Compiler-specific flags in bazel?

Question

I would like to specify linkopts on a cc_library target, but only if the target is built with clang (because gcc doesn't support the flag). How do I go about this?

The specific example in which this need arose is https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/136287 (see also comments on this change). A library produces a warning when linked with clang with the --warn_backrefs flag. This warning can be silenced using --warn-backrefs-exclude, but only clang recognizes this flag: adding it unconditionally will lead gcc to report an error.

Apparently, CMake allows handling such compiler-specific flags via CMAKE_CXX_COMPILER_ID and CMAKE_<LANG>_COMPILER_VERSION. Is there an analogous mechanism in Bazel?

Vertexwahn · Accepted Answer · 2023-04-01 07:32:19Z

You can use a Bazel config. Create in the root directory of your workspace (where WORKSPACE or WORKSPACE.bazel is located) a file name .bazelrc. You can now define different configs, e.g. gcc11, clang14 or vs2022:

# GCC 11
build:gcc11 --cxxopt=-std=c++20
build:gcc11 --cxxopt=-Wall
build:gcc11 --cxxopt=-Werror
build:gcc11 --cxxopt=-Wno-volatile
build:gcc11 --cxxopt=-Wextra


# Clang 14
build:clang14 --cxxopt=-std=c++20
build:clang14 --cxxopt=--warn-backrefs-exclude

# Visual Studio 2022
build:vs2022 --cxxopt=/std:c++20
build:vs2022 --cxxopt=/Zc:__cplusplus
build:vs2022 --enable_runfiles # https://github.com/bazelbuild/bazel/issues/8843
build:vs2022 --copt=-DWIN32_LEAN_AND_MEAN
build:vs2022 --copt=-DNOGDI
build:vs2022 --host_copt=-DWIN32_LEAN_AND_MEAN
build:vs2022 --host_copt=-DNOGDI
build:vs2022 --define compiler=vs2022

Now when you call Bazel you can handover a preferred config, e.g.:

bazel build --config=clang14 //...

Another way could be to make use of platforms.

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