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optimize init_vlc().

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Reduce worst case time from O(N^2) to O(N*log(N)).
Speedup average case by a factor of 10 in ffv2 (total decoding speed +4-25%),
factor of 1.3 in ffvhuff (total +0.5%),
factor of 1.8 in indeo5 (total +1%),
factor of 1.1 in mjpeg (total +0.1%).

Originally committed as revision 22715 to svn://svn.ffmpeg.org/ffmpeg/trunk
This commit is contained in:
Loren Merritt 2010-03-29 02:50:23 +00:00
parent 7ef7ae930c
commit 3224079956
1 changed files with 107 additions and 67 deletions
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174
libavcodec/bitstream.c
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@ -2,6 +2,7 @@
* Common bit i/o utils
* Copyright (c) 2000, 2001 Fabrice Bellard
* Copyright (c) 2002-2004 Michael Niedermayer <michaelni@gmx.at>
* Copyright (c) 2010 Loren Merritt
*
* alternative bitstream reader & writer by Michael Niedermayer <michaelni@gmx.at>
*
@ -116,22 +117,54 @@ static int alloc_table(VLC *vlc, int size, int use_static)
return index;
}
static int build_table(VLC *vlc, int table_nb_bits,
int nb_codes,
const void *bits, int bits_wrap, int bits_size,
const void *codes, int codes_wrap, int codes_size,
const void *symbols, int symbols_wrap, int symbols_size,
uint32_t code_prefix, int n_prefix, int flags)
static av_always_inline uint32_t bitswap_32(uint32_t x) {
return av_reverse[x&0xFF]<<24
| av_reverse[(x>>8)&0xFF]<<16
| av_reverse[(x>>16)&0xFF]<<8
| av_reverse[x>>24];
}
typedef struct {
uint8_t bits;
uint16_t symbol;
/** codeword, with the first bit-to-be-read in the msb
* (even if intended for a little-endian bitstream reader) */
uint32_t code;
} VLCcode;
static int compare_vlcspec(const void *a, const void *b)
{
int i, j, k, n, table_size, table_index, nb, n1, index, code_prefix2, symbol;
const VLCcode *sa=a, *sb=b;
return (sa->code >> 1) - (sb->code >> 1);
}
/**
* Build VLC decoding tables suitable for use with get_vlc().
*
* @param vlc the context to be initted
*
* @param table_nb_bits max length of vlc codes to store directly in this table
* (Longer codes are delegated to subtables.)
*
* @param nb_codes number of elements in codes[]
*
* @param codes descriptions of the vlc codes
* These must be ordered such that codes going into the same subtable are contiguous.
* Sorting by VLCcode.code is sufficient, though not necessary.
*/
static int build_table(VLC *vlc, int table_nb_bits, int nb_codes,
VLCcode *codes, int flags)
{
int table_size, table_index, index, code_prefix, symbol, subtable_bits;
int i, j, k, n, nb, inc;
uint32_t code;
VLC_TYPE (*table)[2];
table_size = 1 << table_nb_bits;
table_index = alloc_table(vlc, table_size, flags & INIT_VLC_USE_NEW_STATIC);
#ifdef DEBUG_VLC
av_log(NULL,AV_LOG_DEBUG,"new table index=%d size=%d code_prefix=%x n=%d\n",
table_index, table_size, code_prefix, n_prefix);
av_log(NULL,AV_LOG_DEBUG,"new table index=%d size=%d\n",
table_index, table_size);
#endif
if (table_index < 0)
return -1;
@ -144,33 +177,22 @@ static int build_table(VLC *vlc, int table_nb_bits,
/* first pass: map codes and compute auxillary table sizes */
for(i=0;i<nb_codes;i++) {
GET_DATA(n, bits, i, bits_wrap, bits_size);
GET_DATA(code, codes, i, codes_wrap, codes_size);
/* we accept tables with holes */
if (n <= 0)
continue;
if (!symbols)
symbol = i;
else
GET_DATA(symbol, symbols, i, symbols_wrap, symbols_size);
n = codes[i].bits;
code = codes[i].code;
symbol = codes[i].symbol;
#if defined(DEBUG_VLC) && 0
av_log(NULL,AV_LOG_DEBUG,"i=%d n=%d code=0x%x\n", i, n, code);
#endif
/* if code matches the prefix, it is in the table */
n -= n_prefix;
if (n > 0) {
if(flags & INIT_VLC_LE)
code_prefix2= code & (n_prefix>=32 ? 0xffffffff : (1 << n_prefix)-1);
else
code_prefix2= code >> n;
if (code_prefix2 == code_prefix) {
if (n <= table_nb_bits) {
/* no need to add another table */
j = (code << (table_nb_bits - n)) & (table_size - 1);
j = code >> (32 - table_nb_bits);
nb = 1 << (table_nb_bits - n);
inc = 1;
if (flags & INIT_VLC_LE) {
j = bitswap_32(code);
inc = 1 << n;
}
for(k=0;k<nb;k++) {
if(flags & INIT_VLC_LE)
j = (code >> n_prefix) + (k<<n);
#ifdef DEBUG_VLC
av_log(NULL, AV_LOG_DEBUG, "%4x: code=%d n=%d\n",
j, i, n);
@ -181,46 +203,41 @@ static int build_table(VLC *vlc, int table_nb_bits,
}
table[j][1] = n; //bits
table[j][0] = symbol;
j++;
j += inc;
}
} else {
/* fill auxiliary table recursively */
n -= table_nb_bits;
j = (code >> ((flags & INIT_VLC_LE) ? n_prefix : n)) & ((1 << table_nb_bits) - 1);
code_prefix = code >> (32 - table_nb_bits);
subtable_bits = n;
codes[i].bits = n;
codes[i].code = code << table_nb_bits;
for (k = i+1; k < nb_codes; k++) {
n = codes[k].bits - table_nb_bits;
if (n <= 0)
break;
code = codes[k].code;
if (code >> (32 - table_nb_bits) != code_prefix)
break;
codes[k].bits = n;
codes[k].code = code << table_nb_bits;
subtable_bits = FFMAX(subtable_bits, n);
}
subtable_bits = FFMIN(subtable_bits, table_nb_bits);
j = (flags & INIT_VLC_LE) ? bitswap_32(code_prefix) >> (32 - table_nb_bits) : code_prefix;
table[j][1] = -subtable_bits;
#ifdef DEBUG_VLC
av_log(NULL,AV_LOG_DEBUG,"%4x: n=%d (subtable)\n",
j, n);
j, codes[i].bits + table_nb_bits);
#endif
/* compute table size */
n1 = -table[j][1]; //bits
if (n > n1)
n1 = n;
table[j][1] = -n1; //bits
index = build_table(vlc, subtable_bits, k-i, codes+i, flags);
if (index < 0)
return -1;
/* note: realloc has been done, so reload tables */
table = &vlc->table[table_index];
table[j][0] = index; //code
i = k-1;
}
}
}
}
/* second pass : fill auxillary tables recursively */
for(i=0;i<table_size;i++) {
n = table[i][1]; //bits
if (n < 0) {
n = -n;
if (n > table_nb_bits) {
n = table_nb_bits;
table[i][1] = -n; //bits
}
index = build_table(vlc, n, nb_codes,
bits, bits_wrap, bits_size,
codes, codes_wrap, codes_size,
symbols, symbols_wrap, symbols_size,
(flags & INIT_VLC_LE) ? (code_prefix | (i << n_prefix)) : ((code_prefix << table_nb_bits) | i),
n_prefix + table_nb_bits, flags);
if (index < 0)
return -1;
/* note: realloc has been done, so reload tables */
table = &vlc->table[table_index];
table[i][0] = index; //code
}
}
return table_index;
}
@ -258,6 +275,9 @@ int init_vlc_sparse(VLC *vlc, int nb_bits, int nb_codes,
const void *symbols, int symbols_wrap, int symbols_size,
int flags)
{
VLCcode buf[nb_codes];
int i, j;
vlc->bits = nb_bits;
if(flags & INIT_VLC_USE_NEW_STATIC){
if(vlc->table_size && vlc->table_size == vlc->table_allocated){
@ -275,11 +295,31 @@ int init_vlc_sparse(VLC *vlc, int nb_bits, int nb_codes,
av_log(NULL,AV_LOG_DEBUG,"build table nb_codes=%d\n", nb_codes);
#endif
if (build_table(vlc, nb_bits, nb_codes,
bits, bits_wrap, bits_size,
codes, codes_wrap, codes_size,
symbols, symbols_wrap, symbols_size,
0, 0, flags) < 0) {
assert(symbols_size <= 2 || !symbols);
j = 0;
#define COPY(condition)\
for (i = 0; i < nb_codes; i++) {\
GET_DATA(buf[j].bits, bits, i, bits_wrap, bits_size);\
if (!(condition))\
continue;\
GET_DATA(buf[j].code, codes, i, codes_wrap, codes_size);\
if (flags & INIT_VLC_LE)\
buf[j].code = bitswap_32(buf[j].code);\
else\
buf[j].code <<= 32 - buf[j].bits;\
if (symbols)\
GET_DATA(buf[j].symbol, symbols, i, symbols_wrap, symbols_size)\
else\
buf[j].symbol = i;\
j++;\
}
COPY(buf[j].bits > nb_bits);
// qsort is the slowest part of init_vlc, and could probably be improved or avoided
qsort(buf, j, sizeof(VLCcode), compare_vlcspec);
COPY(buf[j].bits && buf[j].bits <= nb_bits);
nb_codes = j;
if (build_table(vlc, nb_bits, nb_codes, buf, flags) < 0) {
av_freep(&vlc->table);
return -1;
}