER(MAX_MATCH_BITS))
lzx_flush_bits(os, ALIGNED_CODEWORD_LIMIT);
} else {
+ STATIC_ASSERT(CAN_BUFFER(17));
+
lzx_add_bits(os, extra_bits, num_extra_bits);
if (!CAN_BUFFER(MAX_MATCH_BITS))
lzx_flush_bits(os, 17);
last_seq->adjusted_offset_and_match_hdr = 0x80000000;
}
+/******************************************************************************/
+
+/*
+ * Block splitting algorithm. The problem is to decide when it is worthwhile to
+ * start a new block with new entropy codes. There is a theoretically optimal
+ * solution: recursively consider every possible block split, considering the
+ * exact cost of each block, and choose the minimum cost approach. But this is
+ * far too slow. Instead, as an approximation, we can count symbols and after
+ * every N symbols, compare the expected distribution of symbols based on the
+ * previous data with the actual distribution. If they differ "by enough", then
+ * start a new block.
+ *
+ * As an optimization and heuristic, we don't distinguish between every symbol
+ * but rather we combine many symbols into a single "observation type". For
+ * literals we only look at the high bits and low bits, and for matches we only
+ * look at whether the match is long or not. The assumption is that for typical
+ * "real" data, places that are good block boundaries will tend to be noticable
+ * based only on changes in these aggregate frequencies, without looking for
+ * subtle differences in individual symbols. For example, a change from ASCII
+ * bytes to non-ASCII bytes, or from few matches (generally less compressible)
+ * to many matches (generally more compressible), would be easily noticed based
+ * on the aggregates.
+ *
+ * For determining whether the frequency distributions are "different enough" to
+ * start a new block, the simply heuristic of splitting when the sum of absolute
+ * differences exceeds a constant seems to be good enough. We also add a number
+ * proportional to the block size so that the algorithm is more likely to end
+ * large blocks than small blocks. This reflects the general expectation that
+ * it will become increasingly beneficial to start a new block as the current
+ * blocks grows larger.
+ *
+ * Finally, for an approximation, it is not strictly necessary that the exact
+ * symbols being used are considered. With "near-optimal parsing", for example,
+ * the actual symbols that will be used are unknown until after the block
+ * boundary is chosen and the block has been optimized. Since the final choices
+ * cannot be used, we can use preliminary "greedy" choices instead.
+ */
+
+/* Initialize the block split statistics when starting a new block. */
+static void
+init_block_split_stats(struct block_split_stats *stats)
+{
+ for (int i = 0; i < NUM_OBSERVATION_TYPES; i++) {
+ stats->new_observations[i] = 0;
+ stats->observations[i] = 0;
+ }
+ stats->num_new_observations = 0;
+ stats->num_observations = 0;
+}
+
+/* Literal observation. Heuristic: use the top 2 bits and low 1 bits of the
+ * literal, for 8 possible literal observation types. */
+static inline void
+observe_literal(struct block_split_stats *stats, u8 lit)
+{
+ stats->new_observations[((lit >> 5) & 0x6) | (lit & 1)]++;
+ stats->num_new_observations++;
+}
+
+/* Match observation. Heuristic: use one observation type for "short match" and
+ * one observation type for "long match". */
+static inline void
+observe_match(struct block_split_stats *stats, unsigned length)
+{
+ stats->new_observations[NUM_LITERAL_OBSERVATION_TYPES + (length >= 9)]++;
+ stats->num_new_observations++;
+}
+
+static bool
+do_end_block_check(struct block_split_stats *stats, u32 block_size)
+{
+ if (stats->num_observations > 0) {
+
+ /* Note: to avoid slow divisions, we do not divide by
+ * 'num_observations', but rather do all math with the numbers
+ * multiplied by 'num_observations'. */
+ u32 total_delta = 0;
+ for (int i = 0; i < NUM_OBSERVATION_TYPES; i++) {
+ u32 expected = stats->observations[i] * stats->num_new_observations;
+ u32 actual = stats->new_observations[i] * stats->num_observations;
+ u32 delta = (actual > expected) ? actual - expected :
+ expected - actual;
+ total_delta += delta;
+ }
+
+ /* Ready to end the block? */
+ if (total_delta + (block_size >> 12) * stats->num_observations >=
+ 200 * stats->num_observations)
+ return true;
+ }
+
+ for (int i = 0; i < NUM_OBSERVATION_TYPES; i++) {
+ stats->num_observations += stats->new_observations[i];
+ stats->observations[i] += stats->new_observations[i];
+ stats->new_observations[i] = 0;
+ }
+ stats->num_new_observations = 0;
+ return false;
+}
+
+static inline bool
+should_end_block(struct block_split_stats *stats,
+ const u8 *in_block_begin, const u8 *in_next, const u8 *in_end)
+{
+ /* Ready to check block split statistics? */
+ if (stats->num_new_observations < 250 ||
+ in_next - in_block_begin < MIN_BLOCK_SIZE ||
+ in_end - in_next < MIN_BLOCK_SIZE)
+ return false;
+
+ return do_end_block_check(stats, in_next - in_block_begin);
+}
+
+/******************************************************************************/
+
/*
* Given the minimum-cost path computed through the item graph for the current
* block, walk the path and count how many of each symbol in each Huffman-coded
static void
lzx_compute_match_costs(struct lzx_compressor *c)
{
- unsigned num_offset_slots = (c->num_main_syms - LZX_NUM_CHARS) / LZX_NUM_LEN_HEADERS;
+ unsigned num_offset_slots = (c->num_main_syms - LZX_NUM_CHARS) /
+ LZX_NUM_LEN_HEADERS;
struct lzx_costs *costs = &c->costs;
for (unsigned offset_slot = 0; offset_slot < num_offset_slots; offset_slot++) {
u32 extra_cost = (u32)lzx_extra_offset_bits[offset_slot] * LZX_BIT_COST;
- unsigned main_symbol = LZX_NUM_CHARS + (offset_slot * LZX_NUM_LEN_HEADERS);
+ unsigned main_symbol = LZX_NUM_CHARS + (offset_slot *
+ LZX_NUM_LEN_HEADERS);
unsigned i;
#if LZX_CONSIDER_ALIGNED_COSTS
do {
/* Starting a new block */
const u8 * const in_block_begin = in_next;
- const u8 * const in_block_end =
- in_next + min(LZX_DIV_BLOCK_SIZE, in_end - in_next);
+ const u8 * const in_max_block_end =
+ in_next + min(SOFT_MAX_BLOCK_SIZE, in_end - in_next);
+ const u8 *next_observation = in_next;
+
+ init_block_split_stats(&c->split_stats);
/* Run the block through the matchfinder and cache the matches. */
struct lz_match *cache_ptr = c->match_cache;
if (unlikely(max_len > in_end - in_next)) {
max_len = in_end - in_next;
nice_len = min(max_len, nice_len);
- if (unlikely(max_len < 5)) {
+ if (unlikely(max_len <
+ BT_MATCHFINDER_REQUIRED_NBYTES))
+ {
in_next++;
cache_ptr->length = 0;
cache_ptr++;
}
/* Check for matches. */
- lz_matchptr = CALL_BT_MF(is_16_bit, c, bt_matchfinder_get_matches,
+ lz_matchptr = CALL_BT_MF(is_16_bit, c,
+ bt_matchfinder_get_matches,
in_begin,
in_next - in_begin,
max_len,
next_hashes,
&best_len,
cache_ptr + 1);
+
+ if (in_next >= next_observation) {
+ best_len = 0;
+ if (lz_matchptr > cache_ptr + 1)
+ best_len = (lz_matchptr - 1)->length;
+ if (best_len >= 2) {
+ observe_match(&c->split_stats, best_len);
+ next_observation = in_next + best_len;
+ } else {
+ observe_literal(&c->split_stats, *in_next);
+ next_observation = in_next + 1;
+ }
+ }
+
in_next++;
cache_ptr->length = lz_matchptr - (cache_ptr + 1);
cache_ptr = lz_matchptr;
if (unlikely(max_len > in_end - in_next)) {
max_len = in_end - in_next;
nice_len = min(max_len, nice_len);
- if (unlikely(max_len < 5)) {
+ if (unlikely(max_len <
+ BT_MATCHFINDER_REQUIRED_NBYTES))
+ {
in_next++;
cache_ptr->length = 0;
cache_ptr++;
continue;
}
}
- CALL_BT_MF(is_16_bit, c, bt_matchfinder_skip_position,
+ CALL_BT_MF(is_16_bit, c,
+ bt_matchfinder_skip_position,
in_begin,
in_next - in_begin,
max_len,
cache_ptr++;
} while (--best_len);
}
- } while (in_next < in_block_end &&
- likely(cache_ptr < &c->match_cache[LZX_CACHE_LENGTH]));
+ } while (in_next < in_max_block_end &&
+ likely(cache_ptr < &c->match_cache[LZX_CACHE_LENGTH]) &&
+ !should_end_block(&c->split_stats, in_block_begin, in_next, in_end));
/* We've finished running the block through the matchfinder.
* Now choose a match/literal sequence and write the block. */
/* Starting a new block */
const u8 * const in_block_begin = in_next;
- const u8 * const in_block_end =
- in_next + min(LZX_DIV_BLOCK_SIZE, in_end - in_next);
+ const u8 * const in_max_block_end =
+ in_next + min(SOFT_MAX_BLOCK_SIZE, in_end - in_next);
struct lzx_sequence *next_seq = c->chosen_sequences;
unsigned cur_len;
u32 cur_offset;
u32 litrunlen = 0;
lzx_reset_symbol_frequencies(c);
+ init_block_split_stats(&c->split_stats);
do {
if (unlikely(max_len > in_end - in_next)) {
/* Find the longest match at the current position. */
- cur_len = CALL_HC_MF(is_16_bit, c, hc_matchfinder_longest_match,
+ cur_len = CALL_HC_MF(is_16_bit, c,
+ hc_matchfinder_longest_match,
in_begin,
in_next - in_begin,
2,
{
/* There was no match found, or the only match found
* was a distant length 3 match. Output a literal. */
- lzx_record_literal(c, *in_next++, &litrunlen);
+ lzx_record_literal(c, *in_next, &litrunlen);
+ observe_literal(&c->split_stats, *in_next);
+ in_next++;
continue;
}
+ observe_match(&c->split_stats, cur_len);
+
if (cur_offset == recent_offsets[0]) {
in_next++;
cur_offset_data = 0;
nice_len = min(max_len, nice_len);
}
- next_len = CALL_HC_MF(is_16_bit, c, hc_matchfinder_longest_match,
+ next_len = CALL_HC_MF(is_16_bit, c,
+ hc_matchfinder_longest_match,
in_begin,
in_next - in_begin,
cur_len - 2,
lzx_record_match(c, cur_len, cur_offset_data,
recent_offsets, is_16_bit,
&litrunlen, &next_seq);
- in_next = CALL_HC_MF(is_16_bit, c, hc_matchfinder_skip_positions,
+ in_next = CALL_HC_MF(is_16_bit, c,
+ hc_matchfinder_skip_positions,
in_begin,
in_next - in_begin,
in_end - in_begin,
skip_len,
next_hashes);
- } while (in_next < in_block_end);
+ } while (in_next < in_max_block_end &&
+ !should_end_block(&c->split_stats, in_block_begin, in_next, in_end));
lzx_finish_sequence(next_seq, litrunlen);
git://wimlib.net / wimlib / blobdiff