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lib/deflate_decompress: store decode results pre-shifted

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This slightly speeds up decode table building, since now the decode
results don't need to be shifted at runtime when building the tables.
This commit is contained in:
Eric Biggers 2018-12-25 14:16:38 -06:00
parent eed4829c16
commit 1c3609da7b
1 changed files with 20 additions and 18 deletions
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38
lib/deflate_decompress.c
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@ -364,19 +364,28 @@ do { \
/* Shift to extract the decode result from a decode table entry. */
#define HUFFDEC_RESULT_SHIFT 8
/* Shift a decode result into its position in the decode table entry. */
#define HUFFDEC_RESULT_ENTRY(result) ((u32)(result) << HUFFDEC_RESULT_SHIFT)
/* The decode result for each precode symbol. There is no special optimization
* for the precode; the decode result is simply the symbol value. */
static const u32 precode_decode_results[DEFLATE_NUM_PRECODE_SYMS] = {
0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18,
#define ENTRY(presym) HUFFDEC_RESULT_ENTRY(presym)
ENTRY(0) , ENTRY(1) , ENTRY(2) , ENTRY(3) ,
ENTRY(4) , ENTRY(5) , ENTRY(6) , ENTRY(7) ,
ENTRY(8) , ENTRY(9) , ENTRY(10) , ENTRY(11) ,
ENTRY(12) , ENTRY(13) , ENTRY(14) , ENTRY(15) ,
ENTRY(16) , ENTRY(17) , ENTRY(18) ,
#undef ENTRY
};
/* The decode result for each litlen symbol. For literals, this is the literal
* value itself and the HUFFDEC_LITERAL flag. For lengths, this is the length
* base and the number of extra length bits. */
static const u32 litlen_decode_results[DEFLATE_NUM_LITLEN_SYMS] = {
#define ENTRY(literal) ((HUFFDEC_LITERAL >> HUFFDEC_RESULT_SHIFT) | (literal))
/* Literals */
#define ENTRY(literal) (HUFFDEC_LITERAL | HUFFDEC_RESULT_ENTRY(literal))
ENTRY(0) , ENTRY(1) , ENTRY(2) , ENTRY(3) ,
ENTRY(4) , ENTRY(5) , ENTRY(6) , ENTRY(7) ,
ENTRY(8) , ENTRY(9) , ENTRY(10) , ENTRY(11) ,
@ -447,8 +456,8 @@ static const u32 litlen_decode_results[DEFLATE_NUM_LITLEN_SYMS] = {
#define HUFFDEC_LENGTH_BASE_SHIFT 8
#define HUFFDEC_END_OF_BLOCK_LENGTH 0
#define ENTRY(length_base, num_extra_bits) \
(((u32)(length_base) << HUFFDEC_LENGTH_BASE_SHIFT) | (num_extra_bits))
#define ENTRY(length_base, num_extra_bits) HUFFDEC_RESULT_ENTRY( \
((u32)(length_base) << HUFFDEC_LENGTH_BASE_SHIFT) | (num_extra_bits))
/* End of block */
ENTRY(HUFFDEC_END_OF_BLOCK_LENGTH, 0),
@ -472,8 +481,9 @@ static const u32 offset_decode_results[DEFLATE_NUM_OFFSET_SYMS] = {
#define HUFFDEC_EXTRA_OFFSET_BITS_SHIFT 16
#define HUFFDEC_OFFSET_BASE_MASK (((u32)1 << HUFFDEC_EXTRA_OFFSET_BITS_SHIFT) - 1)
#define ENTRY(offset_base, num_extra_bits) \
((offset_base) | ((u32)(num_extra_bits) << HUFFDEC_EXTRA_OFFSET_BITS_SHIFT))
#define ENTRY(offset_base, num_extra_bits) HUFFDEC_RESULT_ENTRY( \
((u32)(num_extra_bits) << HUFFDEC_EXTRA_OFFSET_BITS_SHIFT) | \
(offset_base))
ENTRY(1 , 0) , ENTRY(2 , 0) , ENTRY(3 , 0) , ENTRY(4 , 0) ,
ENTRY(5 , 1) , ENTRY(7 , 1) , ENTRY(9 , 2) , ENTRY(13 , 2) ,
ENTRY(17 , 3) , ENTRY(25 , 3) , ENTRY(33 , 4) , ENTRY(49 , 4) ,
@ -485,13 +495,6 @@ static const u32 offset_decode_results[DEFLATE_NUM_OFFSET_SYMS] = {
#undef ENTRY
};
/* Construct a decode table entry from a decode result and codeword length. */
static forceinline u32
make_decode_table_entry(u32 result, u32 length)
{
return (result << HUFFDEC_RESULT_SHIFT) | length;
}
/*
* Build a table for fast decoding of symbols from a Huffman code. As input,
* this function takes the codeword length of each symbol which may be used in
@ -596,7 +599,7 @@ build_decode_table(u32 decode_table[],
* decompressing a well-formed stream, these default values will
* never be used. But since a malformed stream might contain
* any bits at all, these entries need to be set anyway. */
u32 entry = make_decode_table_entry(decode_results[0], 1);
u32 entry = decode_results[0] | 1;
for (sym = 0; sym < (1U << table_bits); sym++)
decode_table[sym] = entry;
@ -679,8 +682,8 @@ build_decode_table(u32 decode_table[],
* number of entries it contains). */
decode_table[cur_codeword_prefix] =
HUFFDEC_SUBTABLE_POINTER |
make_decode_table_entry(cur_table_start,
cur_table_bits);
HUFFDEC_RESULT_ENTRY(cur_table_start) |
cur_table_bits;
/* Now that we're filling a subtable, we need to drop
* the first 'table_bits' bits of the codewords. */
@ -690,8 +693,7 @@ build_decode_table(u32 decode_table[],
/* Create the decode table entry, which packs the decode result
* and the codeword length (minus 'table_bits' for subtables)
* together. */
entry = make_decode_table_entry(decode_results[sym],
codeword_len - num_dropped_bits);
entry = decode_results[sym] | (codeword_len - num_dropped_bits);
/* Fill in as many copies of the decode table entry as are
* needed. The number of entries to fill is a power of 2 and