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skia2/toolchain/clang_toolchain_config.bzl
Kevin Lubick 4d41304def [infra] Add hermetic toolchain for C/C++ using Clang+Musl 
This can successfully build a C library:
   bazel build --config=clang //third_party:libpng

This can build and run a statically-linked executable:
   bazel test --config=clang //:bazel_test

For more verbose compile and linking output, add the
`--features diagnostic`
flag to a Bazel command (see _make_diagnostic_flags() in
toolchain/clang_toolchain_config.bzl. Similarly, a
`--features print_search_dirs` can be used to show where
clang is looking for libraries etc to link against.
These features are made available for easier debugging.

Suggested review order:
 - Read https://docs.bazel.build/versions/4.2.1/tutorial/cc-toolchain-config.html
   if unfamiliar with setting up C++ toolchains in Bazel
 - .bazelrc and WORKSPACE.bazel that configure use and download
   of the toolchain (Clang 13, musl 1.2.2)
 - toolchain/build_toolchain.bzl which downloads and assembles
   the toolchain (w/o installing anything on the host machine)
 - toolchain/BUILD.bazel and toolchain/*trampoline.sh to see
   the setup of the toolchain rules.
 - toolchain/clang_toolchain_config.bzl to see the configuration
   of the toolchain. Pay special attention to the various
   command line flags that are set.
 - See that tools/bazel_test.cc has made a new home in
   experimental/bazel_test/bazel_test.cpp, with a companion
   BUILD.bazel. Note the addition of some function calls
   that test use of the C++ standard library.
   The number being used to test the PNG library is the latest
   and greatest that verifies we are compiling the one brought
   in via DEPS (and not a local one).
 - third_party/* to see how png (and its dependent zlib) have
   been built. Pay special attention to the musl_compat hack
   to fix static linking (any idea what the real cause is?)
- //BUILD.bazel to see definition of the bazel_test executable.

Change-Id: I7b0922d0d45cb9be8df2fd5fa5a1f48492654d5f
Bug: skia:12541
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/461178
Reviewed-by: Ben Wagner <bungeman@google.com>
Reviewed-by: Leandro Lovisolo <lovisolo@google.com>
2021-10-21 12:43:49 +00:00

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"""
This file specifies a clang toolchain that can run on a Linux host which does depend on any
installed packages from the host machine.
See build_toolchain.bzl for more details on the creation of the toolchain.
It uses the usr subfolder of the built toolchain as a sysroot
It follows the example of:
- https://docs.bazel.build/versions/4.2.1/tutorial/cc-toolchain-config.html
- https://github.com/emscripten-core/emsdk/blob/7f39d100d8cd207094decea907121df72065517e/bazel/emscripten_toolchain/crosstool.bzl
"""
load(
"@bazel_tools//tools/cpp:cc_toolchain_config_lib.bzl",
"action_config",
"feature",
"flag_group",
"flag_set",
"tool",
"variable_with_value",
)
load("@bazel_tools//tools/build_defs/cc:action_names.bzl", "ACTION_NAMES")
# The location of the created clang toolchain.
EXTERNAL_TOOLCHAIN = "external/clang_linux_amd64_musl"
def _clang_impl(ctx):
action_configs = _make_action_configs()
features = []
features += _make_default_flags()
features += _make_diagnostic_flags()
# https://docs.bazel.build/versions/main/skylark/lib/cc_common.html#create_cc_toolchain_config_info
# Note, this rule is defined in Java code, not Starlark
# https://cs.opensource.google/bazel/bazel/+/master:src/main/java/com/google/devtools/build/lib/starlarkbuildapi/cpp/CcModuleApi.java
return cc_common.create_cc_toolchain_config_info(
ctx = ctx,
features = features,
abi_libc_version = "unknown",
abi_version = "unknown",
action_configs = action_configs,
builtin_sysroot = EXTERNAL_TOOLCHAIN + "/usr",
compiler = "clang",
host_system_name = "local",
target_cpu = "k8",
target_libc = "musl",
target_system_name = "local",
toolchain_identifier = "clang-toolchain",
)
provide_clang_toolchain_config = rule(
attrs = {},
provides = [CcToolchainConfigInfo],
implementation = _clang_impl,
)
def _make_action_configs():
"""
This function sets up the tools needed to perform the various compile/link actions.
Bazel normally restricts us to referring to (and therefore running) executables/scripts
that are in this directory (That is EXEC_ROOT/toolchain). However, the executables we want
to run are brought in via WORKSPACE.bazel and are located in EXEC_ROOT/external/clang....
Therefore, we make use of "trampoline scripts" that will call the binaries from the
toolchain directory.
These action_configs also let us dynamically specify arguments from the Bazel
environment if necessary (see cpp_link_static_library_action).
"""
# https://cs.opensource.google/bazel/bazel/+/master:tools/cpp/cc_toolchain_config_lib.bzl;l=435;drc=3b9e6f201a9a3465720aad8712ab7bcdeaf2e5da
clang_tool = tool(path = "clang_trampoline.sh")
lld_tool = tool(path = "lld_trampoline.sh")
ar_tool = tool(path = "ar_trampoline.sh")
# https://cs.opensource.google/bazel/bazel/+/master:tools/cpp/cc_toolchain_config_lib.bzl;l=488;drc=3b9e6f201a9a3465720aad8712ab7bcdeaf2e5da
assemble_action = action_config(
action_name = ACTION_NAMES.assemble,
tools = [clang_tool],
)
c_compile_action = action_config(
action_name = ACTION_NAMES.c_compile,
tools = [clang_tool],
)
cpp_compile_action = action_config(
action_name = ACTION_NAMES.cpp_compile,
tools = [clang_tool],
)
linkstamp_compile_action = action_config(
action_name = ACTION_NAMES.linkstamp_compile,
tools = [clang_tool],
)
preprocess_assemble_action = action_config(
action_name = ACTION_NAMES.preprocess_assemble,
tools = [clang_tool],
)
cpp_link_dynamic_library_action = action_config(
action_name = ACTION_NAMES.cpp_link_dynamic_library,
tools = [lld_tool],
)
cpp_link_executable_action = action_config(
action_name = ACTION_NAMES.cpp_link_executable,
# Bazel assumes it is talking to clang when building an executable. There are
# "-Wl" flags on the command: https://releases.llvm.org/6.0.1/tools/clang/docs/ClangCommandLineReference.html#cmdoption-clang-Wl
tools = [clang_tool],
)
cpp_link_nodeps_dynamic_library_action = action_config(
action_name = ACTION_NAMES.cpp_link_nodeps_dynamic_library,
tools = [lld_tool],
)
# This is the same rule as
# https://github.com/emscripten-core/emsdk/blob/7f39d100d8cd207094decea907121df72065517e/bazel/emscripten_toolchain/crosstool.bzl#L143
# By default, there are no flags or libraries passed to the llvm-ar tool, so
# we need to specify them. The variables mentioned by expand_if_available are defined
# https://docs.bazel.build/versions/main/cc-toolchain-config-reference.html#cctoolchainconfiginfo-build-variables
cpp_link_static_library_action = action_config(
action_name = ACTION_NAMES.cpp_link_static_library,
flag_sets = [
flag_set(
flag_groups = [
flag_group(
# https://llvm.org/docs/CommandGuide/llvm-ar.html
# replace existing files or insert them if they already exist,
# create the file if it doesn't already exist
# symbol table should be added
# Deterministic timestamps should be used
flags = ["rcsD", "%{output_execpath}"],
# Despite the name, output_execpath just refers to linker output,
# e.g. libFoo.a
expand_if_available = "output_execpath",
),
],
),
flag_set(
flag_groups = [
flag_group(
iterate_over = "libraries_to_link",
flag_groups = [
flag_group(
flags = ["%{libraries_to_link.name}"],
expand_if_equal = variable_with_value(
name = "libraries_to_link.type",
value = "object_file",
),
),
flag_group(
flags = ["%{libraries_to_link.object_files}"],
iterate_over = "libraries_to_link.object_files",
expand_if_equal = variable_with_value(
name = "libraries_to_link.type",
value = "object_file_group",
),
),
],
expand_if_available = "libraries_to_link",
),
],
),
flag_set(
flag_groups = [
flag_group(
flags = ["@%{linker_param_file}"],
expand_if_available = "linker_param_file",
),
],
),
],
tools = [ar_tool],
)
action_configs = [
assemble_action,
c_compile_action,
cpp_compile_action,
cpp_link_dynamic_library_action,
cpp_link_executable_action,
cpp_link_nodeps_dynamic_library_action,
cpp_link_static_library_action,
linkstamp_compile_action,
preprocess_assemble_action,
]
return action_configs
def _make_default_flags():
"""Here we define the flags for certain actions that are always applied."""
cxx_compile_includes = flag_set(
actions = [
ACTION_NAMES.c_compile,
ACTION_NAMES.cpp_compile,
],
flag_groups = [
flag_group(
flags = [
# THIS ORDER MATTERS GREATLY. If these are in the wrong order, the
# #include_next directives will walk off the end of the specified system
# folders here and look in the absolute path that happens to contain
# the clang executable. Because that looks like an absolute path to
# Bazel, it will declare the Build is not properly specified (that is,
# it appears to use headers outside of Bazel's control/view) and fail.
"-isystem",
EXTERNAL_TOOLCHAIN + "/include/c++/v1",
"-isystem",
EXTERNAL_TOOLCHAIN + "/lib/clang/13.0.0/include",
"-isystem",
EXTERNAL_TOOLCHAIN + "/usr/include/x86_64-linux-musl",
],
),
],
)
cpp_compile_includes = flag_set(
actions = [
ACTION_NAMES.cpp_compile,
],
flag_groups = [
flag_group(
flags = [
"-std=c++14",
# This define allows libc++ to work with musl. They were discovered by
# trying to compile without them, reading errors and source code, e.g.
# https://github.com/llvm/llvm-project/blob/f4c1258d5633fcf06385ff3fd1f4bf57ab971964/libcxx/include/__locale#L513
# https://github.com/llvm/llvm-project/blob/f4c1258d5633fcf06385ff3fd1f4bf57ab971964/libcxx/include/__config#L1155
# https://github.com/llvm/llvm-project/blob/f4c1258d5633fcf06385ff3fd1f4bf57ab971964/libcxx/include/__support/musl/xlocale.h
"-D_LIBCPP_HAS_MUSL_LIBC",
],
),
],
)
link_exe_flags = flag_set(
actions = [ACTION_NAMES.cpp_link_executable],
flag_groups = [
flag_group(
flags = [
"-fuse-ld=lld",
# This is the path to libc.a and other libraries.
"-L" + EXTERNAL_TOOLCHAIN + "/usr/lib/x86_64-linux-musl",
"-L" + EXTERNAL_TOOLCHAIN + "/lib",
# We chose to use the llvm runtime, not the gcc one because it is already
# included in the clang binary
"--rtlib=compiler-rt",
# In order to run our executables, they need to be statically linked,
# otherwise, the libc++.so and friends will not be found if they are
# not installed on the user's machine.
"-static",
"-std=c++14",
"-lc++abi",
"-lunwind",
"-lc++",
],
),
],
)
return [feature(
"default_flags",
enabled = True,
flag_sets = [
cxx_compile_includes,
cpp_compile_includes,
link_exe_flags,
],
)]
def _make_diagnostic_flags():
"""Here we define the flags that can be turned on via features to yield debug info."""
cxx_diagnostic = flag_set(
actions = [
ACTION_NAMES.c_compile,
ACTION_NAMES.cpp_compile,
],
flag_groups = [
flag_group(
flags = [
"--trace-includes",
"-v",
],
),
],
)
link_diagnostic = flag_set(
actions = [ACTION_NAMES.cpp_link_executable],
flag_groups = [
flag_group(
flags = [
"-Wl,--verbose",
"-v",
],
),
],
)
link_search_dirs = flag_set(
actions = [ACTION_NAMES.cpp_link_executable],
flag_groups = [
flag_group(
flags = [
"--print-search-dirs",
],
),
],
)
return [
# Running a Bazel command with --features diagnostic will cause the compilation and
# link steps to be more verbose.
feature(
"diagnostic",
enabled = False,
flag_sets = [
cxx_diagnostic,
link_diagnostic,
],
),
# Running a Bazel command with --features print_search_dirs will cause the link to fail
# but directories searched for libraries, etc will be displayed.
feature(
"print_search_dirs",
enabled = False,
flag_sets = [
link_search_dirs,
],
),
]
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