The following macros can be set at compilation time to modify libxxhash's behavior. They are generally disabled by default.
-`XXH_INLINE_ALL`: Make all functions `inline`, with implementations being directly included within `xxhash.h`.
Inlining functions is beneficial for speed on small keys.
It's _extremely effective_ when key length is expressed as _a compile time constant_,
with performance improvements observed in the +200% range .
See [this article](https://fastcompression.blogspot.com/2018/03/xxhash-for-small-keys-impressive-power.html) for details.
-`XXH_PRIVATE_API`: same outcome as `XXH_INLINE_ALL`. Still available for legacy support.
The name underlines that `XXH_*` symbols will not be exported.
-`XXH_NAMESPACE`: Prefixes all symbols with the value of `XXH_NAMESPACE`.
This macro can only use compilable character set.
Useful to evade symbol naming collisions,
in case of multiple inclusions of xxHash's source code.
Client applications still use the regular function names,
as symbols are automatically translated through `xxhash.h`.
-`XXH_FORCE_MEMORY_ACCESS`: The default method `0` uses a portable `memcpy()` notation.
Method `1` uses a gcc-specific `packed` attribute, which can provide better performance for some targets.
Method `2` forces unaligned reads, which is not standards compliant, but might sometimes be the only way to extract better read performance.
Method `3` uses a byteshift operation, which is best for old compilers which don't inline `memcpy()` or big-endian systems without a byteswap instruction
-`XXH_FORCE_ALIGN_CHECK`: Use a faster direct read path when input is aligned.
This option can result in dramatic performance improvement when input to hash is aligned on 32 or 64-bit boundaries,
when running on architectures unable to load memory from unaligned addresses, or suffering a performance penalty from it.
It is (slightly) detrimental on platform with good unaligned memory access performance (same instruction for both aligned and unaligned accesses).
This option is automatically disabled on `x86`, `x64` and `aarch64`, and enabled on all other platforms.
-`XXH_VECTOR` : manually select a vector instruction set (default: auto-selected at compilation time). Available instruction sets are `XXH_SCALAR`, `XXH_SSE2`, `XXH_AVX2`, `XXH_AVX512`, `XXH_NEON` and `XXH_VSX`. Compiler may require additional flags to ensure proper support (for example, `gcc` on linux will require `-mavx2` for AVX2, and `-mavx512f` for AVX512).
-`XXH_NO_INLINE_HINTS`: By default, xxHash uses `__attribute__((always_inline))` and `__forceinline` to improve performance at the cost of code size.
Defining this macro to 1 will mark all internal functions as `static`, allowing the compiler to decide whether to inline a function or not.
This is very useful when optimizing for smallest binary size,
and is automatically defined when compiling with `-O0`, `-Os`, `-Oz`, or `-fno-inline` on GCC and Clang.
This may also increase performance depending on compiler and architecture.
-`XXH_REROLL`: Reduces the size of the generated code by not unrolling some loops.
Impact on performance may vary, depending on platform and algorithm.
-`XXH_ACCEPT_NULL_INPUT_POINTER`: if set to `1`, when input is a `NULL` pointer,
xxHash'd result is the same as a zero-length input
(instead of a dereference segfault).
Adds one branch at the beginning of each hash.
-`XXH_STATIC_LINKING_ONLY`: gives access to the state declaration for static allocation.
Incompatible with dynamic linking, due to risks of ABI changes.
-`XXH_NO_LONG_LONG`: removes compilation of algorithms relying on 64-bit types (XXH3 and XXH64). Only XXH32 will be compiled.
Useful for targets (architectures and compilers) without 64-bit support.
-`XXH_IMPORT`: MSVC specific: should only be defined for dynamic linking, as it prevents linkage errors.
-`XXH_CPU_LITTLE_ENDIAN`: By default, endianess is determined by a runtime test resolved at compile time.
If, for some reason, the compiler cannot simplify the runtime test, it can cost performance.
It's possible to skip auto-detection and simply state that the architecture is little-endian by setting this macro to 1.
Setting it to 0 states big-endian.
For the Command Line Interface `xxhsum`, the following environment variables can also be set :
-`DISPATCH=1` : use `xxh_x86dispatch.c`, to automatically select between `scalar`, `sse2`, `avx2` or `avx512` instruction set at runtime, depending on local host. This option is only valid for `x86`/`x64` systems.
### Building xxHash - Using vcpkg
You can download and install xxHash using the [vcpkg](https://github.com/Microsoft/vcpkg) dependency manager:
git clone https://github.com/Microsoft/vcpkg.git
cd vcpkg
./bootstrap-vcpkg.sh
./vcpkg integrate install
./vcpkg install xxhash
The xxHash port in vcpkg is kept up to date by Microsoft team members and community contributors. If the version is out of date, please [create an issue or pull request](https://github.com/Microsoft/vcpkg) on the vcpkg repository.
