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speed-up trivial one-symbol decoding case for lossless

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We now consider 3 special cases:
 * htree-group has only 1 code (no bit is read from bitstream)
 * htree-group has few enough literal symbols, so that all the bit
   codes can fit into a look-up table of less than 64 entries
 * htree-group has a trivial arb literal (not GREEN!), like before

No overall speed change.

Change-Id: I6077fa0b7e5c31a6c67aa8aca859c22cc50ee254
This commit is contained in:
Pascal Massimino 2015-11-13 19:41:28 +01:00
parent 397863bd66
commit 21735e06f7
2 changed files with 101 additions and 3 deletions
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81
src/dec/vp8l.c
View File

@ -195,6 +195,55 @@ static WEBP_INLINE int ReadSymbol(const HuffmanCode* table,
return table->value;
}
// Reads packed symbol depending on GREEN channel
#define BITS_SPECIAL_MARKER 0x100 // something large enough (and a bit-mask)
#define PACKED_NON_LITERAL_CODE 0 // must be < NUM_LITERAL_CODES
static WEBP_INLINE int ReadPackedSymbols(const HTreeGroup* group,
VP8LBitReader* const br,
uint32_t* const dst) {
const uint32_t val = VP8LPrefetchBits(br) & (HUFFMAN_PACKED_TABLE_SIZE - 1);
const HuffmanCode32 code = group->packed_table[val];
assert(group->use_packed_table);
if (code.bits < BITS_SPECIAL_MARKER) {
VP8LSetBitPos(br, br->bit_pos_ + code.bits);
*dst = code.value;
return PACKED_NON_LITERAL_CODE;
} else {
VP8LSetBitPos(br, br->bit_pos_ + code.bits - BITS_SPECIAL_MARKER);
assert(code.value >= NUM_LITERAL_CODES);
return code.value;
}
}
static int AccumulateHCode(HuffmanCode hcode, int shift,
HuffmanCode32* const huff) {
huff->bits += hcode.bits;
huff->value |= (uint32_t)hcode.value << shift;
assert(huff->bits <= HUFFMAN_TABLE_BITS);
return hcode.bits;
}
static void BuildPackedTable(HTreeGroup* const htree_group) {
uint32_t code;
for (code = 0; code < HUFFMAN_PACKED_TABLE_SIZE; ++code) {
uint32_t bits = code;
HuffmanCode32* const huff = &htree_group->packed_table[bits];
HuffmanCode hcode = htree_group->htrees[GREEN][bits];
if (hcode.value >= NUM_LITERAL_CODES) {
huff->bits = hcode.bits + BITS_SPECIAL_MARKER;
huff->value = hcode.value;
} else {
huff->bits = 0;
huff->value = 0;
bits >>= AccumulateHCode(hcode, 8, huff);
bits >>= AccumulateHCode(htree_group->htrees[RED][bits], 16, huff);
bits >>= AccumulateHCode(htree_group->htrees[BLUE][bits], 0, huff);
bits >>= AccumulateHCode(htree_group->htrees[ALPHA][bits], 24, huff);
(void)bits;
}
}
}
static int ReadHuffmanCodeLengths(
VP8LDecoder* const dec, const int* const code_length_code_lengths,
int num_symbols, int* const code_lengths) {
@ -369,7 +418,9 @@ static int ReadHuffmanCodes(VP8LDecoder* const dec, int xsize, int ysize,
HTreeGroup* const htree_group = &htree_groups[i];
HuffmanCode** const htrees = htree_group->htrees;
int size;
int total_size = 0;
int is_trivial_literal = 1;
int max_bits = 0;
for (j = 0; j < HUFFMAN_CODES_PER_META_CODE; ++j) {
int alphabet_size = kAlphabetSize[j];
htrees[j] = next;
@ -380,20 +431,39 @@ static int ReadHuffmanCodes(VP8LDecoder* const dec, int xsize, int ysize,
if (is_trivial_literal && kLiteralMap[j] == 1) {
is_trivial_literal = (next->bits == 0);
}
total_size += next->bits;
next += size;
if (size == 0) {
goto Error;
}
if (j <= ALPHA) {
int local_max_bits = code_lengths[0];
int k;
for (k = 1; k < alphabet_size; ++k) {
if (code_lengths[k] > local_max_bits) {
local_max_bits = code_lengths[k];
}
}
max_bits += local_max_bits;
}
}
htree_group->is_trivial_literal = is_trivial_literal;
htree_group->is_trivial_code = 0;
if (is_trivial_literal) {
const int red = htrees[RED][0].value;
const int blue = htrees[BLUE][0].value;
const int alpha = htrees[ALPHA][0].value;
htree_group->literal_arb =
((uint32_t)alpha << 24) | (red << 16) | blue;
if (total_size == 0 && htrees[GREEN][0].value < NUM_LITERAL_CODES) {
htree_group->is_trivial_code = 1;
htree_group->literal_arb |= htrees[GREEN][0].value << 8;
}
}
htree_group->use_packed_table = !htree_group->is_trivial_code &&
(max_bits < HUFFMAN_PACKED_BITS);
if (htree_group->use_packed_table) BuildPackedTable(htree_group);
}
WebPSafeFree(code_lengths);
// All OK. Finalize pointers and return.
@ -979,11 +1049,18 @@ static int DecodeImageData(VP8LDecoder* const dec, uint32_t* const data,
// Only update when changing tile. Note we could use this test:
// if "((((prev_col ^ col) | prev_row ^ row)) > mask)" -> tile changed
// but that's actually slower and needs storing the previous col/row.
if ((col & mask) == 0) {
htree_group = GetHtreeGroupForPos(hdr, col, row);
if ((col & mask) == 0) htree_group = GetHtreeGroupForPos(hdr, col, row);
if (htree_group->is_trivial_code) {
*src = htree_group->literal_arb;
goto AdvanceByOne;
}
VP8LFillBitWindow(br);
if (htree_group->use_packed_table) {
code = ReadPackedSymbols(htree_group, br, src);
if (code == PACKED_NON_LITERAL_CODE) goto AdvanceByOne;
} else {
code = ReadSymbol(htree_group->htrees[GREEN], br);
}
if (br->eos_) break; // early out
if (code < NUM_LITERAL_CODES) { // Literal
if (htree_group->is_trivial_literal) {

21
src/utils/huffman.h
View File

@ -35,7 +35,24 @@ typedef struct {
uint16_t value; // symbol value or table offset
} HuffmanCode;
// long version for holding 32b values
typedef struct {
int bits; // number of bits used for this symbol,
// or an impossible value if not a literal code.
uint32_t value; // 32b packed ARGB value if literal,
// or non-literal symbol otherwise
} HuffmanCode32;
#define HUFFMAN_PACKED_BITS 6
#define HUFFMAN_PACKED_TABLE_SIZE (1u << HUFFMAN_PACKED_BITS)
// Huffman table group.
// Includes special handling for the following cases:
// - is_trivial_literal: one common literal base for RED/BLUE/ALPHA (not GREEN)
// - is_trivial_code: only 1 code (no bit is read from bitstream)
// - use_packed_table: few enough literal symbols, so all the bit codes
// can fit into a small look-up table packed_table[]
// The common literal base, if applicable, is stored in 'literal_arb'.
typedef struct HTreeGroup HTreeGroup;
struct HTreeGroup {
HuffmanCode* htrees[HUFFMAN_CODES_PER_META_CODE];
@ -44,6 +61,10 @@ struct HTreeGroup {
uint32_t literal_arb; // If is_trivial_literal is true, this is the
// ARGB value of the pixel, with Green channel
// being set to zero.
int is_trivial_code; // true if is_trivial_literal with only one code
int use_packed_table; // use packed table below for short literal code
// table mapping input bits to a packed values, or escape case to literal code
HuffmanCode32 packed_table[HUFFMAN_PACKED_TABLE_SIZE];
};
// Creates the instance of HTreeGroup with specified number of tree-groups.