pw_toolchain#

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.

GN Toolchains#

pw_toolchain module provides GN toolchains that may be used to build Pigweed. 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#

Bazel toolchains#

Pigweed provides a suite of building blocks for designing a custom C/C++ toolchain in Bazel. See the pw_toolchain_bazel module documentation for more information.

Upstream Pigweed Toolchains#

You can use Pigweed’s upstream toolchains by calling register_pigweed_cxx_toolchains() provided by @pigweed//pw_toolchain:register_toolchains.bzl in your WORKSPACE file.

Note

Pigweed’s upstream toolchains are subject to change without notice. If you would prefer more stability in toolchain configurations, consider declaring custom toolchains in your project.

C/C++ libraries#

pw_toolchain provides some toolchain-related C/C++ libraries.

std:abort wrapper#

The std::abort function is used to terminate a program abnormally. This function may be called by standard library functions, so is often linked into binaries, even if users never intentionally call it.

For embedded builds, the abort implementation likely does not work as intended. For example, it may pull in undesired dependencies (e.g. std::raise) and end in an infinite loop.

pw_toolchain provides the pw_toolchain:wrap_abort library that replaces abort in builds where the default behavior is undesirable. It uses the -Wl,--wrap=abort linker option to redirect to abort calls to PW_CRASH instead.

arm-none-eabi-gcc support#

Targets building with the GNU Arm Embedded Toolchain (arm-none-eabi-gcc) should depend on the pw_toolchain/arm_gcc:arm_none_eabi_gcc_support library. In GN, that target should be included in pw_build_LINK_DEPS. In Bazel, it should be added to link_extra_lib or directly to the deps of any binary being build with that toolchain:

cc_binary(
   deps = [
     # Other deps, omitted
   ] + select({
     "@platforms//cpu:armv7e-m": [
       "@pigweed//pw_toolchain/arm_gcc:arm_none_eabi_gcc_support",
     ],
     "//conditions:default": [],
   }),
)

Newlib OS interface#

Newlib, the C Standard Library implementation provided with arm-none-eabi-gcc, defines a set of OS interface functions that should be implemented. Newlib provides default implementations, but using these results in linker warnings like the following:

readr.c:(.text._read_r+0x10): warning: _read is not implemented and will always fail

Most of the OS interface functions should never be called in embedded builds. The pw_toolchain/arg_gcc:newlib_os_interface_stubs library, which is provided through pw_toolchain/arm_gcc:arm_none_eabi_gcc_support, implements these functions and forces a linker error if they are used. It also automatically includes a wrapper for abort for use of stdout and stderr which abort if they are called.

If you need to use your own wrapper for abort, include the library directly using pw_toolchain/arm_gcc:newlib_os_interface_stubs.

pw_toolchain/no_destructor.h#

template<typename T>
class NoDestructor#

Helper type to create a function-local static variable of type T when T has a non-trivial destructor. Storing a T in a pw::NoDestructor<T> will prevent ~T() from running, even when the variable goes out of scope.

This class is useful when a variable has static storage duration but its type has a non-trivial destructor. Destructor ordering is not defined and can cause issues in multithreaded environments. Additionally, removing destructor calls can save code size.

Except in generic code, do not use pw::NoDestructor<T> with trivially destructible types. Use the type directly instead. If the variable can be constexpr, make it constexpr.

pw::NoDestructor<T> provides a similar API to std::optional. Use * or -> to access the wrapped type.

Example usage: 

pw::sync::Mutex& GetMutex() {
  // Use NoDestructor to avoid running the mutex destructor when exit-time
  // destructors run.
  static const pw::NoDestructor<pw::sync::Mutex> global_mutex;
  return *global_mutex;
}

In Clang, pw::NoDestructor can be replaced with the [[clang::no_destroy]] attribute. pw::NoDestructor<T> is similar to Chromium’s base::NoDestructor<T> in src/base/no_destructor.h.

Warning

Misuse of NoDestructor can cause memory leaks and other problems. Only skip destructors when you know it is safe to do so.

builtins#

builtins are LLVM’s equivalent of libgcc, the compiler will insert calls to these routines. Setting the dir_pw_third_party_builtins gn var to your compiler-rt/builtins checkout will enable building builtins from source instead of relying on the shipped libgcc.