GN build system integrations#

pw_toolchain module provides GN toolchains that may be used to build Pigweed. GN toolchains function both as a set of tools for compilation and as a workspace for evaluating build files. The same compilations and actions can be executed by different toolchains. Each toolchain maintains its own set of build args, and build steps from all toolchains can be executed in parallel.

Various GCC and Clang toolchains for multiple platforms are provided. Toolchains names typically include the compiler (clang or gcc and optimization level (debug, size_optimized, speed_optimized).

Non-C/C++ toolchains#

pw_toolchain/non_c_toolchain.gni provides the pw_non_c_toolchain template. This template creates toolchains that cannot compile C/C++ source code. These toolchains may only be used to execute GN actions or declare groups of targets in other toolchains. Attempting to compile C/C++ code with either of these toolchains results in errors.

Non-C/C++ toolchains can be used to consolidate actions that should only occur once in a multi-toolchain build. Build targets from all toolchains can refer to these actions in a non-C/C++ toolchain so they only execute once instead of once per toolchain.

For example, Pigweed runs protobuf compilation and Python package actions like installation and Pylint in toolchains created with pw_non_c_toolchain. This allows all toolchains to cleanly share the same protobuf and Python declarations without any duplicated work.

Testing other compiler versions#

The clang-based toolchain provided by Pigweed can be substituted with another version by modifying the pw_toolchain_CLANG_PREFIX GN build argument to point to the directory that contains the desired clang, clang++, and llvm-ar binaries. This should only be used for debugging purposes. Pigweed does not officially support any compilers other than those provided by Pigweed.

Running static analysis checks#

clang-tidy can be run as a compiler replacement, to analyze all sources built for a target. pw_toolchain/static_analysis_toolchain.gni provides the pw_static_analysis_toolchain template. This template creates toolchains that execute clang-tidy for C/C++ sources, and mock implementations of the link, alink and solink tools.

In addition to the standard toolchain requirements (cc, cxx, etc.), the pw_static_analysis_toolchain template requires a scope static_analysis to be defined on the invoker.

static_analysis = {
 # Configure whether static_analysis should be enabled for invoker toolchain.
 # This is must be set true if using pw_static_analysis_toolchain.
 enabled = true
 # Optionally override clang-tidy binary to use by setting to proper path.
 clang_tidy_path = ""
 # Optionally specify additional command(s) to run as part of cc tool.
 cc_post = ""
 # Optionally specify additional command(s) to run as part of cxx tool.
 cxx_post = ""
}

The generate_toolchain supports the above mentioned static_analysis scope, which if specified must at the very least define the bool enabled within the scope. If the static_analysis scope is provided and static_analysis.enabled = true, the derived toolchain ${target_name}.static_analysis will be generated using pw_generate_static_analysis_toolchain and the toolchain options.

An example on the utility of the static_analysis scope args is shown in the snippet below where we enable clang-tidy caching and add //.clang-tidy as a dependency to the generated .d files for the pw_static_analysis_toolchain.

static_analysis = {
 clang_tidy_path = "//third_party/ctcache/clang-tidy"
 _clang_tidy_cfg_path = rebase_path("//.clang-tidy", root_build_dir)
 cc_post = "echo '-: $_clang_tidy_cfg_path' >> {{output}}.d"
 cxx_post = "echo '-: $_clang_tidy_cfg_path' >> {{output}}.d"
}

Excluding files from checks#

The build argument pw_toolchain_STATIC_ANALYSIS_SKIP_SOURCES_RES is used to exclude source files from the analysis. The list must contain regular expressions matching individual files, rather than directories. For example, provide "the_path/.*" to exclude all files in all directories under the_path.

The build argument pw_toolchain_STATIC_ANALYSIS_SKIP_INCLUDE_PATHS is used used to exclude header files from the analysis. This argument must be a list of POSIX-style path suffixes for include paths, or regular expressions matching include paths. For example, passing the_path/include excludes all header files that are accessed from include paths ending in the_path/include, while passing .*/third_party/.* excludes all third-party header files.

Note that pw_toolchain_STATIC_ANALYSIS_SKIP_INCLUDE_PATHS operates on include paths, not header file paths. For example, say your compile commands include -Idrivers, and this results in a file at drivers/public/i2c.h being included. You can skip this header by adding drivers or drivers.* to pw_toolchain_STATIC_ANALYSIS_SKIP_INCLUDE_PATHS, but not by adding drivers/.*: this last regex matches the header file path, but not the include path.

Provided toolchains#

pw_toolchain provides static analysis GN toolchains that may be used to test host targets:

  • pw_toolchain_host_clang.debug.static_analysis

  • pw_toolchain_host_clang.speed_optimized.static_analysis

  • pw_toolchain_host_clang.size_optimized.static_analysis

  • pw_toolchain_host_clang.fuzz.static_analysis (if pw_toolchain_OSS_FUZZ_ENABLED is false)

  • pw_toolchain_arm_clang.debug.static_analysis

  • pw_toolchain_arm_clang.speed_optimized.static_analysis

  • pw_toolchain_arm_clang.size_optimized.static_analysis

For example, to run clang-tidy on all source dependencies of the default target:

generate_toolchain("my_toolchain") {
  ..
  static_analysis = {
   enabled = true
  }
}

group("static_analysis") {
  deps = [ ":default(my_toolchain.static_analysis)" ]
}

Warning

The status of the static analysis checks might change when any relevant .clang-tidy file is updated. You should clean the output directory before invoking clang-tidy.

Target traits#

Pigweed targets expose a set of constants that describe properties of the target or the toolchain compiling code for it. These are referred to as target traits.

In GN, these traits are exposed as GN args and are prefixed with pw_toolchain_ (e.g. pw_toolchain_CXX_STANDARD). They are defined in pw_toolchain/traits.gni.

Traits must never be set by the user (e.g. with gn args). Traits are always set by the target.

Warning

This feature is under development and is likely to change significantly. See b/234883746.

List of traits#

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