mirror of https://git.tukaani.org/xz.git
liblzma: LZMA decoder: Get rid of next_state[].
It's not completely obvious if this is better in the decoder. It should be good if compiler can avoid creating a branch (like using CMOV on x86). This also makes lzma_encoder.c use the new macros.
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@ -83,6 +83,20 @@ typedef enum {
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? (state) - 3 \
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: (state) - 6))
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/// Like update_literal(state) but when it is already known that
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/// is_literal_state(state) is true.
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#define update_literal_normal(state) \
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state = ((state) <= STATE_SHORTREP_LIT_LIT \
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? STATE_LIT_LIT \
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: (state) - 3);
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/// Like update_literal(state) but when it is already known that
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/// is_literal_state(state) is false.
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#define update_literal_matched(state) \
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state = ((state) <= STATE_LIT_SHORTREP \
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? (state) - 3 \
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: (state) - 6);
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/// Indicate that the latest state was a match.
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#define update_match(state) \
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state = ((state) < LIT_STATES ? STATE_LIT_MATCH : STATE_NONLIT_MATCH)
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@ -307,24 +307,6 @@ lzma_decode(void *coder_ptr, lzma_dict *restrict dictptr,
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might_finish_without_eopm = true;
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}
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// Lookup table used to update the literal state.
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// Compared to other state updates, this would need two branches.
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// The lookup table is used by both Resumable and Non-resumable modes.
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static const lzma_lzma_state next_state[] = {
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STATE_LIT_LIT,
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STATE_LIT_LIT,
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STATE_LIT_LIT,
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STATE_LIT_LIT,
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STATE_MATCH_LIT_LIT,
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STATE_REP_LIT_LIT,
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STATE_SHORTREP_LIT_LIT,
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STATE_MATCH_LIT,
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STATE_REP_LIT,
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STATE_SHORTREP_LIT,
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STATE_MATCH_LIT,
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STATE_REP_LIT
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};
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// The main decoder loop. The "switch" is used to resume the decoder at
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// correct location. Once resumed, the "switch" is no longer used.
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// The decoder loops is split into two modes:
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@ -381,16 +363,18 @@ lzma_decode(void *coder_ptr, lzma_dict *restrict dictptr,
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dict.pos, dict_get(&dict, 0));
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if (is_literal_state(state)) {
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update_literal_normal(state);
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// Decode literal without match byte.
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rc_bittree8(probs, 0);
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} else {
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update_literal_matched(state);
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// Decode literal with match byte.
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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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// Write decoded literal to dictionary
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dict_put(&dict, symbol);
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continue;
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@ -705,6 +689,8 @@ slow:
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symbol = 1;
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if (is_literal_state(state)) {
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update_literal_normal(state);
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// Decode literal without match byte.
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// The "slow" version does not unroll
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// the loop.
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@ -714,6 +700,8 @@ slow:
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SEQ_LITERAL);
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} while (symbol < (1 << 8));
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} else {
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update_literal_matched(state);
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// Decode literal with match byte.
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len = (uint32_t)(dict_get(&dict, rep0)) << 1;
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@ -742,8 +730,6 @@ slow:
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} while (symbol < (1 << 8));
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}
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state = next_state[state];
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case SEQ_LITERAL_WRITE:
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if (dict_put_safe(&dict, symbol)) {
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coder->sequence = SEQ_LITERAL_WRITE;
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@ -54,18 +54,18 @@ literal(lzma_lzma1_encoder *coder, lzma_mf *mf, uint32_t position)
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if (is_literal_state(coder->state)) {
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// Previous LZMA-symbol was a literal. Encode a normal
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// literal without a match byte.
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update_literal_normal(coder->state);
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rc_bittree(&coder->rc, subcoder, 8, cur_byte);
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} else {
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// Previous LZMA-symbol was a match. Use the last byte of
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// the match as a "match byte". That is, compare the bits
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// of the current literal and the match byte.
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update_literal_matched(coder->state);
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const uint8_t match_byte = mf->buffer[
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mf->read_pos - coder->reps[0] - 1
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- mf->read_ahead];
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literal_matched(&coder->rc, subcoder, match_byte, cur_byte);
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}
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update_literal(coder->state);
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}
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