Efficient and performance-portable vector software

Highway is a C++ library that provides portable SIMD/vector intrinsics.

Why

We are passionate about high-performance software. We see major untapped potential in CPUs (servers, mobile, desktops). Highway is for engineers who want to reliably and economically push the boundaries of what is possible in software.

How

CPUs provide SIMD/vector instructions that apply the same operation to multiple data items. This can reduce energy usage e.g. fivefold because fewer instructions are executed. We also often see 5-10x speedups.

Highway makes SIMD/vector programming practical and workable according to these guiding principles:

Does what you expect: Highway is a C++ library with carefully-chosen functions that map well to CPU instructions without extensive compiler transformations. The resulting code is more predictable and robust to code changes/compiler updates than autovectorization.

Works on widely-used platforms: Highway supports four architectures; the same application code can target eight instruction sets, including those with 'scalable' vectors (size unknown at compile time). Highway only requires C++11 and supports four families of compilers. If you would like to use Highway on other platforms, please raise an issue.

Flexible to deploy: Applications using Highway can run on heterogeneous clouds or client devices, choosing the best available instruction set at runtime. Alternatively, developers may choose to target a single instruction set without any runtime overhead. In both cases, the application code is the same except for swapping HWY_STATIC_DISPATCH with HWY_DYNAMIC_DISPATCH plus one line of code.

Suitable for a variety of domains: Highway provides an extensive set of operations, used for image processing (floating-point), compression, video analysis, linear algebra, cryptography, sorting and random generation. We recognise that new use-cases may require additional ops and are happy to add them where it makes sense (e.g. no performance cliffs on some architectures). If you would like to discuss, please file an issue.

Rewards data-parallel design: Highway provides tools such as Gather, MaskedLoad, and FixedTag to enable speedups for legacy data structures. However, the biggest gains are unlocked by designing algorithms and data structures for scalable vectors. Helpful techniques include batching, structure-of-array layouts, and aligned/padded allocations.

Examples

Online demos using Compiler Explorer:

generating code for multiple targets (recommended)
single target using -m flags

Projects using Highway: (to add yours, feel free to raise an issue or contact us via the below email)

Current status

Targets

Supported targets: scalar, S-SSE3, SSE4, AVX2, AVX-512, AVX3_DL (~Icelake, requires opt-in by defining HWY_WANT_AVX3_DL), NEON (ARMv7 and v8), SVE, SVE2, WASM SIMD, RISC-V V.

SVE was initially tested using farm_sve (see acknowledgments).

Versioning

Highway releases aim to follow the semver.org system (MAJOR.MINOR.PATCH), incrementing MINOR after backward-compatible additions and PATCH after backward-compatible fixes. We recommend using releases (rather than the Git tip) because they are tested more extensively, see below.

Version 0.11 is considered stable enough to use in other projects. Version 1.0 will signal an increased focus on backwards compatibility and is planned for 2022H1 now that all targets are feature-complete.

Testing

Continuous integration tests build with a recent version of Clang (running on native x86, Spike for RVV, and QEMU for ARM) and MSVC from VS2015 (running on native x86).

Before releases, we also test on x86 with Clang and GCC, and ARMv7/8 via GCC cross-compile and QEMU. See the testing process for details.

Related modules

The contrib directory contains SIMD-related utilities: an image class with aligned rows, a math library (16 functions already implemented, mostly trigonometry), and functions for computing dot products and sorting.

Installation

This project uses CMake to generate and build. In a Debian-based system you can install it via:

sudo apt install cmake

Highway's unit tests use googletest. By default, Highway's CMake downloads this dependency at configuration time. You can disable this by setting the HWY_SYSTEM_GTEST CMake variable to ON and installing gtest separately:

sudo apt install libgtest-dev

To build Highway as a shared or static library (depending on BUILD_SHARED_LIBS), the standard CMake workflow can be used:

mkdir -p build && cd build
cmake ..
make -j && make test

Or you can run run_tests.sh (run_tests.bat on Windows).

Bazel is also supported for building, but it is not as widely used/tested.

Quick start

You can use the benchmark inside examples/ as a starting point.

A quick-reference page briefly lists all operations and their parameters, and the instruction_matrix indicates the number of instructions per operation.

We recommend using full SIMD vectors whenever possible for maximum performance portability. To obtain them, pass a ScalableTag<float> (or equivalently HWY_FULL(float)) tag to functions such as Zero/Set/Load. There are two alternatives for use-cases requiring an upper bound on the lanes:

For up to a power of two N, specify CappedTag<T, N> (or equivalently HWY_CAPPED(T, N)). This is useful for data structures such as a narrow matrix. A loop is still required because vectors may actually have fewer than N lanes.
For exactly a power of two N lanes, specify FixedTag<T, N>. The largest supported N depends on the target, but is guaranteed to be at least 16/sizeof(T).

Due to ADL restrictions, user code calling Highway ops must either:

Reside inside namespace hwy { namespace HWY_NAMESPACE {; or
prefix each op with an alias such as namespace hn = hwy::HWY_NAMESPACE; hn::Add(); or
add using-declarations for each op used: using hwy::HWY_NAMESPACE::Add;.

Additionally, each function that calls Highway ops must either be prefixed with HWY_ATTR, OR reside between HWY_BEFORE_NAMESPACE() and HWY_AFTER_NAMESPACE(). Lambda functions currently require HWY_ATTR before their opening brace.

The entry points into code using Highway differ slightly depending on whether they use static or dynamic dispatch.

For static dispatch, HWY_TARGET will be the best available target among HWY_BASELINE_TARGETS, i.e. those allowed for use by the compiler (see quick-reference). Functions inside HWY_NAMESPACE can be called using HWY_STATIC_DISPATCH(func)(args) within the same module they are defined in. You can call the function from other modules by wrapping it in a regular function and declaring the regular function in a header.
For dynamic dispatch, a table of function pointers is generated via the HWY_EXPORT macro that is used by HWY_DYNAMIC_DISPATCH(func)(args) to call the best function pointer for the current CPU's supported targets. A module is automatically compiled for each target in HWY_TARGETS (see quick-reference) if HWY_TARGET_INCLUDE is defined and foreach_target.h is included.

Compiler flags

Applications should be compiled with optimizations enabled - without inlining, SIMD code may slow down by factors of 10 to 100. For clang and GCC, -O2 is generally sufficient.

For MSVC, we recommend compiling with /Gv to allow non-inlined functions to pass vector arguments in registers. If intending to use the AVX2 target together with half-width vectors (e.g. for PromoteTo), it is also important to compile with /arch:AVX2. This seems to be the only way to generate VEX-encoded SSE4 instructions on MSVC. Otherwise, mixing VEX-encoded AVX2 instructions and non-VEX SSE4 may cause severe performance degradation. Unfortunately, the resulting binary will then require AVX2. Note that no such flag is needed for clang and GCC because they support target-specific attributes, which we use to ensure proper VEX code generation for AVX2 targets.

Strip-mining loops

To vectorize a loop, "strip-mining" transforms it into an outer loop and inner loop with number of iterations matching the preferred vector width.

In this section, let T denote the element type, d = ScalableTag<T>, count the number of elements to process, and N = Lanes(d) the number of lanes in a full vector. Assume the loop body is given as a function template<bool partial, class D> void LoopBody(D d, size_t index, size_t max_n).

Highway offers several ways to express loops where N need not divide count:

Ensure all inputs/outputs are padded. Then the loop is simply
```
for (size_t i = 0; i < count; i += N) LoopBody<false>(d, i, 0);
```
Here, the template parameter and second function argument are not needed.

This is the preferred option, unless N is in the thousands and vector operations are pipelined with long latencies. This was the case for supercomputers in the 90s, but nowadays ALUs are cheap and we see most implementations split vectors into 1, 2 or 4 parts, so there is little cost to processing entire vectors even if we do not need all their lanes. Indeed this avoids the (potentially large) cost of predication or partial loads/stores on older targets, and does not duplicate code.
Use the Transform* functions in hwy/contrib/algo/transform-inl.h. This takes care of the loop and remainder handling and you simply define a generic lambda function (C++14) or functor which receives the current vector from the input/output array, plus optionally vectors from up to two extra input arrays, and returns the value to write to the input/output array.

Here is an example implementing the BLAS function SAXPY (alpha * x + y): `Transform1(d, x, n, y, [](auto d, const auto v, const auto v1) HWY_ATTR { return MulAdd(Set(d, alpha), v, v1); });``
Process whole vectors as above, followed by a scalar loop:
```
size_t i = 0;
for (; i + N <= count; i += N) LoopBody<false>(d, i, 0);
for (; i < count; ++i) LoopBody<false>(HWY_CAPPED(T, 1)(), i, 0);
```
The template parameter and second function arguments are again not needed.

This avoids duplicating code, and is reasonable if count is large. If count is small, the second loop may be slower than the next option.
Process whole vectors as above, followed by a single call to a modified LoopBody with masking:
```
size_t i = 0;
for (; i + N <= count; i += N) {
  LoopBody<false>(d, i, 0);
}
if (i < count) {
  LoopBody<true>(d, i, count - i);
}
```
Now the template parameter and third function argument can be used inside LoopBody to non-atomically 'blend' the first num_remaining lanes of v with the previous contents of memory at subsequent locations: BlendedStore(v, FirstN(d, num_remaining), d, pointer);. Similarly, MaskedLoad(FirstN(d, num_remaining), d, pointer) loads the first num_remaining elements and returns zero in other lanes.

This is a good default when it is infeasible to ensure vectors are padded, but is only safe #if !HWY_MEM_OPS_MIGHT_FAULT! In contrast to the scalar loop, only a single final iteration is needed. The increased code size from two loop bodies is expected to be worthwhile because it avoids the cost of masking in all but the final iteration.

Additional resources

Acknowledgments

We have used farm-sve by Berenger Bramas; it has proved useful for checking the SVE port on an x86 development machine.

This is not an officially supported Google product. Contact: janwas@google.com

jonsneyers / highway