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[huf] Improve fast huffman decoding speed in linux kernel #3826

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merged 1 commit into from Nov 20, 2023
Merged

[huf] Improve fast huffman decoding speed in linux kernel #3826

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4 changes: 4 additions & 0 deletions lib/README.md
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Expand Up @@ -178,6 +178,10 @@ The file structure is designed to make this selection manually achievable for an
`ZSTDERRORLIB_VSIBILITY`, and `ZDICTLIB_VISIBILITY` if unset, for backwards compatibility
with the old macro names.

- The C compiler macro `HUF_DISABLE_FAST_DECODE` disables the newer Huffman fast C
and assembly decoding loops. You may want to use this macro if these loops are
slower on your platform.

#### Windows : using MinGW+MSYS to create DLL

DLL can be created using MinGW+MSYS with the `make libzstd` command.
Expand Down
171 changes: 105 additions & 66 deletions lib/decompress/huf_decompress.c
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Expand Up @@ -34,6 +34,12 @@
* Macros
****************************************************************/

#ifdef HUF_DISABLE_FAST_DECODE
# define HUF_ENABLE_FAST_DECODE 0
#else
# define HUF_ENABLE_FAST_DECODE 1
#endif

/* These two optional macros force the use one way or another of the two
* Huffman decompression implementations. You can't force in both directions
* at the same time.
Expand Down Expand Up @@ -292,6 +298,24 @@ static size_t HUF_initRemainingDStream(BIT_DStream_t* bit, HUF_DecompressFastArg
return 0;
}

/* Calls X(N) for each stream 0, 1, 2, 3. */
#define HUF_4X_FOR_EACH_STREAM(X) \
{ \
X(0) \
X(1) \
X(2) \
X(3) \
}

/* Calls X(N, var) for each stream 0, 1, 2, 3. */
#define HUF_4X_FOR_EACH_STREAM_WITH_VAR(X, var) \
{ \
X(0, (var)) \
X(1, (var)) \
X(2, (var)) \
X(3, (var)) \
}


#ifndef HUF_FORCE_DECOMPRESS_X2

Expand Down Expand Up @@ -706,7 +730,6 @@ void HUF_decompress4X1_usingDTable_internal_fast_c_loop(HUF_DecompressFastArgs*
for (;;) {
BYTE* olimit;
int stream;
int symbol;

/* Assert loop preconditions */
#ifndef NDEBUG
Expand Down Expand Up @@ -753,27 +776,42 @@ void HUF_decompress4X1_usingDTable_internal_fast_c_loop(HUF_DecompressFastArgs*
}
#endif

#define HUF_4X1_DECODE_SYMBOL(_stream, _symbol) \
{ \
int const index = (int)(bits[(_stream)] >> 53); \
int const entry = (int)dtable[index]; \
bits[(_stream)] <<= (entry & 0x3F); \
op[(_stream)][(_symbol)] = (BYTE)((entry >> 8) & 0xFF); \
}

#define HUF_4X1_RELOAD_STREAM(_stream) \
{ \
int const ctz = ZSTD_countTrailingZeros64(bits[(_stream)]); \
int const nbBits = ctz & 7; \
int const nbBytes = ctz >> 3; \
op[(_stream)] += 5; \
ip[(_stream)] -= nbBytes; \
bits[(_stream)] = MEM_read64(ip[(_stream)]) | 1; \
bits[(_stream)] <<= nbBits; \
}

/* Manually unroll the loop because compilers don't consistently
* unroll the inner loops, which destroys performance.
*/
do {
/* Decode 5 symbols in each of the 4 streams */
for (symbol = 0; symbol < 5; ++symbol) {
for (stream = 0; stream < 4; ++stream) {
int const index = (int)(bits[stream] >> 53);
int const entry = (int)dtable[index];
bits[stream] <<= (entry & 63);
op[stream][symbol] = (BYTE)((entry >> 8) & 0xFF);
}
}
/* Reload the bitstreams */
for (stream = 0; stream < 4; ++stream) {
int const ctz = ZSTD_countTrailingZeros64(bits[stream]);
int const nbBits = ctz & 7;
int const nbBytes = ctz >> 3;
op[stream] += 5;
ip[stream] -= nbBytes;
bits[stream] = MEM_read64(ip[stream]) | 1;
bits[stream] <<= nbBits;
}
HUF_4X_FOR_EACH_STREAM_WITH_VAR(HUF_4X1_DECODE_SYMBOL, 0)
HUF_4X_FOR_EACH_STREAM_WITH_VAR(HUF_4X1_DECODE_SYMBOL, 1)
HUF_4X_FOR_EACH_STREAM_WITH_VAR(HUF_4X1_DECODE_SYMBOL, 2)
HUF_4X_FOR_EACH_STREAM_WITH_VAR(HUF_4X1_DECODE_SYMBOL, 3)
HUF_4X_FOR_EACH_STREAM_WITH_VAR(HUF_4X1_DECODE_SYMBOL, 4)

/* Reload each of the 4 the bitstreams */
HUF_4X_FOR_EACH_STREAM(HUF_4X1_RELOAD_STREAM)
} while (op[3] < olimit);

#undef HUF_4X1_DECODE_SYMBOL
#undef HUF_4X1_RELOAD_STREAM
}

_out:
Expand Down Expand Up @@ -869,7 +907,7 @@ static size_t HUF_decompress4X1_usingDTable_internal(void* dst, size_t dstSize,
}
#endif

if (!(flags & HUF_flags_disableFast)) {
if (HUF_ENABLE_FAST_DECODE && !(flags & HUF_flags_disableFast)) {
size_t const ret = HUF_decompress4X1_usingDTable_internal_fast(dst, dstSize, cSrc, cSrcSize, DTable, loopFn);
if (ret != 0)
return ret;
Expand Down Expand Up @@ -1492,7 +1530,6 @@ void HUF_decompress4X2_usingDTable_internal_fast_c_loop(HUF_DecompressFastArgs*
for (;;) {
BYTE* olimit;
int stream;
int symbol;

/* Assert loop preconditions */
#ifndef NDEBUG
Expand Down Expand Up @@ -1549,54 +1586,56 @@ void HUF_decompress4X2_usingDTable_internal_fast_c_loop(HUF_DecompressFastArgs*
}
#endif

#define HUF_4X2_DECODE_SYMBOL(_stream, _decode3) \
if ((_decode3) || (_stream) != 3) { \
int const index = (int)(bits[(_stream)] >> 53); \
HUF_DEltX2 const entry = dtable[index]; \
MEM_write16(op[(_stream)], entry.sequence); \
bits[(_stream)] <<= (entry.nbBits) & 0x3F; \
op[(_stream)] += (entry.length); \
}

#define HUF_4X2_RELOAD_STREAM(_stream) \
{ \
HUF_4X2_DECODE_SYMBOL(3, 1) \
{ \
int const ctz = ZSTD_countTrailingZeros64(bits[(_stream)]); \
int const nbBits = ctz & 7; \
int const nbBytes = ctz >> 3; \
ip[(_stream)] -= nbBytes; \
bits[(_stream)] = MEM_read64(ip[(_stream)]) | 1; \
bits[(_stream)] <<= nbBits; \
} \
}

/* Manually unroll the loop because compilers don't consistently
* unroll the inner loops, which destroys performance.
*/
do {
/* Do 5 table lookups for each of the first 3 streams */
for (symbol = 0; symbol < 5; ++symbol) {
for (stream = 0; stream < 3; ++stream) {
int const index = (int)(bits[stream] >> 53);
HUF_DEltX2 const entry = dtable[index];
MEM_write16(op[stream], entry.sequence);
bits[stream] <<= (entry.nbBits);
op[stream] += (entry.length);
}
}
/* Do 1 table lookup from the final stream */
{
int const index = (int)(bits[3] >> 53);
HUF_DEltX2 const entry = dtable[index];
MEM_write16(op[3], entry.sequence);
bits[3] <<= (entry.nbBits);
op[3] += (entry.length);
}
/* Do 4 table lookups from the final stream & reload bitstreams */
for (stream = 0; stream < 4; ++stream) {
/* Do a table lookup from the final stream.
* This is interleaved with the reloading to reduce register
* pressure. This shouldn't be necessary, but compilers can
* struggle with codegen with high register pressure.
*/
{
int const index = (int)(bits[3] >> 53);
HUF_DEltX2 const entry = dtable[index];
MEM_write16(op[3], entry.sequence);
bits[3] <<= (entry.nbBits);
op[3] += (entry.length);
}
/* Reload the bistreams. The final bitstream must be reloaded
* after the 5th symbol was decoded.
*/
{
int const ctz = ZSTD_countTrailingZeros64(bits[stream]);
int const nbBits = ctz & 7;
int const nbBytes = ctz >> 3;
ip[stream] -= nbBytes;
bits[stream] = MEM_read64(ip[stream]) | 1;
bits[stream] <<= nbBits;
}
}
/* Decode 5 symbols from each of the first 3 streams.
* The final stream will be decoded during the reload phase
* to reduce register pressure.
*/
HUF_4X_FOR_EACH_STREAM_WITH_VAR(HUF_4X2_DECODE_SYMBOL, 0)
HUF_4X_FOR_EACH_STREAM_WITH_VAR(HUF_4X2_DECODE_SYMBOL, 0)
HUF_4X_FOR_EACH_STREAM_WITH_VAR(HUF_4X2_DECODE_SYMBOL, 0)
HUF_4X_FOR_EACH_STREAM_WITH_VAR(HUF_4X2_DECODE_SYMBOL, 0)
HUF_4X_FOR_EACH_STREAM_WITH_VAR(HUF_4X2_DECODE_SYMBOL, 0)

/* Decode one symbol from the final stream */
HUF_4X2_DECODE_SYMBOL(3, 1)

/* Decode 4 symbols from the final stream & reload bitstreams.
* The final stream is reloaded last, meaning that all 5 symbols
* are decoded from the final stream before it is reloaded.
*/
HUF_4X_FOR_EACH_STREAM(HUF_4X2_RELOAD_STREAM)
} while (op[3] < olimit);
}

#undef HUF_4X2_DECODE_SYMBOL
#undef HUF_4X2_RELOAD_STREAM

_out:

/* Save the final values of each of the state variables back to args. */
Expand Down Expand Up @@ -1681,7 +1720,7 @@ static size_t HUF_decompress4X2_usingDTable_internal(void* dst, size_t dstSize,
}
#endif

if (!(flags & HUF_flags_disableFast)) {
if (HUF_ENABLE_FAST_DECODE && !(flags & HUF_flags_disableFast)) {
size_t const ret = HUF_decompress4X2_usingDTable_internal_fast(dst, dstSize, cSrc, cSrcSize, DTable, loopFn);
if (ret != 0)
return ret;
Expand Down