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CMake Build Rules for Halide

Overview

The primary support for Halide in CMake is the halide_library rule, which allows you to build a Generator into an executable for various architectures.

Halide code is produced in a two-stage build that may seem a bit unusual at first, so before documenting the build rules, let's recap: a Generator is a C++ class in which you define a Halide pipeline and schedule, as well as well-defined input and output parameters; it can be compiled into an executable (often referred to as a Filter) that efficiently runs the Halide pipeline.

Halide Compilation Process

The Halide compilation process is actually several steps:

  1. Your Generator is compiled using a standard C++ compiler.
  2. It's linked with libHalide, LLVM, and a simple command-line wrapper. This results in an executable known as the Generator Executable.
  3. The Generator Executable is run as a tool, at build time; this produces a .a, .h, and a few other Halide-specific files.
  4. The resulting .a and .h are bundled into a CMake add_library() rule.

The halide_library rule (mostly) hides the details of this, with some unavoidable warts, the main one being that there are two distinct sort of dependencies involved:

  1. Generator dependencies: dependencies that are necessary to compile and/or link the Generator itself.
  2. Filter dependencies: dependencies that are necessary to link and/or run the Filter. (Generally speaking, you only need Filter dependencies if you use Halide's define_extern directive.)

Using the Halide Build Rules

The easiest way to use CMake rules for Halide is via a prebuilt Halide distribution that includes them. In your CMakeLists.txt file, just set HALIDE_DISTRIB_DIR to point to the distribution directory, then include halide.cmake:

set(HALIDE_DISTRIB_DIR "/path/to/halide/distrib")
include("${HALIDE_DISTRIB_DIR}/halide.cmake")

Then, using a Generator can be as simple as

halide_library(coolness SRCS coolness_generator.cpp)

add_executable(my_app my_app.cpp)
target_link_libraries(my_app PUBLIC coolness)

For an example of "standalone" use of the CMake rules, see apps/wavelet.

Build Rules

halide_library

halide_library(name, srcs, extra_outputs, filter_deps, function_name,
generator_args, generator_deps, generator_name, halide_target,
halide_target_features, includes)

  • name (Name; required) The name of the library target.

  • srcs (List of C++ source files; required) source file(s) to compile into the Generator Executable.

  • extra_outputs (List of strings; optional) A list of extra Halide outputs to generate at build time; this is exclusively intended for debugging (e.g. to examine Halide code generation) and currently supports:

    • "assembly" (generate assembly listings for the generated functions)
    • "bitcode" (emit the LLVM bitcode for generation functions)
    • "stmt" (generate Halide .stmt files for generated functions)
    • "stmt_html" (like "stmt", but generated with HTML-formatted wrapping)

  • filter_deps (List of CMake targets; optional) optional list of dependencies needed by the Filter.

  • function_name (String; optional) The name of the generated C function for the filter. If omitted, defaults to the CMake rule name.

  • generator_args (String; optional) Arguments to pass to the Generator, used to define the compile-time values of GeneratorParams defined by the Generator. If any undefined GeneratorParam names (or illegal GeneratorParam values) are specified, a compile error will result.

  • generator_deps (List of CMake targets; optional) optional list of dependencies needed by the Generator. (These dependencies are not included in the filter produced by halide_library, nor in any executable that depends on it.)

  • generator_name (String; optional) The registered name of the Halide generator (i.e., the name passed as the second argument to HALIDE_REGISTER_GENERATOR in the Generator source file). If omitted, defaults to the CMake rule name.

  • halide_target (String; optional) The Halide target-string to use when generating code; if omitted, defaults to "host". If you want to target for a specific architechture, or to enable certain features, you can specify specific values, e.g. "x86-64-linux-sse41-avx" compiles for x86-64 on Linux, also enabling SSE4.1 and AVX. You can also specify multiple values, separated by commas, to generate several variants that are selected at runtime; e.g. "x86-64-linux-sse41-avx,x86-64-linux-sse41,x86-64-linux" will generate three versions of the code (for SSE4.1+AVX, for SSE4.1, and for plain x86-64), and select the first matching variants at runtime. (See also halide_target vs halide_target_features below.)

  • halide_target_features (List of strings; optional) A list of extra Halide Features to enable in the code. This can be any feature listed in feature_name_map in src/Target.cpp. This is typically where flags to enable GPU features should go. (See also halide_target vs halide_target_features below.)

  • includes (List of strings; optional) The list of paths to search include files when building the Generator.

halide_target vs halide_target_features

What's the difference between these two? It's a little subtle, and their uses overlap a bit. The Halide target string used to generate code found by appending all the features in halide_target_features onto halide_target; this can be useful in complex builds in which you target multiple architectures, with additional features tacked on. For instance, say we want to generate libraries for x86 and ARM architectures, with OpenGL supported on all of them:

foreach(ARCH x86 arm)
    foreach(BITS 32 64)
        halide_library(spiffy_${ARCH}_${BITS}
                       SRCS spiffy_generator.cpp
                       HALIDE_TARGET ${ARCH}-${BITS}-android
                       HALIDE_TARGET_FEATURES opengl)
    endforeach()
endforeach()

The distinction can also be useful if you are generating a multi-architecture library:

halide_library(nifty
               SRCS nifty_generator.cpp
               HALIDE_TARGET x86-64-osx-sse41-avx,x86-64-osx-sse41,x86-64-osx-sse41
               HALIDE_TARGET_FEATURES metal)

This says "Generate the library with specializations for AVX+SSE4.1, SSE4.1, and plain x86-64, and select the proper one at runtime... but enable the Metal feature for all three."

Build Rules (Advanced)

Most code can use the halide_library rule directly, but for some usage, it can be desirable to directly use two lower-level rules.

halide_generator

halide_generator(name, srcs, deps, generator_name, includes)

The halide_generator rule compiles Generator source code into an executable; it corresponds to steps 1 and 2 in the Halide Compilation Process.

  • name (Name; required) The name of the executable target. Note that target names for halide_generator are required to end in .generator.

  • srcs (List of C++ source files; required) source file(s) to compile into the Generator Executable.

  • deps (List of CMake targets; optional) optional list of dependencies needed by the Generator.

  • generator_name (String; optional) The registered name of the Halide generator (i.e., the name passed as the second argument to HALIDE_REGISTER_GENERATOR in the Generator source file). If omitted, defaults to the CMake rule name.

  • includes (List of paths; optional) The list of paths to search include files when building the Generator.

halide_library_from_generator

halide_library_from_generator(name, generator, deps, extra_outputs,
function_name, generator_args, halide_target, halide_target_features)

The halide_library_from_generator rule executes a compiled Generator to produce final object code and related files; it corresponds to steps 3 and 4 in the Halide Compilation Process.

  • name (Name; required) The name of the library target.

  • generator (CMake target; required) The CMake target for a halide_generator rule.

  • deps (List of CMake targets; optional) optional list of dependencies needed by the Filter.

  • extra_outputs (List of strings; optional) A list of extra Halide outputs to generate at build time; this is exclusively intended for debugging (e.g. to examine Halide code generation) and currently supports:

    • "assembly" (generate assembly listings for the generated functions)
    • "bitcode" (emit the LLVM bitcode for generation functions)
    • "stmt" (generate Halide .stmt files for generated functions)
    • "stmt_html" (like "stmt", but generated with HTML-formatted wrapping)

  • function_name (String; optional) The name of the generated C function for the filter. If omitted, defaults to the CMake rule name.

  • generator_args (String; optional) Arguments to pass to the Generator, used to define the compile-time values of GeneratorParams defined by the Generator. If any undefined GeneratorParam names (or illegal GeneratorParam values) are specified, a compile error will result.

  • halide_target (String; optional) The Halide target-string to use when generating code; if omitted, defaults to "host". See halide_library for more information.

  • halide_target_features (List of strings; optional) A list of extra Halide Features to enable in the code. See halide_library for more information.