SIMD Vector Classes for C++
mattkretz Identify appleclang via __apple_build_version__
Fixes: gh-213

Signed-off-by: Matthias Kretz <kretz@kde.org>
Latest commit 0a7836f Oct 12, 2018
Permalink
Failed to load latest commit information.
.github Add design guidelines and be clearer on naming Apr 7, 2016
Vc Identify appleclang via __apple_build_version__ Oct 12, 2018
cmake Modernize CMake package and add exported targets Sep 3, 2018
doc after release: version 1.4.0-dev Oct 1, 2018
examples Fix includes after director reorganization Sep 12, 2018
math Octave, bc, and gnuplot scripts supporting math dev Aug 21, 2018
src Doxygen cleanup Sep 4, 2018
tests Test operator failures via sfinae_is_callable Sep 25, 2018
.appveyor.yml AppVeyor: use MSVC2017 and init git submodules Sep 18, 2018
.clang-format clang-format: set BinPackParameters Jun 25, 2014
.gitignore Port to latest virtest; add some std::simd compat Sep 3, 2018
.gitmodules Port remaining test to new test interface Sep 3, 2018
.travis.yml Travis: Bump MacOS GCC versions and allow them to fail Sep 13, 2018
CMakeLists.txt after release: version 1.4.0-dev Oct 1, 2018
CTestConfig.cmake CMake: USE_LAUNCHERS must not be set in CTestConfig Jan 16, 2017
CTestCustom.cmake CTest: drop old warning suppression Apr 18, 2018
INSTALL add README.md Jul 2, 2015
LICENSE Drop 'all rights reserved' Apr 11, 2016
Makefile Standalone toplevel Makefile Aug 22, 2018
README.md release: version 1.4.0 Oct 1, 2018
Test_all_compilers.sh CTest: Quick exit for ICC<18 & less useless error codes Sep 13, 2018
Test_vc.sh Remove unnecessary grep Mar 31, 2016
changeVersion.sh Bump version to 1.3.80-dev Sep 3, 2018
configure-icc.sh simple script to help setup a icc build dir Aug 13, 2009
godbolt.h Update godbolt header Oct 11, 2018
makeApidox.sh docs: remove .qhp file when done Aug 6, 2014
makeApidoxLoop.sh add simple script to rebuild api docs automatically Oct 22, 2014
makeInternalDox.sh dox: new script to build internal docs Aug 5, 2014
makeRelease.sh Adjust to new version.h path and fix commit Oct 1, 2018
prepare_single_header.sh Update godbolt header Oct 11, 2018
print_target_architecture.cmake ctest: fix print_target_architecture usage Nov 24, 2016
test.cmake CTest: No "install/fast" with ninja Oct 1, 2018

README.md

Vc: portable, zero-overhead C++ types for explicitly data-parallel programming

Recent generations of CPUs, and GPUs in particular, require data-parallel codes for full efficiency. Data parallelism requires that the same sequence of operations is applied to different input data. CPUs and GPUs can thus reduce the necessary hardware for instruction decoding and scheduling in favor of more arithmetic and logic units, which execute the same instructions synchronously. On CPU architectures this is implemented via SIMD registers and instructions. A single SIMD register can store N values and a single SIMD instruction can execute N operations on those values. On GPU architectures N threads run in perfect sync, fed by a single instruction decoder/scheduler. Each thread has local memory and a given index to calculate the offsets in memory for loads and stores.

Current C++ compilers can do automatic transformation of scalar codes to SIMD instructions (auto-vectorization). However, the compiler must reconstruct an intrinsic property of the algorithm that was lost when the developer wrote a purely scalar implementation in C++. Consequently, C++ compilers cannot vectorize any given code to its most efficient data-parallel variant. Especially larger data-parallel loops, spanning over multiple functions or even translation units, will often not be transformed into efficient SIMD code.

The Vc library provides the missing link. Its types enable explicitly stating data-parallel operations on multiple values. The parallelism is therefore added via the type system. Competing approaches state the parallelism via new control structures and consequently new semantics inside the body of these control structures.

Vc is a free software library to ease explicit vectorization of C++ code. It has an intuitive API and provides portability between different compilers and compiler versions as well as portability between different vector instruction sets. Thus an application written with Vc can be compiled for:

  • AVX and AVX2
  • SSE2 up to SSE4.2 or SSE4a
  • Scalar
  • AVX-512 (Vc 2 development)
  • NEON (in development)
  • NVIDIA GPUs / CUDA (research)

After Intel dropped MIC support with ICC 18, Vc 1.4 also removes support for it.

Examples

Usage on Compiler Explorer

Scalar Product

Let's start from the code for calculating a 3D scalar product using builtin floats:

using Vec3D = std::array<float, 3>;
float scalar_product(Vec3D a, Vec3D b) {
  return a[0] * b[0] + a[1] * b[1] + a[2] * b[2];
}

Using Vc, we can easily vectorize the code using the float_v type:

using Vc::float_v
using Vec3D = std::array<float_v, 3>;
float_v scalar_product(Vec3D a, Vec3D b) {
  return a[0] * b[0] + a[1] * b[1] + a[2] * b[2];
}

The above will scale to 1, 4, 8, 16, etc. scalar products calculated in parallel, depending on the target hardware's capabilities.

For comparison, the same vectorization using Intel SSE intrinsics is more verbose and uses prefix notation (i.e. function calls):

using Vec3D = std::array<__m128, 3>;
__m128 scalar_product(Vec3D a, Vec3D b) {
  return _mm_add_ps(_mm_add_ps(_mm_mul_ps(a[0], b[0]), _mm_mul_ps(a[1], b[1])),
                    _mm_mul_ps(a[2], b[2]));
}

The above will neither scale to AVX, AVX-512, etc. nor is it portable to other SIMD ISAs.

Build Requirements

cmake >= 3.0

C++11 Compiler:

  • GCC >= 4.8.1
  • clang >= 3.4
  • ICC >= 18
  • Visual Studio 2015 (64-bit target)

Building and Installing Vc

  • After cloning, you need to initialize Vc's git submodules:
git submodule update --init
  • Create a build directory:
$ mkdir build
$ cd build
  • Call cmake with the relevant options:
$ cmake -DCMAKE_INSTALL_PREFIX=/opt/Vc -DBUILD_TESTING=OFF <srcdir>
  • Build and install:
$ make -j16
$ make install

Documentation

The documentation is generated via doxygen. You can build the documentation by running doxygen in the doc subdirectory. Alternatively, you can find nightly builds of the documentation at:

Publications

Work on integrating the functionality of Vc in the C++ standard library.

Communication

A channel on the freenode IRC network is reserved for discussions on Vc: ##vc on freenode (via SSL)

Feel free to use the GitHub issue tracker for questions. Alternatively, there's a mailinglist for users of Vc

License

Vc is released under the terms of the 3-clause BSD license.