Update zstd to version 1.5.0

C/zstd/README.md

@@ -19,12 +19,16 @@ The scope can be reduced on demand (see paragraph _modular build_).
 
 #### Multithreading support
 
-Multithreading is disabled by default when building with `make`.
+When building with `make`, by default the dynamic library is multithreaded and static library is single-threaded (for compatibility reasons).
+
 Enabling multithreading requires 2 conditions :
 - set build macro `ZSTD_MULTITHREAD` (`-DZSTD_MULTITHREAD` for `gcc`)
 - for POSIX systems : compile with pthread (`-pthread` compilation flag for `gcc`)
 
-Both conditions are automatically applied when invoking `make lib-mt` target.
+For convenience, we provide a build target to generate multi and single threaded libraries:
+- Force enable multithreading on both dynamic and static libraries by appending `-mt` to the target, e.g. `make lib-mt`.
+- Force disable multithreading on both dynamic and static libraries by appending `-nomt` to the target, e.g. `make lib-nomt`.
+- By default, as mentioned before, dynamic library is multithreaded, and static library is single-threaded, e.g. `make lib`.
 
 When linking a POSIX program with a multithreaded version of `libzstd`,
 note that it's necessary to invoke the `-pthread` flag during link stage.
@@ -42,8 +46,8 @@ Zstandard's stable API is exposed within [lib/zstd.h](zstd.h).
 
 Optional advanced features are exposed via :
 
-- `lib/common/zstd_errors.h` : translates `size_t` function results
-                               into a `ZSTD_ErrorCode`, for accurate error handling.
+- `lib/zstd_errors.h` : translates `size_t` function results
+                        into a `ZSTD_ErrorCode`, for accurate error handling.
 
 - `ZSTD_STATIC_LINKING_ONLY` : if this macro is defined _before_ including `zstd.h`,
                           it unlocks access to the experimental API,
@@ -143,6 +147,14 @@ The file structure is designed to make this selection manually achievable for an
   Setting this macro will either force to generate the BMI2 dispatcher (1)
   or prevent it (0). It overrides automatic detection.
 
+- The build macro `ZSTD_NO_UNUSED_FUNCTIONS` can be defined to hide the definitions of functions
+  that zstd does not use. Not all unused functions are hidden, but they can be if needed.
+  Currently, this macro will hide function definitions in FSE and HUF that use an excessive
+  amount of stack space.
+
+- The build macro `ZSTD_NO_INTRINSICS` can be defined to disable all explicit intrinsics.
+  Compiler builtins are still used.
+
 
 #### Windows : using MinGW+MSYS to create DLL
 
@@ -160,6 +172,26 @@ file it should be linked with `dll\libzstd.dll`. For example:
 The compiled executable will require ZSTD DLL which is available at `dll\libzstd.dll`.
 
 
+#### Advanced Build options
+
+The build system requires a hash function in order to
+separate object files created with different compilation flags.
+By default, it tries to use `md5sum` or equivalent.
+The hash function can be manually switched by setting the `HASH` variable.
+For example : `make HASH=xxhsum`
+The hash function needs to generate at least 64-bit using hexadecimal format.
+When no hash function is found,
+the Makefile just generates all object files into the same default directory,
+irrespective of compilation flags.
+This functionality only matters if `libzstd` is compiled multiple times
+with different build flags.
+
+The build directory, where object files are stored
+can also be manually controlled using variable `BUILD_DIR`,
+for example `make BUILD_DIR=objectDir/v1`.
+In which case, the hash function doesn't matter.
+
+
 #### Deprecated API
 
 Obsolete API on their way out are stored in directory `lib/deprecated`.