added documentation about LZ4_FORCE_SW_BITCOUNT
Also : added memory-frugal software byte count for big endian 64-bit cpus.
Disabled by default.
diff --git a/lib/README.md b/lib/README.md
index 707d777..3653c81 100644
--- a/lib/README.md
+++ b/lib/README.md
@@ -46,11 +46,11 @@
#### Build macros
-The following build macro can be selected at compilation time :
+The following build macro can be selected to adjust source code behavior at compilation time :
-- `LZ4_FAST_DEC_LOOP` : this triggers the optimized decompression loop.
- This loops works great on x86/x64 cpus, and is automatically enabled on this platform.
- It's possible to enable or disable it manually, by passing `LZ4_FAST_DEC_LOOP=1` or `0` to the preprocessor.
+- `LZ4_FAST_DEC_LOOP` : this triggers a speed optimized decompression loop, more powerful on modern cpus.
+ This loop works great on x86, x64 and aarch64 cpus, and is automatically enabled for them.
+ It's also possible to enable or disable it manually, by passing `LZ4_FAST_DEC_LOOP=1` or `0` to the preprocessor.
For example, with `gcc` : `-DLZ4_FAST_DEC_LOOP=1`,
and with `make` : `CPPFLAGS+=-DLZ4_FAST_DEC_LOOP=1 make lz4`.
@@ -66,9 +66,17 @@
Should this be a problem, it's generally possible to make the compiler ignore these warnings,
for example with `-Wno-deprecated-declarations` on `gcc`,
or `_CRT_SECURE_NO_WARNINGS` for Visual Studio.
- Another method is to define `LZ4_DISABLE_DEPRECATE_WARNINGS`
+ Another project-specific method is to define `LZ4_DISABLE_DEPRECATE_WARNINGS`
before including the LZ4 header files.
+- `LZ4_FORCE_SW_BITCOUNT` : by default, the compression algorithm tries to determine lengths
+ by using bitcount instructions, generally implemented as fast single instructions in many cpus.
+ In case the target cpus doesn't support it, or compiler intrinsic doesn't work, or feature bad performance,
+ it's possible to use an optimized software path instead.
+ This is achieved by setting this build macros .
+ In most cases, it's not expected to be necessary,
+ but it can be legitimately considered for less common platforms.
+
#### Amalgamation
@@ -103,7 +111,7 @@
#### Miscellaneous
-Other files present in the directory are not source code. There are :
+Other files present in the directory are not source code. They are :
- `LICENSE` : contains the BSD license text
- `Makefile` : `make` script to compile and install lz4 library (static and dynamic)
diff --git a/lib/lz4.c b/lib/lz4.c
index e10b58e..0ca7b21 100644
--- a/lib/lz4.c
+++ b/lib/lz4.c
@@ -88,6 +88,7 @@
* Define this parameter if your target system or compiler does not support hardware bit count
*/
#if defined(_MSC_VER) && defined(_WIN32_WCE) /* Visual Studio for WinCE doesn't support Hardware bit count */
+# undef LZ4_FORCE_SW_BITCOUNT /* avoid double def */
# define LZ4_FORCE_SW_BITCOUNT
#endif
@@ -527,6 +528,9 @@
!defined(LZ4_FORCE_SW_BITCOUNT)
return (unsigned)__builtin_clzll((U64)val) >> 3;
# else
+#if 1
+ /* this method is probably faster,
+ * but adds a 128 bytes lookup table */
static const unsigned char ctz7_tab[128] = {
7, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,
4, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,
@@ -537,9 +541,22 @@
5, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,
4, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,
};
- const U64 mask = 0x0101010101010101ULL;
- U64 t = (((val >> 8) - mask) | val) & mask;
+ U64 const mask = 0x0101010101010101ULL;
+ U64 const t = (((val >> 8) - mask) | val) & mask;
return ctz7_tab[(t * 0x0080402010080402ULL) >> 57];
+#else
+ /* this method doesn't consume memory space like the previous one,
+ * but it contains several branches,
+ * that may end up slowing execution */
+ static const U32 by32 = sizeof(val)*4; /* 32 on 64 bits (goal), 16 on 32 bits.
+ Just to avoid some static analyzer complaining about shift by 32 on 32-bits target.
+ Note that this code path is never triggered in 32-bits mode. */
+ unsigned r;
+ if (!(val>>by32)) { r=4; } else { r=0; val>>=by32; }
+ if (!(val>>16)) { r+=2; val>>=8; } else { val>>=24; }
+ r += (!val);
+ return r;
+#endif
# endif
} else /* 32 bits */ {
# if (defined(__clang__) || (defined(__GNUC__) && ((__GNUC__ > 3) || \
