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lz4.c: refactor the decoding routines

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I noticed that LZ4_decompress_generic is sometimes instantiated with
identical set of parameters, or (what's worse) with a subtly different
sets of parameters.  For example, LZ4_decompress_fast_withPrefix64k is
instantiated as follows:

    return LZ4_decompress_generic(source, dest, 0, originalSize, endOnOutputSize,
		full, 0, withPrefix64k, (BYTE*)dest - 64 KB, NULL, 64 KB);

while the equivalent withPrefix64k call in LZ4_decompress_usingDict_generic
passes 0 for the last argument instead of 64 KB.  It turns out that there
is no difference in this case: if you change 64 KB to 0 KB in
LZ4_decompress_fast_withPrefix64k, you get the same binary code.

Moreover, because it's been clarified that LZ4_decompress_fast doesn't
check match offsets, it is now obvious that both of these fast/withPrefix64k
instantiations are simply redundant.  Exactly because LZ4_decompress_fast
doesn't check offsets, it serves well with any prefixed dictionary.

There's a difference, though, with LZ4_decompress_safe_withPrefix64k.
It also passes 64 KB as the last argument, and if you change that to 0,
as in LZ4_decompress_usingDict_generic, you get a completely different
binary code.  It seems that passing 0 enables offset checking:

    const int checkOffset = ((safeDecode) && (dictSize < (int)(64 KB)));

However, the resulting code seems to run a bit faster.  How come
enabling extra checks can make the code run faster?  Curiouser and
curiouser!  This needs extra study.  Currently I take the view that
the dictSize should be set to non-zero when nothing else will do,
i.e. when passing the external dictionary via dictStart.  Otherwise,
lowPrefix betrays just enough information about the dictionary.

    * * *

Anyway, with this change, I instantiate all the necessary cases as
functions with distinctive names, which also take fewer arguments and
are therefore less error-prone.  I also make the functions non-inline.
(The compiler won't inline the functions because they are used more than
once.  Hence I attach LZ4_FORCE_O2_GCC_PPC64LE to the instances while
removing from the callers.)  The number of instances is now is reduced
from 18 (safe+fast+partial+4*continue+4*prefix+4*dict+2*prefix64+forceExtDict)
down to 7 (safe+fast+partial+2*prefix+2*dict).  The size of the code is
not the only issue here.  Separate helper function are much more
amenable to profile-guided optimization: it is enough to profile only
a few basic functions, while the other less-often used functions, such
as LZ4_decompress_*_continue, will benefit automatically.

This is the list of LZ4_decompress* functions in liblz4.so, sorted by size.
Exported functions are marked with a capital T.

$ nm -S lib/liblz4.so |grep -wi T |grep LZ4_decompress |sort -k2
0000000000016260 0000000000000005 T LZ4_decompress_fast_withPrefix64k
0000000000016dc0 0000000000000025 T LZ4_decompress_fast_usingDict
0000000000016d80 0000000000000040 T LZ4_decompress_safe_usingDict
0000000000016d10 000000000000006b T LZ4_decompress_fast_continue
0000000000016c70 000000000000009f T LZ4_decompress_safe_continue
00000000000156c0 000000000000059c T LZ4_decompress_fast
0000000000014a90 00000000000005fa T LZ4_decompress_safe
0000000000015c60 00000000000005fa T LZ4_decompress_safe_withPrefix64k
0000000000002280 00000000000005fa t LZ4_decompress_safe_withSmallPrefix
0000000000015090 000000000000062f T LZ4_decompress_safe_partial
0000000000002880 00000000000008ea t LZ4_decompress_fast_extDict
0000000000016270 0000000000000993 t LZ4_decompress_safe_forceExtDict
This commit is contained in:
Alexey Tourbin 2018-04-23 08:37:44 +03:00
parent 62d7cdcc74
commit b4eda8d08f
1 changed files with 79 additions and 53 deletions
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132
lib/lz4.c
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@ -92,6 +92,7 @@
* Dependency
**************************************/
#define LZ4_STATIC_LINKING_ONLY
#define LZ4_DISABLE_DEPRECATE_WARNINGS /* due to LZ4_decompress_safe_withPrefix64k */
#include "lz4.h"
/* see also "memory routines" below */
@ -1666,6 +1667,8 @@ _output_error:
}
/*===== Instantiate the API decoding functions. =====*/
LZ4_FORCE_O2_GCC_PPC64LE
int LZ4_decompress_safe(const char* source, char* dest, int compressedSize, int maxDecompressedSize)
{
@ -1687,9 +1690,63 @@ int LZ4_decompress_fast(const char* source, char* dest, int originalSize)
{
return LZ4_decompress_generic(source, dest, 0, originalSize,
endOnOutputSize, full, 0, withPrefix64k,
(BYTE*)(dest - 64 KB), NULL, 64 KB);
(BYTE*)dest - 64 KB, NULL, 0);
}
/*===== Instantiate a few more decoding cases, used more than once. =====*/
LZ4_FORCE_O2_GCC_PPC64LE /* Exported, an obsolete API function. */
int LZ4_decompress_safe_withPrefix64k(const char* source, char* dest, int compressedSize, int maxOutputSize)
{
return LZ4_decompress_generic(source, dest, compressedSize, maxOutputSize,
endOnInputSize, full, 0, withPrefix64k,
(BYTE*)dest - 64 KB, NULL, 0);
}
/* Another obsolete API function, paired with the previous one. */
int LZ4_decompress_fast_withPrefix64k(const char* source, char* dest, int originalSize)
{
/* LZ4_decompress_fast doesn't validate match offsets,
* and thus serves well with any prefixed dictionary. */
return LZ4_decompress_fast(source, dest, originalSize);
}
LZ4_FORCE_O2_GCC_PPC64LE
static int LZ4_decompress_safe_withSmallPrefix(const char* source, char* dest, int compressedSize, int maxOutputSize,
size_t dictSize)
{
return LZ4_decompress_generic(source, dest, compressedSize, maxOutputSize,
endOnInputSize, full, 0, noDict,
(BYTE*)dest-dictSize, NULL, 0);
}
LZ4_FORCE_INLINE
int LZ4_decompress_safe_withPrefix(const char* source, char* dest, int compressedSize, int maxOutputSize,
size_t dictSize)
{
if (dictSize >= 64 KB - 1)
return LZ4_decompress_safe_withPrefix64k(source, dest, compressedSize, maxOutputSize);
return LZ4_decompress_safe_withSmallPrefix(source, dest, compressedSize, maxOutputSize, dictSize);
}
LZ4_FORCE_O2_GCC_PPC64LE /* Exported under another name, for tests/fullbench.c */
#define LZ4_decompress_safe_extDict LZ4_decompress_safe_forceExtDict
int LZ4_decompress_safe_extDict(const char* source, char* dest, int compressedSize, int maxOutputSize,
const char* dictStart, size_t dictSize)
{
return LZ4_decompress_generic(source, dest, compressedSize, maxOutputSize,
endOnInputSize, full, 0, usingExtDict,
(BYTE*)dest, (const BYTE*)dictStart, dictSize);
}
LZ4_FORCE_O2_GCC_PPC64LE
static int LZ4_decompress_fast_extDict(const char* source, char* dest, int originalSize,
const char* dictStart, size_t dictSize)
{
return LZ4_decompress_generic(source, dest, 0, originalSize,
endOnOutputSize, full, 0, usingExtDict,
(BYTE*)dest, (const BYTE*)dictStart, dictSize);
}
/*===== streaming decompression functions =====*/
@ -1730,25 +1787,26 @@ int LZ4_setStreamDecode (LZ4_streamDecode_t* LZ4_streamDecode, const char* dicti
If it's not possible, save the relevant part of decoded data into a safe buffer,
and indicate where it stands using LZ4_setStreamDecode()
*/
LZ4_FORCE_O2_GCC_PPC64LE
int LZ4_decompress_safe_continue (LZ4_streamDecode_t* LZ4_streamDecode, const char* source, char* dest, int compressedSize, int maxOutputSize)
{
LZ4_streamDecode_t_internal* lz4sd = &LZ4_streamDecode->internal_donotuse;
int result;
if (lz4sd->prefixEnd == (BYTE*)dest) {
result = LZ4_decompress_generic(source, dest, compressedSize, maxOutputSize,
endOnInputSize, full, 0,
usingExtDict, lz4sd->prefixEnd - lz4sd->prefixSize, lz4sd->externalDict, lz4sd->extDictSize);
if (lz4sd->prefixSize == 0) {
result = LZ4_decompress_safe(source, dest, compressedSize, maxOutputSize);
if (result <= 0) return result;
lz4sd->prefixSize = result;
lz4sd->prefixEnd = (BYTE*)dest + result;
} else if (lz4sd->prefixEnd == (BYTE*)dest) {
result = LZ4_decompress_safe_withPrefix(source, dest, compressedSize, maxOutputSize, lz4sd->prefixSize);
if (result <= 0) return result;
lz4sd->prefixSize += result;
lz4sd->prefixEnd += result;
} else {
lz4sd->extDictSize = lz4sd->prefixSize;
lz4sd->externalDict = lz4sd->prefixEnd - lz4sd->extDictSize;
result = LZ4_decompress_generic(source, dest, compressedSize, maxOutputSize,
endOnInputSize, full, 0,
usingExtDict, (BYTE*)dest, lz4sd->externalDict, lz4sd->extDictSize);
result = LZ4_decompress_safe_extDict(source, dest, compressedSize, maxOutputSize,
(const char*)lz4sd->externalDict, lz4sd->extDictSize);
if (result <= 0) return result;
lz4sd->prefixSize = result;
lz4sd->prefixEnd = (BYTE*)dest + result;
@ -1757,25 +1815,21 @@ int LZ4_decompress_safe_continue (LZ4_streamDecode_t* LZ4_streamDecode, const ch
return result;
}
LZ4_FORCE_O2_GCC_PPC64LE
int LZ4_decompress_fast_continue (LZ4_streamDecode_t* LZ4_streamDecode, const char* source, char* dest, int originalSize)
{
LZ4_streamDecode_t_internal* lz4sd = &LZ4_streamDecode->internal_donotuse;
int result;
if (lz4sd->prefixEnd == (BYTE*)dest) {
result = LZ4_decompress_generic(source, dest, 0, originalSize,
endOnOutputSize, full, 0,
usingExtDict, lz4sd->prefixEnd - lz4sd->prefixSize, lz4sd->externalDict, lz4sd->extDictSize);
if (lz4sd->prefixSize == 0 || lz4sd->prefixEnd == (BYTE*)dest) {
result = LZ4_decompress_fast(source, dest, originalSize);
if (result <= 0) return result;
lz4sd->prefixSize += originalSize;
lz4sd->prefixEnd += originalSize;
} else {
lz4sd->extDictSize = lz4sd->prefixSize;
lz4sd->externalDict = lz4sd->prefixEnd - lz4sd->extDictSize;
result = LZ4_decompress_generic(source, dest, 0, originalSize,
endOnOutputSize, full, 0,
usingExtDict, (BYTE*)dest, lz4sd->externalDict, lz4sd->extDictSize);
result = LZ4_decompress_fast_extDict(source, dest, originalSize,
(const char*)lz4sd->externalDict, lz4sd->extDictSize);
if (result <= 0) return result;
lz4sd->prefixSize = originalSize;
lz4sd->prefixEnd = (BYTE*)dest + originalSize;
@ -1792,36 +1846,20 @@ Advanced decoding functions :
the dictionary must be explicitly provided within parameters
*/
LZ4_FORCE_O2_GCC_PPC64LE
LZ4_FORCE_INLINE int LZ4_decompress_usingDict_generic(const char* source, char* dest, int compressedSize, int maxOutputSize, int safe, const char* dictStart, int dictSize)
{
if (dictSize==0)
return LZ4_decompress_generic(source, dest, compressedSize, maxOutputSize, safe, full, 0, noDict, (BYTE*)dest, NULL, 0);
if (dictStart+dictSize == dest) {
if (dictSize >= (int)(64 KB - 1))
return LZ4_decompress_generic(source, dest, compressedSize, maxOutputSize, safe, full, 0, withPrefix64k, (BYTE*)dest-64 KB, NULL, 0);
return LZ4_decompress_generic(source, dest, compressedSize, maxOutputSize, safe, full, 0, noDict, (BYTE*)dest-dictSize, NULL, 0);
}
return LZ4_decompress_generic(source, dest, compressedSize, maxOutputSize, safe, full, 0, usingExtDict, (BYTE*)dest, (const BYTE*)dictStart, dictSize);
}
LZ4_FORCE_O2_GCC_PPC64LE
int LZ4_decompress_safe_usingDict(const char* source, char* dest, int compressedSize, int maxOutputSize, const char* dictStart, int dictSize)
{
return LZ4_decompress_usingDict_generic(source, dest, compressedSize, maxOutputSize, 1, dictStart, dictSize);
if (dictSize==0)
return LZ4_decompress_safe(source, dest, compressedSize, maxOutputSize);
if (dictStart+dictSize == dest)
return LZ4_decompress_safe_withPrefix(source, dest, compressedSize, maxOutputSize, dictSize);
return LZ4_decompress_safe_extDict(source, dest, compressedSize, maxOutputSize, dictStart, dictSize);
}
LZ4_FORCE_O2_GCC_PPC64LE
int LZ4_decompress_fast_usingDict(const char* source, char* dest, int originalSize, const char* dictStart, int dictSize)
{
return LZ4_decompress_usingDict_generic(source, dest, 0, originalSize, 0, dictStart, dictSize);
}
/* debug function */
LZ4_FORCE_O2_GCC_PPC64LE
int LZ4_decompress_safe_forceExtDict(const char* source, char* dest, int compressedSize, int maxOutputSize, const char* dictStart, int dictSize)
{
return LZ4_decompress_generic(source, dest, compressedSize, maxOutputSize, endOnInputSize, full, 0, usingExtDict, (BYTE*)dest, (const BYTE*)dictStart, dictSize);
if (dictSize==0 || dictStart+dictSize == dest)
return LZ4_decompress_fast(source, dest, originalSize);
return LZ4_decompress_fast_extDict(source, dest, originalSize, dictStart, dictSize);
}
@ -1892,16 +1930,4 @@ char* LZ4_slideInputBuffer (void* state)
return (char *)(uptrval)((LZ4_stream_t*)state)->internal_donotuse.dictionary;
}
/* Obsolete streaming decompression functions */
int LZ4_decompress_safe_withPrefix64k(const char* source, char* dest, int compressedSize, int maxOutputSize)
{
return LZ4_decompress_generic(source, dest, compressedSize, maxOutputSize, endOnInputSize, full, 0, withPrefix64k, (BYTE*)dest - 64 KB, NULL, 64 KB);
}
int LZ4_decompress_fast_withPrefix64k(const char* source, char* dest, int originalSize)
{
return LZ4_decompress_generic(source, dest, 0, originalSize, endOnOutputSize, full, 0, withPrefix64k, (BYTE*)dest - 64 KB, NULL, 64 KB);
}
#endif /* LZ4_COMMONDEFS_ONLY */