This is a comprehensive guide to the three main usage stories of the Halide CMake build.
- Compiling or packaging Halide from source.
- Building Halide programs using the official CMake package.
- Contributing to Halide and updating the build files.
The following sections cover each in detail.
- Halide and CMake
- Getting started
- Building Halide with CMake
- Using Halide from your CMake build
- Contributing CMake code to Halide
This section covers installing a recent version of CMake and the correct dependencies for building and using Halide. If you have not used CMake before, we strongly suggest reading through the CMake documentation first.
Halide requires at least version 3.22, which was released in November 2021. Fortunately, getting a recent version of CMake couldn't be easier, and there are multiple good options on any system to do so. Generally, one should always have the most recent version of CMake installed system-wide. CMake is committed to backwards compatibility and even the most recent release can build projects over a decade old.
The Python package manager pip3 has the newest version of CMake at all times.
This might be the most convenient method since Python 3 is an optional
dependency for Halide, anyway.
$ pip3 install --upgrade cmake
See the PyPI website for more details.
On Windows, there are three primary methods for installing an up-to-date CMake:
- If you have Visual Studio 2019 installed, you can get CMake 3.17 through the Visual Studio installer. This is the recommended way of getting CMake if you are able to use Visual Studio 2019. See Microsoft's documentation for more details.
- If you use Chocolatey, its CMake package is kept
up to date. It should be as simple as
choco install cmake. - Otherwise, you should install CMake from Kitware's website.
On macOS, the Homebrew CMake package is kept up to date. Simply run:
$ brew update
$ brew install cmake
to install the newest version of CMake. If your environment prevents you from installing Homebrew, the binary release on Kitware's website is also a viable option.
There are a few good ways to install a modern CMake on Ubuntu:
- If you're on Ubuntu Linux 22.04 (Jammy Jellyfish), then simply running
sudo apt install cmakewill get you CMake 3.22. - If you are on an older Ubuntu release or would like to use the newest CMake,
try installing via the snap store:
snap install cmake. Be sure you do not already havecmakeinstalled via APT. The snap package automatically stays up to date. - For older versions of Debian, Ubuntu, Mint, and derivatives, Kitware provides an APT repository with up-to-date releases. Note that this is still useful for Ubuntu 20.04 because it will remain up to date.
- If all else fails, you might need to build CMake from source (eg. on old Ubuntu versions running on ARM). In that case, follow the directions posted on Kitware's website.
For other Linux distributions, check with your distribution's package manager or use pip as detailed above. Snap packages might also be available.
Note: On WSL 1, the snap service is not available; in this case, prefer to use the APT repository. On WSL 2, all methods are available.
We generally recommend using a package manager to fetch Halide's dependencies. Except where noted, we recommend using vcpkg on Windows, Homebrew on macOS, and APT on Ubuntu 20.04 LTS.
Only LLVM and Clang are absolutely required to build Halide. Halide always supports three LLVM versions: the current major version, the previous major version, and trunk. The LLVM and Clang versions must match exactly. For most users, we recommend using a binary release of LLVM rather than building it yourself.
However, to run all of the tests and apps, an extended set is needed. This includes lld, Python 3, libpng, libjpeg, Doxygen, OpenBLAS, ATLAS, and Eigen3. While not required to build any part of Halide, we find that Ninja is the best backend build tool across all platforms.
Note that CMake has many special variables for overriding the locations of packages and executables. A partial list can be found in the "find module options" section below, and more can be found in the documentation for the CMake find_package command. Normally, you should prefer to make sure your environment is set up so that CMake can find dependencies automatically. For instance, if you want CMake to use a particular version of Python, create a virtual environment and activate it before configuring Halide.
We assume you have vcpkg installed at D:\vcpkg. Follow the instructions in the
vcpkg README to install. Start by installing LLVM.
D:\vcpkg> .\vcpkg install llvm[target-all,enable-assertions,clang-tools-extra]:x64-windows
D:\vcpkg> .\vcpkg install llvm[target-all,enable-assertions,clang-tools-extra]:x86-windows
This will also install Clang and LLD. The enable-assertions option is not
strictly necessary but will make debugging during development much smoother.
These builds will take a long time and a lot of disk space. After they are
built, it is safe to delete the intermediate build files and caches in
D:\vcpkg\buildtrees and %APPDATA%\local\vcpkg.
Then install the other libraries:
D:\vcpkg> .\vcpkg install libpng:x64-windows libjpeg-turbo:x64-windows openblas:x64-windows eigen3:x64-windows
D:\vcpkg> .\vcpkg install libpng:x86-windows libjpeg-turbo:x86-windows openblas:x86-windows eigen3:x86-windows
To build the documentation, you will need to install Doxygen. This can be done either through Chocolatey or from the Doxygen website.
> choco install doxygen
To build the Python bindings, you will need to install Python 3. This should be
done by running the official installer from the Python website. Be
sure to download the debugging symbols through the installer. This will require
using the "Advanced Installation" workflow. Although it is not strictly
necessary, it is convenient to install Python system-wide on Windows (ie.
C:\Program Files). This makes it easy for CMake to find without needing to
manually set the PATH.
Once Python is installed, you can install the Python module dependencies either globally or in a virtual environment by running
> pip3 install -r .\python_bindings\requirements.txt
from the root of the repository.
If you would like to use Ninja, note that it is installed alongside CMake when using the Visual Studio 2019 installer. Alternatively, you can install via Chocolatey or place the pre-built binary from their website in the PATH.
> choco install ninja
On macOS, it is possible to install all dependencies via Homebrew:
$ brew install llvm libpng libjpeg [email protected] openblas doxygen ninja
The llvm package includes clang, clang-format, and lld, too. Don't
forget to install the Python module dependencies:
$ pip3 install -r python_bindings/requirements.txt
Finally, on Ubuntu 20.04 LTS, you should install the following packages (this includes the Python module dependencies):
dev@ubuntu:~$ sudo apt install \
clang-tools lld llvm-dev libclang-dev liblld-10-dev \
libpng-dev libjpeg-dev libgl-dev \
python3-dev python3-numpy python3-scipy python3-imageio python3-pybind11 \
libopenblas-dev libeigen3-dev libatlas-base-dev \
doxygen ninja-build
These instructions assume that your working directory is the Halide repo root.
If you plan to use the Ninja generator, be sure to be in the developer command prompt corresponding to your intended environment. Note that whatever your intended target system (x86, x64, or arm), you must use the 64-bit host tools because the 32-bit tools run out of memory during the linking step with LLVM. More information is available from Microsoft's documentation.
You should either open the correct Developer Command Prompt directly or run the
vcvarsall.bat script with the correct argument, ie. one of the
following:
D:\> "C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Auxiliary\Build\vcvarsall.bat" x64
D:\> "C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Auxiliary\Build\vcvarsall.bat" x64_x86
D:\> "C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Auxiliary\Build\vcvarsall.bat" x64_arm
Then, assuming that vcpkg is installed to D:\vcpkg, simply run:
> cmake -G Ninja -DCMAKE_BUILD_TYPE=Release -DCMAKE_TOOLCHAIN_FILE=D:\vcpkg\scripts\buildsystems\vcpkg.cmake -S . -B build
> cmake --build .\build
Valid values of CMAKE_BUILD_TYPE are Debug,
RelWithDebInfo, MinSizeRel, and Release. When using a single-configuration
generator (like Ninja) you must specify a build type when configuring Halide (or
any other CMake project).
Otherwise, if you wish to create a Visual Studio based build system, you can configure with:
> cmake -G "Visual Studio 16 2019" -Thost=x64 -A x64 ^
-DCMAKE_TOOLCHAIN_FILE=D:\vcpkg\scripts\buildsystems\vcpkg.cmake ^
-S . -B build
> cmake --build .\build --config Release -j %NUMBER_OF_PROCESSORS%
Because the Visual Studio generator is a multi-config generator, you don't set
CMAKE_BUILD_TYPE at configure-time, but instead pass the configuration to the
build (and test/install) commands with the --config flag. More documentation
is available in the CMake User Interaction Guide.
The process is similar for 32-bit:
> cmake -G "Visual Studio 16 2019" -Thost=x64 -A Win32 ^
-DCMAKE_TOOLCHAIN_FILE=D:\vcpkg\scripts\buildsystems\vcpkg.cmake ^
-S . -B build
> cmake --build .\build --config Release -j %NUMBER_OF_PROCESSORS%
In both cases, the -Thost=x64 flag ensures that the correct host tools are
used.
Note: due to limitations in MSBuild, incremental builds using the VS
generators will not detect changes to headers in the src/runtime folder. We
recommend using Ninja for day-to-day development and use Visual Studio only if
you need it for packaging.
The instructions here are straightforward. Assuming your environment is set up correctly, just run:
dev@host:~/Halide$ cmake -G Ninja -DCMAKE_BUILD_TYPE=Release -S . -B build
dev@host:~/Halide$ cmake --build ./build
If you omit -G Ninja, a Makefile-based generator will likely be used instead.
In either case, CMAKE_BUILD_TYPE must be set to one of the
standard types: Debug, RelWithDebInfo, MinSizeRel, or Release.
If you are using CMake 3.21+, we provide several presets to make the above commands more convenient. The following CMake preset commands correspond to the longer ones above.
> cmake --preset=win64 # VS 2019 generator, 64-bit build, vcpkg deps
> cmake --preset=win32 # VS 2019 generator, 32-bit build, vcpkg deps
> cmake --preset=release # Release mode, any single-config generator / compiler
$ cmake --list-presets # Get full list of presets.
The Windows presets assume that the environment variable VCPKG_ROOT is set and
points to the root of the vcpkg installation.
There are also presets to use some Clang sanitizers with the CMake build; at present, only Fuzzer and ASAN (Address Sanitizer) are supported, and only on linux-x86-64. To use these, you must build LLVM with additional options:
-D LLVM_ENABLE_PROJECTS="clang;lld;clang-tools-extra"
-D LLVM_ENABLE_RUNTIMES="compiler-rt;libcxx;libcxxabi;libunwind"
To build / test with ASAN, use --preset linux-x64-asan.
To build / test with the Fuzzer, use --preset linux-x64-fuzzer.
Once built, Halide will need to be installed somewhere before using it in a
separate project. On any platform, this means running the
cmake --install command in one of two ways. For a
single-configuration generator (like Ninja), run either:
dev@host:~/Halide$ cmake --install ./build --prefix /path/to/Halide-install
> cmake --install .\build --prefix X:\path\to\Halide-install
For a multi-configuration generator (like Visual Studio) run:
dev@host:~/Halide$ cmake --install ./build --prefix /path/to/Halide-install --config Release
> cmake --install .\build --prefix X:\path\to\Halide-install --config Release
Of course, make sure that you build the corresponding config before attempting to install it.
Halide reads and understands several options that can configure the build. The following are the most consequential and control how Halide is actually compiled.
| Option | Default | Description |
|---|---|---|
BUILD_SHARED_LIBS |
ON |
Standard CMake variable that chooses whether to build as a static or shared library. |
Halide_BUNDLE_LLVM |
OFF |
When building Halide as a static library, unpack the LLVM static libraries and add those objects to libHalide.a. |
Halide_SHARED_LLVM |
OFF |
Link to the shared version of LLVM. Not available on Windows. |
Halide_ENABLE_RTTI |
inherited from LLVM | Enable RTTI when building Halide. Recommended to be set to ON |
Halide_ENABLE_EXCEPTIONS |
ON |
Enable exceptions when building Halide |
Halide_TARGET |
empty | The default target triple to use for add_halide_library (and the generator tests, by extension) |
The following options are advanced and should not be required in typical workflows. Generally, these are used by Halide's own CI infrastructure, or as escape hatches for third-party packagers.
| Option | Default | Description |
|---|---|---|
Halide_CLANG_TIDY_BUILD |
OFF |
Used internally to generate fake compile jobs for runtime files when running clang-tidy. |
Halide_CCACHE_BUILD |
OFF |
Use ccache with Halide-recommended settings to accelerate rebuilds. |
Halide_CCACHE_PARAMS |
CCACHE_CPP2=yes CCACHE_HASHDIR=yes CCACHE_SLOPPINESS=pch_defines |
Options to pass to ccache when using Halide_CCACHE_BUILD. |
Halide_SOVERSION_OVERRIDE |
${Halide_VERSION_MAJOR} |
Override the SOVERSION for libHalide. Expects a positive integer (i.e. not a version). |
The following options are only available when building Halide directly, ie. not
through the add_subdirectory or
FetchContent mechanisms. They control whether non-essential
targets (like tests and documentation) are built.
| Option | Default | Description |
|---|---|---|
WITH_TESTS |
ON |
Enable building unit and integration tests |
WITH_PYTHON_BINDINGS |
ON if Python found |
Enable building Python 3.x bindings |
WITH_DOCS |
OFF |
Enable building the documentation via Doxygen |
WITH_UTILS |
ON |
Enable building various utilities including the trace visualizer |
WITH_TUTORIALS |
ON |
Enable building the tutorials |
The following options control whether to build certain test subsets. They only
apply when WITH_TESTS=ON:
| Option | Default | Description |
|---|---|---|
WITH_TEST_AUTO_SCHEDULE |
ON |
enable the auto-scheduling tests |
WITH_TEST_CORRECTNESS |
ON |
enable the correctness tests |
WITH_TEST_ERROR |
ON |
enable the expected-error tests |
WITH_TEST_WARNING |
ON |
enable the expected-warning tests |
WITH_TEST_PERFORMANCE |
ON |
enable performance testing |
WITH_TEST_GENERATOR |
ON |
enable the AOT generator tests |
The following options enable/disable various LLVM backends (they correspond to LLVM component names):
| Option | Default | Description |
|---|---|---|
TARGET_AARCH64 |
ON, if available |
Enable the AArch64 backend |
TARGET_AMDGPU |
ON, if available |
Enable the AMD GPU backend |
TARGET_ARM |
ON, if available |
Enable the ARM backend |
TARGET_HEXAGON |
ON, if available |
Enable the Hexagon backend |
TARGET_NVPTX |
ON, if available |
Enable the NVidia PTX backend |
TARGET_POWERPC |
ON, if available |
Enable the PowerPC backend |
TARGET_RISCV |
ON, if available |
Enable the RISC V backend |
TARGET_WEBASSEMBLY |
ON, if available |
Enable the WebAssembly backend. |
TARGET_X86 |
ON, if available |
Enable the x86 (and x86_64) backend |
The following options enable/disable various Halide-specific backends:
| Option | Default | Description |
|---|---|---|
TARGET_OPENCL |
ON |
Enable the OpenCL-C backend |
TARGET_METAL |
ON |
Enable the Metal backend |
TARGET_D3D12COMPUTE |
ON |
Enable the Direct3D 12 Compute backend |
The following options are WebAssembly-specific. They only apply when
TARGET_WEBASSEMBLY=ON:
| Option | Default | Description |
|---|---|---|
WITH_WABT |
ON |
Include WABT Interpreter for WASM testing |
Halide uses the following find modules to search for certain dependencies. These
modules accept certain variables containing hints for the search process. Before
setting any of these variables, closely study the find_package
documentation.
All of these variables should be set at the CMake command line via the -D
flag.
First, Halide expects to find LLVM and Clang through the CONFIG mode of
find_package. You can tell Halide where to find these dependencies by setting
the corresponding _DIR variables:
| Variable | Description |
|---|---|
LLVM_DIR |
$LLVM_ROOT/lib/cmake/LLVM/LLVMConfig.cmake |
Clang_DIR |
$LLVM_ROOT/lib/cmake/Clang/ClangConfig.cmake |
Here, $LLVM_ROOT is assumed to point to the root of an LLVM installation tree.
This is either a system path or one produced by running cmake --install (as
detailed in the main README.md). When building LLVM (and any other CONFIG
packages) manually, it is a common mistake to point CMake to a build tree
rather than an install tree. Doing so often produces inscrutable errors.
When using CMake 3.18 or above, some of Halide's tests will search for CUDA
using the FindCUDAToolkit module. If it doesn't find your
CUDA installation automatically, you can point it to it by setting:
| Variable | Description |
|---|---|
CUDAToolkit_ROOT |
Path to the directory containing bin/nvcc[.exe] |
CUDA_PATH |
Environment variable, same as above. |
If the CMake version is lower than 3.18, the deprecated FindCUDA
module will be used instead. It reads the variable CUDA_TOOLKIT_ROOT_DIR
instead of CUDAToolkit_ROOT above.
Halide also searches for libpng and libjpeg-turbo through the
FindPNG and FindJPEG modules, respectively. They can
be overridden by setting the following variables.
| Variable | Description |
|---|---|
PNG_LIBRARIES |
Paths to the libraries to link against to use PNG. |
PNG_INCLUDE_DIRS |
Path to png.h, etc. |
JPEG_LIBRARIES |
Paths to the libraries needed to use JPEG. |
JPEG_INCLUDE_DIRS |
Paths to jpeglib.h, etc. |
When WITH_DOCS is set to ON, Halide searches for Doxygen using the
FindDoxygen module. It can be overridden by setting the
following variable.
| Variable | Description |
|---|---|
DOXYGEN_EXECUTABLE |
Path to the Doxygen executable. |
When compiling for an OpenCL target, Halide uses the FindOpenCL
target to locate the libraries and include paths. These can be overridden by
setting the following variables:
| Variable | Description |
|---|---|
OpenCL_LIBRARIES |
Paths to the libraries to link against to use OpenCL. |
OpenCL_INCLUDE_DIRS |
Include directories for OpenCL. |
Lastly, Halide searches for Python 3 using the FindPython3
module, not the deprecated FindPythonInterp and FindPythonLibs modules,
like other projects you might have encountered. You can select which Python
installation to use by setting the following variable.
| Variable | Description |
|---|---|
Python3_ROOT_DIR |
Define the root directory of a Python 3 installation. |
This section assumes some basic familiarity with CMake but tries to be explicit in all its examples. To learn more about CMake, consult the documentation and engage with the community on the CMake Discourse.
Note: previous releases bundled a halide.cmake module that was meant to be
include()-ed into your project. This has been removed. Please
upgrade to the new package config module.
There are two main ways to use Halide in your application: as a JIT compiler for dynamic pipelines or an ahead-of-time (AOT) compiler for static pipelines. CMake provides robust support for both use cases.
No matter how you intend to use Halide, you will need some basic CMake boilerplate.
cmake_minimum_required(VERSION 3.22)
project(HalideExample)
set(CMAKE_CXX_STANDARD 17) # or newer
set(CMAKE_CXX_STANDARD_REQUIRED YES)
set(CMAKE_CXX_EXTENSIONS NO)
find_package(Halide REQUIRED)The cmake_minimum_required command is required to be
the first command executed in a CMake program. It disables all of the deprecated
behavior ("policies" in CMake lingo) from earlier versions. The
project command sets the name of the project (and has arguments for
versioning, language support, etc.) and is required by CMake to be called
immediately after setting the minimum version.
The next three variables set the project-wide C++ standard. The first,
CMAKE_CXX_STANDARD, simply sets the standard version.
Halide requires at least C++17. The second,
CMAKE_CXX_STANDARD_REQUIRED, tells CMake to
fail if the compiler cannot provide the requested standard version. Lastly,
CMAKE_CXX_EXTENSIONS tells CMake to disable
vendor-specific extensions to C++. This is not necessary to simply use Halide,
but we require it when authoring new code in the Halide repo.
Finally, we use find_package to locate Halide on your system.
If Halide is not globally installed, you will need to add the root of the Halide
installation directory to CMAKE_PREFIX_PATH at the CMake
command line.
dev@ubuntu:~/myproj$ cmake -G Ninja -DCMAKE_BUILD_TYPE=Release -DCMAKE_PREFIX_PATH="/path/to/Halide-install" -S . -B build
To use Halide in JIT mode (like the tutorials do, for
example), you can simply link to Halide::Halide.
# ... same project setup as before ...
add_executable(my_halide_app main.cpp)
target_link_libraries(my_halide_app PRIVATE Halide::Halide)Then Halide.h will be available to your code and everything should just work.
That's it!
Using Halide in AOT mode is more complicated so we'll walk through it step by
step. Note that this only applies to Halide generators, so it might be useful to
re-read the tutorial on generators. Assume (like in
the tutorial) that you have a source file named my_generators.cpp and that in
it you have generator classes MyFirstGenerator and MySecondGenerator with
registered names my_first_generator and my_second_generator respectively.
Then the first step is to add a generator executable to your build:
# ... same project setup as before ...
add_executable(my_generators my_generators.cpp)
target_link_libraries(my_generators PRIVATE Halide::Generator)Using the generator executable, we can add a Halide library corresponding to
MyFirstGenerator.
# ... continuing from above
add_halide_library(my_first_generator FROM my_generators)This will create a static library target in CMake that corresponds to the output of running your generator. The second generator in the file requires generator parameters to be passed to it. These are also easy to handle:
# ... continuing from above
add_halide_library(my_second_generator FROM my_generators
PARAMS parallel=false scale=3.0 rotation=ccw output.type=uint16)Adding multiple configurations is easy, too:
# ... continuing from above
add_halide_library(my_second_generator_2 FROM my_generators
GENERATOR my_second_generator
PARAMS scale=9.0 rotation=ccw output.type=float32)
add_halide_library(my_second_generator_3 FROM my_generators
GENERATOR my_second_generator
PARAMS parallel=false output.type=float64)Here, we had to specify which generator to use (my_second_generator) since it
uses the target name by default. The functions in these libraries will be named
after the target names, my_second_generator_2 and my_second_generator_3, by
default, but it is possible to control this via the FUNCTION_NAME parameter.
Each one of these targets, <GEN>, carries an associated <GEN>.runtime
target, which is also a static library containing the Halide runtime. It is
transitively linked through <GEN> to targets that link to <GEN>. On an
operating system like Linux, where weak linking is available, this is not an
issue. However, on Windows, this can fail due to symbol redefinitions. In these
cases, you must declare that two Halide libraries share a runtime, like so:
# ... updating above
add_halide_library(my_second_generator_2 FROM my_generators
GENERATOR my_second_generator
USE_RUNTIME my_first_generator.runtime
PARAMS scale=9.0 rotation=ccw output.type=float32)
add_halide_library(my_second_generator_3 FROM my_generators
GENERATOR my_second_generator
USE_RUNTIME my_first_generator.runtime
PARAMS parallel=false output.type=float64)This will even work correctly when different combinations of targets are specified for each halide library. A "greatest common denominator" target will be chosen that is compatible with all of them (or the build will fail).
When the autoschedulers are included in the release package, they are very
simple to apply to your own generators. For example, we could update the
definition of the my_first_generator library above to use the Adams2019
autoscheduler:
add_halide_library(my_second_generator FROM my_generators
AUTOSCHEDULER Halide::Adams2019)Halide provides a generic driver for generators to be used during development
for benchmarking and debugging. Suppose you have a generator executable called
my_gen and a generator within called my_filter. Then you can pass a variable
name to the REGISTRATION parameter of add_halide_library which will contain
the name of a generated C++ source that should be linked to Halide::RunGenMain
and my_filter.
For example:
add_halide_library(my_filter FROM my_gen
REGISTRATION filter_reg_cpp)
add_executable(runner ${filter_reg_cpp})
target_link_libraries(runner PRIVATE my_filter Halide::RunGenMain)Then you can run, debug, and benchmark your generator through the runner
executable.
Halide provides a CMake package configuration module. The intended way to use
the CMake build is to run find_package(Halide ...) in your CMakeLists.txt
file. Closely read the find_package documentation before
proceeding.
The Halide package script understands a handful of optional components when loading the package.
First, if you plan to use the Halide Image IO library, you will want to include
the png and jpeg components when loading Halide.
Second, Halide releases can contain a variety of configurations: static, shared, debug, release, etc. CMake handles Debug/Release configurations automatically, but generally only allows one type of library to be loaded.
The package understands two components, static and shared, that specify
which type of library you would like to load. For example, if you want to make
sure that you link against shared Halide, you can write:
find_package(Halide REQUIRED COMPONENTS shared)If the shared libraries are not available, this will result in a failure.
If no component is specified, then the Halide_SHARED_LIBS variable is checked.
If it is defined and set to true, then the shared libraries will be loaded or
the package loading will fail. Similarly, if it is defined and set to false, the
static libraries will be loaded.
If no component is specified and Halide_SHARED_LIBS is not defined, then the
BUILD_SHARED_LIBS variable will be inspected. If it is
not defined or defined and set to true, then it will attempt to load the
shared libs and fall back to the static libs if they are not available.
Similarly, if BUILD_SHARED_LIBS is defined and set to false, then it will
try the static libs first then fall back to the shared libs.
Variables that control package loading:
| Variable | Description |
|---|---|
Halide_SHARED_LIBS |
override BUILD_SHARED_LIBS when loading the Halide package via find_package. Has no effect when using Halide via add_subdirectory as a Git or FetchContent submodule. |
Halide_RUNTIME_NO_THREADS |
skip linking of Threads library to runtime. Should be set if your toolchain does not support it (e.g. baremetal). |
Halide_RUNTIME_NO_DL_LIBS |
skip linking of DL library to runtime. Should be set if your toolchain does not support it (e.g. baremetal). |
Variables set by the package:
| Variable | Description |
|---|---|
Halide_VERSION |
The full version string of the loaded Halide package |
Halide_VERSION_MAJOR |
The major version of the loaded Halide package |
Halide_VERSION_MINOR |
The minor version of the loaded Halide package |
Halide_VERSION_PATCH |
The patch version of the loaded Halide package |
Halide_VERSION_TWEAK |
The tweak version of the loaded Halide package |
Halide_HOST_TARGET |
The Halide target triple corresponding to "host" for this build. |
Halide_CMAKE_TARGET |
The Halide target triple corresponding to the active CMake target. |
Halide_ENABLE_EXCEPTIONS |
Whether Halide was compiled with exception support |
Halide_ENABLE_RTTI |
Whether Halide was compiled with RTTI |
Variables that control package behavior:
| Variable | Description |
|---|---|
Halide_PYTHON_LAUNCHER |
Semicolon separated list containing a command to launch the Python interpreter. Can be used to set environment variables for Python generators. |
Halide_NO_DEFAULT_FLAGS |
Off by default. When enabled, suppresses recommended compiler flags that would be added by add_halide_generator |
Halide defines the following targets that are available to users:
| Imported target | Description |
|---|---|
Halide::Halide |
this is the JIT-mode library to use when using Halide from C++. |
Halide::Generator |
this is the target to use when defining a generator executable. It supplies a main() function. |
Halide::Runtime |
adds include paths to the Halide runtime headers |
Halide::Tools |
adds include paths to the Halide tools, including the benchmarking utility. |
Halide::ImageIO |
adds include paths to the Halide image IO utility. Depends on PNG::PNG and JPEG::JPEG if they exist or were loaded through the corresponding package components. |
Halide::ThreadPool |
adds include paths to the Halide simple thread pool utility library. This is not the same as the runtime's thread pool and is intended only for use by tests. Depends on Threads::Threads. |
Halide::RunGenMain |
used with the REGISTRATION parameter of add_halide_library to create simple runners and benchmarking tools for Halide libraries. |
The following targets are not guaranteed to be available:
| Imported target | Description |
|---|---|
Halide::Python |
this is a Python 3 package that can be referenced as $<TARGET_FILE_DIR:Halide::Python>/.. when setting up PYTHONPATH for Python tests or the like from CMake. |
Halide::Adams19 |
the Adams et.al. 2019 autoscheduler (no GPU support) |
Halide::Li18 |
the Li et.al. 2018 gradient autoscheduler (limited GPU support) |
Halide::Mullapudi2016 |
the Mullapudi et.al. 2016 autoscheduler (no GPU support) |
Currently, only two functions are defined:
This is the main function for managing generators in AOT compilation. The full signature follows:
add_halide_library(<target> FROM <generator-target>
[GENERATOR generator-name]
[FUNCTION_NAME function-name]
[NAMESPACE cpp-namespace]
[USE_RUNTIME hl-target]
[PARAMS param1 [param2 ...]]
[TARGETS target1 [target2 ...]]
[FEATURES feature1 [feature2 ...]]
[PLUGINS plugin1 [plugin2 ...]]
[AUTOSCHEDULER scheduler-name]
[GRADIENT_DESCENT]
[C_BACKEND]
[REGISTRATION OUTVAR]
[HEADER OUTVAR]
[FUNCTION_INFO_HEADER OUTVAR]
[<extra-output> OUTVAR])
extra-output = ASSEMBLY | BITCODE | COMPILER_LOG | FEATURIZATION
| LLVM_ASSEMBLY | PYTHON_EXTENSION
| PYTORCH_WRAPPER | SCHEDULE | STMT | STMT_HTML
This function creates a called <target> corresponding to running the
<generator-target> (an executable target which links to Halide::Generator)
one time, using command line arguments derived from the other parameters.
The arguments GENERATOR and FUNCTION_NAME default to <target>. They
correspond to the -g and -f command line flags, respectively.
NAMESPACE is syntactic sugar to specify the C++ namespace (if any) of the
generated function; you can also specify the C++ namespace (if any) directly
in the FUNCTION_NAME argument, but for repeated declarations or very long
namespaces, specifying this separately can provide more readable build files.
If USE_RUNTIME is not specified, this function will create another target
called <target>.runtime which corresponds to running the generator with -r
and a compatible list of targets. This runtime target is an INTERFACE dependency
of <target>. If multiple runtime targets need to be linked together, setting
USE_RUNTIME to another Halide library, <target2> will prevent the generation
of <target>.runtime and instead use <target2>.runtime. This argument is
most commonly used in conjunction with add_halide_runtime.
Parameters can be passed to a generator via the PARAMS argument. Parameters
should be space-separated. Similarly, TARGETS is a space-separated list of
targets for which to generate code in a single function. They must all share the
same platform/bits/os triple (eg. arm-32-linux). Features that are in common
among all targets, including device libraries (like cuda) should go in
FEATURES. If TARGETS is not specified, the value of Halide_TARGET specified
at configure time will be used.
Every element of TARGETS must begin with the same arch-bits-os triple. This
function understands two meta-triples, host and cmake. The meta-triple
host is equal to the arch-bits-os triple used to compile Halide along with
all of the supported instruction set extensions. On platforms that support
running both 32 and 64-bit programs, this will not necessarily equal the
platform the compiler is running on or that CMake is targeting.
The meta-triple cmake is equal to the arch-bits-os of the current CMake
target. This is useful if you want to make sure you are not unintentionally
cross-compiling, which would result in an IMPORTED target
being created. When TARGETS is empty and the host target would not
cross-compile, then host will be used. Otherwise, cmake will be used and an
author warning will be issued.
To use an autoscheduler, set the AUTOSCHEDULER argument to a target
named like Namespace::Scheduler, for example Halide::Adams19. This will set
the autoscheduler GeneratorParam on the generator command line to Scheduler
and add the target to the list of plugins. Additional plugins can be loaded by
setting the PLUGINS argument. If the argument to AUTOSCHEDULER does not
contain :: or it does not name a target, it will be passed to the -s flag
verbatim.
If GRADIENT_DESCENT is set, then the module will be built suitably for
gradient descent calculation in TensorFlow or PyTorch. See
Generator::build_gradient_module() for more documentation. This corresponds to
passing -d 1 at the generator command line.
If the C_BACKEND option is set, this command will invoke the configured C++
compiler on a generated source. Note that a <target>.runtime target is not
created in this case, and the USE_RUNTIME option is ignored. Other options
work as expected.
If REGISTRATION is set, the path (relative to CMAKE_CURRENT_BINARY_DIR)
to the generated .registration.cpp file will be set in OUTVAR. This can be
used to generate a runner for a Halide library that is useful for benchmarking
and testing, as documented above. This is equivalent to setting
-e registration at the generator command line.
If HEADER is set, the path (relative to CMAKE_CURRENT_BINARY_DIR) to the
generated .h header file will be set in OUTVAR. This can be used with
install(FILES) to conveniently deploy the generated header along with your
library.
If FUNCTION_INFO_HEADER is set, the path (relative to
CMAKE_CURRENT_BINARY_DIR) to the generated .function_info.h header file
will be set in OUTVAR. This produces a file that contains constexpr
descriptions of information about the generated functions (e.g., argument
type and information). It is generated separately from the normal HEADER
file because HEADER is intended to work with basic extern "C" linkage,
while FUNCTION_INFO_HEADER requires C++17 or later to use effectively.
(This can be quite useful for advanced usages, such as producing automatic
call wrappers, etc.) Examples of usage can be found in the generated file.
Lastly, each of the extra-output arguments directly correspond to an extra
output (via -e) from the generator. The value OUTVAR names a variable into
which a path (relative to
CMAKE_CURRENT_BINARY_DIR) to the extra file will
be written.
This function aids in creating cross-compilable builds that use Halide generators.
add_halide_generator(
target
[PACKAGE_NAME package-name]
[PACKAGE_NAMESPACE namespace]
[EXPORT_FILE export-file]
[PYSTUB generator-name]
[[SOURCES] source1 ...]
)
Every named argument is optional, and the function uses the following default arguments:
- If
PACKAGE_NAMEis not provided, it defaults to${PROJECT_NAME}-halide_generators. - If
PACKAGE_NAMESPACEis not provided, it defaults to${PROJECT_NAME}::halide_generators::. - If
EXPORT_FILEis not provided, it defaults to${PROJECT_BINARY_DIR}/cmake/${ARG_PACKAGE_NAME}-config.cmake
The SOURCES keyword marks the beginning of sources to be used to build
<target>, if it is not loaded. All unparsed arguments will be interpreted as
sources.
This function guarantees that a Halide generator target named
<namespace><target> is available. It will first search for a package named
<package-name> using find_package; if it is found, it is assumed that it
provides the target. Otherwise, it will create an executable target named
target and an ALIAS target <namespace><target>. This function also
creates a custom target named <package-name> if it does not exist and
<target> would exist. In this case, <package-name> will depend on
<target>, this enables easy building of just the Halide generators managed
by this function.
After the call, <PACKAGE_NAME>_FOUND will be set to true if the host
generators were imported (and hence won't be built). Otherwise, it will be set
to false. This variable may be used to conditionally set properties on
<target>.
Please see test/integration/xc for a simple example and apps/hannk for a complete app that uses it extensively.
If PYSTUB is specified, then a Python Extension will be built that
wraps the Generator with CPython glue to allow use of the Generator
Python 3.x. The result will be a a shared library of the form
<target>_pystub.<soabi>.so, where describes the specific Python
version and platform (e.g., cpython-310-darwin for Python 3.10 on macOS.).
See README_python.md for examples of use.
This function wraps the outputs of one or more add_halide_library targets with glue code to produce
a Python Extension library.
add_halide_python_extension_library(
target
[MODULE_NAME module-name]
HALIDE_LIBRARIES library1 ...
)
FROM specifies any valid Generator target. If omitted,
HALIDE_LIBRARIES is a list of one of more add_halide_library targets. Each will be added to the
extension as a callable method of the module. Note that every library specified must be built with
the PYTHON_EXTENSION keyword specified, and all libraries must use the same Halide runtime.
The result will be a a shared library of the form
<target>.<soabi>.so, where describes the specific Python version and
platform (e.g., cpython-310-darwin for Python 3.10 on macOS.)
This function generates a library containing a Halide runtime. Most user code will never
need to use this, as add_halide_library() will call it for you if necessary. The most common
use case is usually in conjunction with add_halide_python_extension_library(), as a way to
ensure that all the halide libraries share an identical runtime.
add_halide_runtime(
target
[TARGETS target1 [target2 ...]]
)
The TARGETS argument has identical semantics to the argument of the same name
for add_halide_library.
Cross-compiling in CMake can be tricky, since CMake doesn't easily support compiling for both the host platform and the cross-platform within the same build. Unfortunately, Halide generator executables are just about always designed to run on the host platform. Each project will be set up differently and have different requirements, but here are some suggestions for effective use of CMake in these scenarios.
If you are writing new programs that use Halide, you might wish to use our
helper, add_halide_generator. When using this helper, you are expected to
build your project twice: once for your build host and again for your intended
target.
When building the host build, you can use the <package-name> (see the
documentation above) target to build just the generators. Then, in the
target build, set <package-name>_ROOT to the host build directory.
For example:
$ cmake -G Ninja -S . -B build-host -DCMAKE_BUILD_TYPE=Release
$ cmake --build build-host --target <package-name>
$ cmake -G Ninja -S . -B build-target -DCMAKE_BUILD_TYPE=Release \
-DCMAKE_TOOLCHAIN_FILE=/path/to/target-tc.cmake \
-D<package-name>_ROOT:FILEPATH=$PWD/build-host
$ cmake --build build-target
A CMake super-build consists of breaking down a project into sub-projects that
are isolated by toolchain. The basic structure is to have an
outermost project that only coordinates the sub-builds via the
ExternalProject module.
One would then use Halide to build a generator executable in one self-contained
project, then export that target to be used in a separate project. The second
project would be configured with the target toolchain and
would call add_halide_library with no TARGETS option and set FROM equal to
the name of the imported generator executable. Obviously, this is a significant
increase in complexity over a typical CMake project.
This is very compatible with the add_halide_generator strategy above.
A lighter weight alternative to the above is to use
ExternalProject directly in your parent build. Configure
the parent build with the target toolchain, and configure
the inner project to use the host toolchain. Then, manually create an
IMPORTED target for your generator executable and call
add_halide_library as described above.
The main drawback of this approach is that creating accurate IMPORTED targets
is difficult since predicting the names and locations of your binaries across
all possible platform and CMake project generators is difficult. In particular,
it is hard to predict executable extensions in cross-OS builds.
The CMAKE_CROSSCOMPILING_EMULATOR variable
allows one to specify a command prefix to run a target-system binary on the
host machine. One could set this to a custom shell script that uploads the
generator executable, runs it on the device and copies back the results.
The previous two options ensure that the targets generated by
add_halide_library will be normal static libraries. This approach does not
use ExternalProject, but instead produces IMPORTED
targets. The main drawback of IMPORTED targets is that they are considered
second-class in CMake. In particular, they cannot be installed with the typical
install(TARGETS) command. Instead, they must be installed
using install(FILES) and the
$<TARGET_FILE:tgt> generator expression.
When contributing new CMake code to Halide, keep in mind that the minimum version is 3.22. Therefore, it is possible (and indeed required) to use modern CMake best practices.
Like any large and complex system with a dedication to preserving backwards compatibility, CMake is difficult to learn and full of traps. While not comprehensive, the following serves as a guide for writing quality CMake code and outlines the code quality expectations we have as they apply to CMake.
The following are some common mistakes that lead to subtly broken builds.
- Reading the build directory. While setting up the build, the build directory should be considered write only. Using the build directory as a read/write temporary directory is acceptable as long as all temp files are cleaned up by the end of configuration.
- Not using generator expressions. Declarative is better than imperative and this is no exception. Conditionally adding to a target property can leak unwanted details about the build environment into packages. Some information is not accurate or available except via generator expressions, eg. the build configuration.
- Using the wrong variable.
CMAKE_SOURCE_DIRdoesn't always point to the Halide source root. When someone uses Halide viaFetchContent, it will point to their source root instead. The correct variable isHalide_SOURCE_DIR. If you want to know if the compiler is MSVC, check it directly with theMSVCvariable; don't useWIN32. That will be wrong when compiling with clang on Windows. In most cases, however, a generator expression will be more appropriate. - Using directory properties. Directory properties have vexing behavior and are essentially deprecated from CMake 3.0+. Propagating target properties is the way of the future.
- Using the wrong visibility. Target properties can be
PRIVATE,INTERFACE, or both (akaPUBLIC). Pick the most conservative one for each scenario. Refer to the transitive usage requirements docs for more information. - Needlessly expanding variables The
ifandforeachcommands generally expand variables when provided by name. Expanding such variables manually can unintentionally change the behavior of the command. Useforeach (item IN LISTS list)instead offoreach (item ${list}). Similarly, useif (varA STREQUAL varB)instead ofif ("${varA}" STREQUAL "${varB}")and definitely don't useif (${varA} STREQUAL ${varB})since that will fail (in the best case) if either variable's value contains a semi-colon (due to argument expansion).
As mentioned above, using directory properties is brittle and they are therefore not allowed. The following functions may not appear in any new CMake code.
| Command | Alternative |
|---|---|
add_compile_definitions |
Use target_compile_definitions |
add_compile_options |
Use target_compile_options |
add_definitions |
Use target_compile_definitions |
add_link_options |
Use target_link_options, but prefer not to use either |
get_directory_property |
Use cache variables or target properties |
get_property(... DIRECTORY) |
Use cache variables or target properties |
include_directories |
Use target_include_directories |
link_directories |
Use target_link_libraries |
link_libraries |
Use target_link_libraries |
remove_definitions |
Generator expressions in target_compile_definitions |
set_directory_properties |
Use cache variables or target properties |
set_property(... DIRECTORY) |
Use cache variables or target properties |
target_link_libraries(target lib) |
Use target_link_libraries with a visibility specifier (eg. PRIVATE) |
As an example, it was once common practice to write code similar to this:
# WRONG: do not do this
include_directories(include)
add_library(my_lib source1.cpp ..)However, this has two major pitfalls. First, it applies to all targets created
in that directory, even those before the call to include_directories and those
created in include()-ed CMake files. As CMake files get larger and
more complex, this behavior gets harder to pinpoint. This is particularly vexing
when using the link_libraries or add_defintions commands. Second, this form
does not provide a way to propagate the include directory to consumers of
my_lib. The correct way to do this is:
# CORRECT
add_library(my_lib source1.cpp ...)
target_include_directories(my_lib PUBLIC $<BUILD_INTERFACE:include>)This is better in many ways. It only affects the target in question. It
propagates the include path to the targets linking to it (via PUBLIC). It also
does not incorrectly export the host-filesystem-specific include path when
installing or packaging the target (via $<BUILD_INTERFACE>).
If common properties need to be grouped together, use an INTERFACE target (better) or write a function (worse). There are also several functions that are disallowed for other reasons:
| Command | Reason | Alternative |
|---|---|---|
aux_source_directory |
Interacts poorly with incremental builds and Git | List source files explicitly |
build_command |
CTest internal function | Use CTest build-and-test mode via CMAKE_CTEST_COMMAND |
cmake_host_system_information |
Usually misleading information. | Inspect toolchain variables and use generator expressions. |
cmake_policy(... OLD) |
OLD policies are deprecated by definition. | Instead, fix the code to work with the new policy. |
create_test_sourcelist |
We use our own unit testing solution | See the adding tests section. |
define_property |
Adds unnecessary complexity | Use a cache variable. Exceptions under special circumstances. |
enable_language |
Halide is C/C++ only | FindCUDAToolkit or FindCUDA, appropriately guarded. |
file(GLOB ...) |
Interacts poorly with incremental builds and Git | List source files explicitly. Allowed if not globbing for source files. |
fltk_wrap_ui |
Halide does not use FLTK | None |
include_external_msproject |
Halide must remain portable | Write a CMake package config file or find module. |
include_guard |
Use of recursive inclusion is not allowed | Write (recursive) functions. |
include_regular_expression |
Changes default dependency checking behavior | None |
load_cache |
Superseded by FetchContent/ExternalProject |
Use aforementioned modules |
macro |
CMake macros are not hygienic and are therefore error-prone | Use functions instead. |
site_name |
Privacy: do not want leak host name information | Provide a cache variable, generate a unique name. |
variable_watch |
Debugging helper | None. Not needed in production. |
Lastly, do not introduce any dependencies via find_package
without broader approval. Confine dependencies to the dependencies/ subtree.
Any variables that are specific to languages that are not enabled should, of course, be avoided. But of greater concern are variables that are easy to misuse or should not be overridden for our end-users. The following (non-exhaustive) list of variables shall not be used in code merged into main.
| Variable | Reason | Alternative |
|---|---|---|
CMAKE_ROOT |
Code smell | Rely on find_package search options; include HINTS if necessary |
CMAKE_DEBUG_TARGET_PROPERTIES |
Debugging helper | None |
CMAKE_FIND_DEBUG_MODE |
Debugging helper | None |
CMAKE_RULE_MESSAGES |
Debugging helper | None |
CMAKE_VERBOSE_MAKEFILE |
Debugging helper | None |
CMAKE_BACKWARDS_COMPATIBILITY |
Deprecated | None |
CMAKE_BUILD_TOOL |
Deprecated | ${CMAKE_COMMAND} --build or CMAKE_MAKE_PROGRAM (but see below) |
CMAKE_CACHEFILE_DIR |
Deprecated | CMAKE_BINARY_DIR, but see below |
CMAKE_CFG_INTDIR |
Deprecated | $<CONFIG>, $<TARGET_FILE:..>, target resolution of add_custom_command, etc. |
CMAKE_CL_64 |
Deprecated | CMAKE_SIZEOF_VOID_P |
CMAKE_COMPILER_IS_* |
Deprecated | CMAKE_<LANG>_COMPILER_ID |
CMAKE_HOME_DIRECTORY |
Deprecated | CMAKE_SOURCE_DIR, but see below |
CMAKE_DIRECTORY_LABELS |
Directory property | None |
CMAKE_BUILD_TYPE |
Only applies to single-config generators. | $<CONFIG> |
CMAKE_*_FLAGS* (w/o _INIT) |
User-only | Write a toolchain file with the corresponding _INIT variable |
CMAKE_COLOR_MAKEFILE |
User-only | None |
CMAKE_ERROR_DEPRECATED |
User-only | None |
CMAKE_CONFIGURATION_TYPES |
We only support the four standard build types | None |
Of course feel free to insert debugging helpers while developing but please remove them before review. Finally, the following variables are allowed, but their use must be motivated:
| Variable | Reason | Alternative |
|---|---|---|
CMAKE_SOURCE_DIR |
Points to global source root, not Halide's. | Halide_SOURCE_DIR or PROJECT_SOURCE_DIR |
CMAKE_BINARY_DIR |
Points to global build root, not Halide's | Halide_BINARY_DIR or PROJECT_BINARY_DIR |
CMAKE_MAKE_PROGRAM |
CMake abstracts over differences in the build tool. | Prefer CTest's build and test mode or CMake's --build mode |
CMAKE_CROSSCOMPILING |
Often misleading. | Inspect relevant variables directly, eg. CMAKE_SYSTEM_NAME |
BUILD_SHARED_LIBS |
Could override user setting | None, but be careful to restore value when overriding for a dependency |
Any use of these functions and variables will block a PR.
When adding a file to any of the folders under test, be aware that CI expects
that every .c and .cpp appears in the CMakeLists.txt file on its own
line, possibly as a comment. This is to avoid globbing and also to ensure that
added files are not missed.
For most test types, it should be as simple as adding to the existing lists, which must remain in alphabetical order. Generator tests are trickier, but following the existing examples is a safe way to go.
If you're contributing a new app to Halide: great! Thank you! There are a few guidelines you should follow when writing a new app.
- Write the app as if it were a top-level project. You should call
find_package(Halide)and set the C++ version to 11. - Call
enable_testing()and add a small test that runs the app. - Don't assume your app will have access to a GPU. Write your schedules to be robust to varying buildbot hardware.
- Don't assume your app will be run on a specific OS, architecture, or bitness. Write your apps to be robust (ideally efficient) on all supported platforms.
- If you rely on any additional packages, don't include them as
REQUIRED, instead test to see if their targets are available and, if not, callreturn()before creating any targets. In this case, print amessage(STATUS "[SKIP] ..."), too. - Look at the existing apps for examples.
- Test your app with ctest before opening a PR. Apps are built as part of the test, rather than the main build.