mirror of https://git.tukaani.org/xz.git
liblzma: LZMA decoder improvements.
This adds macros for bittree decoding which prepares the code for alternative C versions and inline assembly.
This commit is contained in:
Lasse Collin
parent
de5c5e4176
commit
e0c0ee475c
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@ -24,9 +24,8 @@
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// Minimum number of input bytes to safely decode one LZMA symbol.
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// The worst case is that we decode 22 bits using probabilities and 26
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// direct bits. This may decode at maximum 20 bytes of input plus one
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// extra byte after the final EOPM normalization.
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#define LZMA_IN_REQUIRED 21
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// direct bits. This may decode at maximum 20 bytes of input.
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#define LZMA_IN_REQUIRED 20
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// Macros for (somewhat) size-optimized code.
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@ -73,32 +72,22 @@ do { \
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symbol = 1; \
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rc_if_0(ld.choice) { \
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rc_update_0(ld.choice); \
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rc_bit(ld.low[pos_state][symbol], , ); \
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rc_bit(ld.low[pos_state][symbol], , ); \
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rc_bit(ld.low[pos_state][symbol], , ); \
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target = symbol - LEN_LOW_SYMBOLS + MATCH_LEN_MIN; \
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rc_bittree3(ld.low[pos_state], \
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-LEN_LOW_SYMBOLS + MATCH_LEN_MIN); \
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target = symbol; \
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} else { \
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rc_update_1(ld.choice); \
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rc_if_0(ld.choice2) { \
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rc_update_0(ld.choice2); \
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rc_bit(ld.mid[pos_state][symbol], , ); \
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rc_bit(ld.mid[pos_state][symbol], , ); \
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rc_bit(ld.mid[pos_state][symbol], , ); \
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target = symbol - LEN_MID_SYMBOLS \
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+ MATCH_LEN_MIN + LEN_LOW_SYMBOLS; \
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rc_bittree3(ld.mid[pos_state], -LEN_MID_SYMBOLS \
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+ MATCH_LEN_MIN + LEN_LOW_SYMBOLS); \
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target = symbol; \
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} else { \
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rc_update_1(ld.choice2); \
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rc_bit(ld.high[symbol], , ); \
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rc_bit(ld.high[symbol], , ); \
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rc_bit(ld.high[symbol], , ); \
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rc_bit(ld.high[symbol], , ); \
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rc_bit(ld.high[symbol], , ); \
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rc_bit(ld.high[symbol], , ); \
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rc_bit(ld.high[symbol], , ); \
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rc_bit(ld.high[symbol], , ); \
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target = symbol - LEN_HIGH_SYMBOLS \
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rc_bittree8(ld.high, -LEN_HIGH_SYMBOLS \
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+ MATCH_LEN_MIN \
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+ LEN_LOW_SYMBOLS + LEN_MID_SYMBOLS; \
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+ LEN_LOW_SYMBOLS + LEN_MID_SYMBOLS); \
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target = symbol; \
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} \
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} \
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} while (0)
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@ -369,8 +358,8 @@ lzma_decode(void *coder_ptr, lzma_dict *restrict dictptr,
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// If there is not enough room for another LZMA symbol
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// go to Resumable mode.
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if (rc_in_pos + LZMA_IN_REQUIRED > in_size
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|| dict.pos == dict.limit)
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if (unlikely(rc_in_end - rc_in_ptr < LZMA_IN_REQUIRED
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|| dict.pos == dict.limit))
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goto slow;
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// Decode the first bit from the next LZMA symbol.
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@ -390,64 +379,14 @@ lzma_decode(void *coder_ptr, lzma_dict *restrict dictptr,
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probs = literal_subcoder(coder->literal,
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literal_context_bits, literal_pos_mask,
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dict.pos, dict_get(&dict, 0));
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symbol = 1;
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if (is_literal_state(state)) {
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// Decode literal without match byte.
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// We need to decode 8 bits, so instead
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// of looping from 1 - 8, we unroll the
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// loop for a speed optimization.
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rc_bit(probs[symbol], , );
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rc_bit(probs[symbol], , );
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rc_bit(probs[symbol], , );
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rc_bit(probs[symbol], , );
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rc_bit(probs[symbol], , );
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rc_bit(probs[symbol], , );
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rc_bit(probs[symbol], , );
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rc_bit(probs[symbol], , );
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rc_bittree8(probs, 0);
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} else {
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// Decode literal with match byte.
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//
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// We store the byte we compare against
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// ("match byte") to "len" to minimize the
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// number of variables we need to store
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// between decoder calls.
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len = (uint32_t)(dict_get(&dict, rep0)) << 1;
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// The usage of "offset" allows omitting some
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// branches, which should give tiny speed
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// improvement on some CPUs. "offset" gets
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// set to zero if match_bit didn't match.
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offset = 0x100;
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// Unroll the loop.
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uint32_t match_bit;
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uint32_t subcoder_index;
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# define decode_with_match_bit \
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match_bit = len & offset; \
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subcoder_index = offset + match_bit + symbol; \
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rc_bit(probs[subcoder_index], \
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offset &= ~match_bit, \
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offset &= match_bit)
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decode_with_match_bit;
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len <<= 1;
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decode_with_match_bit;
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len <<= 1;
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decode_with_match_bit;
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len <<= 1;
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decode_with_match_bit;
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len <<= 1;
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decode_with_match_bit;
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len <<= 1;
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decode_with_match_bit;
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len <<= 1;
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decode_with_match_bit;
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len <<= 1;
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decode_with_match_bit;
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# undef decode_match_bit
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rc_matched_literal(probs,
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dict_get(&dict, rep0));
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}
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state = next_state[state];
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@ -501,18 +440,8 @@ lzma_decode(void *coder_ptr, lzma_dict *restrict dictptr,
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// The next 6 bits determine how to decode the
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// rest of the distance.
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probs = coder->dist_slot[get_dist_state(len)];
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symbol = 1;
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rc_bit(probs[symbol], , );
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rc_bit(probs[symbol], , );
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rc_bit(probs[symbol], , );
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rc_bit(probs[symbol], , );
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rc_bit(probs[symbol], , );
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rc_bit(probs[symbol], , );
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// Get rid of the highest bit that was needed for
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// indexing of the probability array.
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symbol -= DIST_SLOTS;
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rc_bittree6(probs, -DIST_SLOTS);
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assert(symbol <= 63);
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if (symbol < DIST_MODEL_START) {
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@ -540,6 +469,7 @@ lzma_decode(void *coder_ptr, lzma_dict *restrict dictptr,
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assert(limit <= 5);
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rep0 <<= limit;
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assert(rep0 <= 96);
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// -1 is fine, because we start
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// decoding at probs[1], not probs[0].
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// NOTE: This violates the C standard,
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@ -553,106 +483,51 @@ lzma_decode(void *coder_ptr, lzma_dict *restrict dictptr,
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probs = coder->pos_special + rep0
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- symbol - 1;
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symbol = 1;
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offset = 0;
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offset = 1;
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switch (limit) {
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case 5:
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assert(offset == 0);
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rc_bit(probs[symbol], ,
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rep0 += 1U);
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++offset;
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--limit;
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case 4:
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rc_bit(probs[symbol], ,
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rep0 += 1U << offset);
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++offset;
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--limit;
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case 3:
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rc_bit(probs[symbol], ,
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rep0 += 1U << offset);
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++offset;
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--limit;
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case 2:
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rc_bit(probs[symbol], ,
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rep0 += 1U << offset);
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++offset;
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--limit;
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case 1:
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// We need "symbol" only for
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// indexing the probability
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// array, thus we can use
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// rc_bit_last() here to
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// omit the unneeded updating
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// of "symbol".
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rc_bit_last(probs[symbol], ,
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rep0 += 1U << offset);
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}
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// Variable number (1-5) of bits
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// from a reverse bittree. This
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// isn't worth manual unrolling.
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do {
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rc_bit_add_if_1(probs,
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rep0, offset);
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offset <<= 1;
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} while (--limit > 0);
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} else {
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// The distance is >= 128. Decode the
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// lower bits without probabilities
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// except the lowest four bits.
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assert(symbol >= 14);
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assert(limit >= 6);
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limit -= ALIGN_BITS;
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assert(limit >= 2);
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// Not worth manual unrolling
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do {
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rc_direct(rep0);
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} while (--limit > 0);
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rc_direct(rep0, limit);
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// Decode the lowest four bits using
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// probabilities.
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rep0 <<= ALIGN_BITS;
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symbol = 1;
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rc_bittree_rev4(coder->pos_align);
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rep0 += symbol;
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rc_bit(coder->pos_align[symbol], ,
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rep0 += 1);
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rc_bit(coder->pos_align[symbol], ,
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rep0 += 2);
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rc_bit(coder->pos_align[symbol], ,
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rep0 += 4);
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// Like when distance [4, 127], we
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// don't need "symbol" for anything
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// other than indexing the probability
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// array.
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rc_bit_last(
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coder->pos_align[symbol], ,
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rep0 += 8);
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if (rep0 == UINT32_MAX) {
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///////////////////////////
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// End of payload marker //
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///////////////////////////
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// End of payload marker was
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// found. It may only be
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// present if
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// - uncompressed size is
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// unknown or
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// - after known uncompressed
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// size amount of bytes has
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// been decompressed and
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// caller has indicated
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// that EOPM might be used
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// (it's not allowed in
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// LZMA2).
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if (!eopm_is_valid) {
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ret = LZMA_DATA_ERROR;
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goto out;
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}
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// LZMA1 stream with
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// end-of-payload marker.
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rc_normalize();
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ret = rc_is_finished(rc)
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? LZMA_STREAM_END
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: LZMA_DATA_ERROR;
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goto out;
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}
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// If the end of payload marker (EOPM)
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// is detected, jump to the safe code.
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// The EOPM handling isn't speed
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// critical at all.
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//
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// A final normalization is needed
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// after the EOPM (there can be a
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// dummy byte to read in some cases).
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// If the normalization was done here
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// in the fast code, it would need to
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// be taken into account in the value
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// of LZMA_IN_REQUIRED. Using the
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// safe code allows keeping
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// LZMA_IN_REQUIRED as 20 instead of
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// 21.
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if (rep0 == UINT32_MAX)
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goto eopm;
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}
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}
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@ -948,31 +823,48 @@ slow:
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limit -= ALIGN_BITS;
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assert(limit >= 2);
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case SEQ_DIRECT:
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do {
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rc_direct_safe(rep0,
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rc_direct_safe(rep0, limit,
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SEQ_DIRECT);
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} while (--limit > 0);
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rep0 <<= ALIGN_BITS;
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symbol = 1;
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offset = 0;
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symbol = 0;
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offset = 1;
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case SEQ_ALIGN:
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do {
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rc_bit_safe(coder->pos_align[
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symbol], ,
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rep0 += 1U << offset,
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rc_bit_last_safe(
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coder->pos_align[
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offset
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+ symbol],
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,
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symbol += offset,
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SEQ_ALIGN);
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} while (++offset < ALIGN_BITS);
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offset <<= 1;
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} while (offset < ALIGN_SIZE);
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rep0 += symbol;
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// End of payload marker
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if (rep0 == UINT32_MAX) {
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// End of payload marker was
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// found. It may only be
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// present if
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// - uncompressed size is
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// unknown or
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// - after known uncompressed
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// size amount of bytes has
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// been decompressed and
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// caller has indicated
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// that EOPM might be used
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// (it's not allowed in
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// LZMA2).
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eopm:
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if (!eopm_is_valid) {
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ret = LZMA_DATA_ERROR;
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goto out;
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}
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case SEQ_EOPM:
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// LZMA1 stream with
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// end-of-payload marker.
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rc_normalize_safe(SEQ_EOPM);
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ret = rc_is_finished(rc)
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? LZMA_STREAM_END
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@ -68,6 +68,10 @@
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///
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/// I will be sticking to uint16_t unless some specific architectures
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/// are *much* faster (20-50 %) with uint32_t.
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///
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/// Update in 2024: The branchless C and x86-64 assembly was written so that
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/// probability is assumed to be uint16_t. (In contrast, LZMA SDK 23.01
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/// assembly supports both types.)
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typedef uint16_t probability;
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#endif
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@ -16,6 +16,17 @@
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#include "range_common.h"
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// Negative RC_BIT_MODEL_TOTAL but the lowest RC_MOVE_BITS are flipped.
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// This is useful for updating probability variables in branchless decoding:
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//
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// uint32_t decoded_bit = ...;
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// probability tmp = RC_BIT_MODEL_OFFSET;
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// tmp &= decoded_bit - 1;
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// prob -= (prob + tmp) >> RC_MOVE_BITS;
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#define RC_BIT_MODEL_OFFSET \
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((UINT32_C(1) << RC_MOVE_BITS) - 1 - RC_BIT_MODEL_TOTAL)
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typedef struct {
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uint32_t range;
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uint32_t code;
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@ -52,7 +63,8 @@ rc_read_init(lzma_range_decoder *rc, const uint8_t *restrict in,
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/// variables 'in' and 'in_size' to be defined.
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#define rc_to_local(range_decoder, in_pos) \
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lzma_range_decoder rc = range_decoder; \
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size_t rc_in_pos = (in_pos); \
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const uint8_t *rc_in_ptr = in + (in_pos); \
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const uint8_t *rc_in_end = in + in_size; \
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uint32_t rc_bound
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@ -60,7 +72,7 @@ rc_read_init(lzma_range_decoder *rc, const uint8_t *restrict in,
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#define rc_from_local(range_decoder, in_pos) \
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do { \
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range_decoder = rc; \
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in_pos = rc_in_pos; \
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in_pos = (size_t)(rc_in_ptr - in); \
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} while (0)
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@ -85,7 +97,7 @@ do { \
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do { \
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if (rc.range < RC_TOP_VALUE) { \
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rc.range <<= RC_SHIFT_BITS; \
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rc.code = (rc.code << RC_SHIFT_BITS) | in[rc_in_pos++]; \
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rc.code = (rc.code << RC_SHIFT_BITS) | *rc_in_ptr++; \
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} \
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} while (0)
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@ -98,12 +110,12 @@ do { \
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#define rc_normalize_safe(seq) \
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do { \
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if (rc.range < RC_TOP_VALUE) { \
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if (unlikely(rc_in_pos == in_size)) { \
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if (rc_in_ptr == rc_in_end) { \
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coder->sequence = seq; \
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goto out; \
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} \
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rc.range <<= RC_SHIFT_BITS; \
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rc.code = (rc.code << RC_SHIFT_BITS) | in[rc_in_pos++]; \
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rc.code = (rc.code << RC_SHIFT_BITS) | *rc_in_ptr++; \
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} \
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} while (0)
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@ -133,10 +145,14 @@ do { \
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/// Update the range decoder state and the used probability variable to
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/// match a decoded bit of 0.
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///
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/// The x86-64 assemly uses the commented method but it seems that,
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/// at least on x86-64, the first version is slightly faster as C code.
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#define rc_update_0(prob) \
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do { \
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rc.range = rc_bound; \
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prob += (RC_BIT_MODEL_TOTAL - (prob)) >> RC_MOVE_BITS; \
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/* prob -= ((prob) + RC_BIT_MODEL_OFFSET) >> RC_MOVE_BITS; */ \
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} while (0)
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@ -192,19 +208,121 @@ do { \
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symbol = (symbol << 1) + 1; action1, \
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seq);
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/// Decode a bit without using a probability.
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#define rc_direct(dest) \
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// Unroll fixed-sized bittree decoding.
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//
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// A compile-time constant in final_add can be used to get rid of the high bit
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// from symbol that is used for the array indexing (1U << bittree_bits).
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// final_add may also be used to add offset to the result (LZMA length
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// decoder does that).
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//
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// The reason to have final_add here is that in the asm code the addition
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// can be done for free: in x86-64 there is SBB instruction with -1 as
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// the immediate value, and final_add is combined with that value.
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#define rc_bittree_bit(prob) \
|
||||
rc_bit(prob, , )
|
||||
|
||||
#define rc_bittree3(probs, final_add) \
|
||||
do { \
|
||||
symbol = 1; \
|
||||
rc_bittree_bit(probs[symbol]); \
|
||||
rc_bittree_bit(probs[symbol]); \
|
||||
rc_bittree_bit(probs[symbol]); \
|
||||
symbol += (uint32_t)(final_add); \
|
||||
} while (0)
|
||||
|
||||
#define rc_bittree6(probs, final_add) \
|
||||
do { \
|
||||
symbol = 1; \
|
||||
rc_bittree_bit(probs[symbol]); \
|
||||
rc_bittree_bit(probs[symbol]); \
|
||||
rc_bittree_bit(probs[symbol]); \
|
||||
rc_bittree_bit(probs[symbol]); \
|
||||
rc_bittree_bit(probs[symbol]); \
|
||||
rc_bittree_bit(probs[symbol]); \
|
||||
symbol += (uint32_t)(final_add); \
|
||||
} while (0)
|
||||
|
||||
#define rc_bittree8(probs, final_add) \
|
||||
do { \
|
||||
symbol = 1; \
|
||||
rc_bittree_bit(probs[symbol]); \
|
||||
rc_bittree_bit(probs[symbol]); \
|
||||
rc_bittree_bit(probs[symbol]); \
|
||||
rc_bittree_bit(probs[symbol]); \
|
||||
rc_bittree_bit(probs[symbol]); \
|
||||
rc_bittree_bit(probs[symbol]); \
|
||||
rc_bittree_bit(probs[symbol]); \
|
||||
rc_bittree_bit(probs[symbol]); \
|
||||
symbol += (uint32_t)(final_add); \
|
||||
} while (0)
|
||||
|
||||
|
||||
// Fixed-sized reverse bittree
|
||||
#define rc_bittree_rev4(probs) \
|
||||
do { \
|
||||
symbol = 0; \
|
||||
rc_bit_last(probs[symbol + 1], , symbol += 1); \
|
||||
rc_bit_last(probs[symbol + 2], , symbol += 2); \
|
||||
rc_bit_last(probs[symbol + 4], , symbol += 4); \
|
||||
rc_bit_last(probs[symbol + 8], , symbol += 8); \
|
||||
} while (0)
|
||||
|
||||
|
||||
// Decode one bit from variable-sized reverse bittree.
|
||||
// The loop is done in the code that uses this macro.
|
||||
#define rc_bit_add_if_1(probs, dest, value_to_add_if_1) \
|
||||
rc_bit(probs[symbol], \
|
||||
, \
|
||||
dest += value_to_add_if_1);
|
||||
|
||||
|
||||
// Matched literal
|
||||
#define decode_with_match_bit \
|
||||
t_match_byte <<= 1; \
|
||||
t_match_bit = t_match_byte & t_offset; \
|
||||
t_subcoder_index = t_offset + t_match_bit + symbol; \
|
||||
rc_bit(probs[t_subcoder_index], \
|
||||
t_offset &= ~t_match_bit, \
|
||||
t_offset &= t_match_bit)
|
||||
|
||||
#define rc_matched_literal(probs_base_var, match_byte) \
|
||||
do { \
|
||||
uint32_t t_match_byte = (match_byte); \
|
||||
uint32_t t_match_bit; \
|
||||
uint32_t t_subcoder_index; \
|
||||
uint32_t t_offset = 0x100; \
|
||||
symbol = 1; \
|
||||
decode_with_match_bit; \
|
||||
decode_with_match_bit; \
|
||||
decode_with_match_bit; \
|
||||
decode_with_match_bit; \
|
||||
decode_with_match_bit; \
|
||||
decode_with_match_bit; \
|
||||
decode_with_match_bit; \
|
||||
decode_with_match_bit; \
|
||||
} while (0)
|
||||
|
||||
|
||||
/// Decode a bit without using a probability.
|
||||
//
|
||||
// NOTE: GCC 13 and Clang/LLVM 16 can, at least on x86-64, optimize the bound
|
||||
// calculation to use an arithmetic right shift so there's no need to provide
|
||||
// the alternative code which, according to C99/C11/C23 6.3.1.3-p3 isn't
|
||||
// perfectly portable: rc_bound = (uint32_t)((int32_t)rc.code >> 31);
|
||||
#define rc_direct(dest, count_var) \
|
||||
do { \
|
||||
dest = (dest << 1) + 1; \
|
||||
rc_normalize(); \
|
||||
rc.range >>= 1; \
|
||||
rc.code -= rc.range; \
|
||||
rc_bound = UINT32_C(0) - (rc.code >> 31); \
|
||||
dest += rc_bound; \
|
||||
rc.code += rc.range & rc_bound; \
|
||||
dest = (dest << 1) + (rc_bound + 1); \
|
||||
} while (0)
|
||||
} while (--count_var > 0)
|
||||
|
||||
|
||||
#define rc_direct_safe(dest, seq) \
|
||||
|
||||
#define rc_direct_safe(dest, count_var, seq) \
|
||||
do { \
|
||||
rc_normalize_safe(seq); \
|
||||
rc.range >>= 1; \
|
||||
|
|
@ -212,10 +330,6 @@ do { \
|
|||
rc_bound = UINT32_C(0) - (rc.code >> 31); \
|
||||
rc.code += rc.range & rc_bound; \
|
||||
dest = (dest << 1) + (rc_bound + 1); \
|
||||
} while (0)
|
||||
|
||||
|
||||
// NOTE: No macros are provided for bittree decoding. It seems to be simpler
|
||||
// to just write them open in the code.
|
||||
} while (--count_var > 0)
|
||||
|
||||
#endif
|
||||
|
|
|
|||
Loading…
Reference in New Issue