C++20以Bazel&Clang开始

C++20 如何以 Bazel & Clang 进行构建呢？

本文将介绍：

• Bazel 构建系统的安装
• LLVM 编译系统的安装
  • Clang is an "LLVM native" C/C++/Objective-C compiler
• Bazel Clang 工具链的配置
• C++20 库与应用的构建

本文示例可见： https://github.com/ikuokuo/start-cpp20

本文是于 Ubuntu 20 上进行的实践，Windows 可以用 WSL 准备环境。

安装 Bazel，以二进制方式

Bazelisk 是安装 Bazel 的推荐方式，我们安装它的二进制发布即可：

cd ~wget https://github.com/bazelbuild/bazelisk/releases/download/v1.12.0/bazelisk-linux-amd64 -O bazelisk-1.12.0-linux-amd64chmod a+x bazelisk-*sudo ln -s $(pwd)/bazelisk-1.12.0-linux-amd64 /usr/local/bin/bazeltouch WORKSPACE# 国内下载 Bazel 可能遇到如下问题，配置 .bazeliskrc 解决# could not resolve the version 'latest' to an actual version number#  https://github.com/bazelbuild/bazelisk/issues/220cat <<-EOF > .bazeliskrcBAZELISK_BASE_URL=https://github.com/bazelbuild/bazel/releases/downloadUSE_BAZEL_VERSION=5.2.0EOFbazel version

更多方式，可见官方文档。进一步，推荐安装 buildtools，下载后软链一下：

sudo ln -s $(pwd)/buildifier-5.1.0-linux-amd64 /usr/local/bin/buildifiersudo ln -s $(pwd)/buildozer-5.1.0-linux-amd64 /usr/local/bin/buildozer

Bazel 如何构建 C++ 项目，可见我的 Start Bazel 笔记。

安装 LLVM，以源码方式

Clang 有关 std::fromat 文本格式化的特性，默认未开启：

The paper is implemented but still marked as an incomplete feature (the feature-test macro is not set and the libary is only available when built with LIBCXX_ENABLE_INCOMPLETE_FEATURES). Not yet implemented LWG-issues will cause API and ABI breakage.

C++20 特性，编译器支持情况：

• C++ compiler support
• libc++ C++20 Status

因此，这里以源码方式安装 LLVM，需要构建 Clang & libc++：

• Building Clang
• Building libc++

git clone -b llvmorg-14.0.6 --depth 1 https://github.com/llvm/llvm-project.gitcd llvm-projectmkdir _buildcd _build# llvm install path, such as /usr/local/llvmLLVM_PREFIX=$HOME/Apps/llvm-14.0.6cmake -DCMAKE_BUILD_TYPE=Release \-DCMAKE_INSTALL_PREFIX=$LLVM_PREFIX \-DLLVM_ENABLE_PROJECTS=clang \-DLLVM_ENABLE_RUNTIMES="libcxx;libcxxabi" \-DLIBCXX_ENABLE_INCOMPLETE_FEATURES=ON \../llvmmake -j`nproc`make installsudo ln -s $LLVM_PREFIX /usr/local/llvmcat <<-EOF >> ~/.bashrc# llvmexport LLVM_HOME=/usr/local/llvmexport PATH=\$LLVM_HOME/bin:\$PATHexport LD_LIBRARY_PATH=\$LLVM_HOME/lib/x86_64-unknown-linux-gnu:\$LD_LIBRARY_PATHEOFllvm-config --versionclang --version

LLVM_PREFIX 安装路径自己决定。最后，编译测试：

cat <<-EOF > hello.cc#include <format>#include <iostream>int main() {  std::string message = std::format("The answer is {}.", 42);  std::cout << message << std::endl;}EOFclang++ -std=c++20 -stdlib=libc++ hello.cc -o hello./hello

安装 LLVM，以二进制方式

可省略该节。本文实践未用此方式，因为想开启更多 C++20 特性。这里仅作记录，有需要可参考。

方式 1. 安装二进制发布：

cd ~wget https://github.com/llvm/llvm-project/releases/download/llvmorg-13.0.0/clang+llvm-13.0.0-x86_64-linux-gnu-ubuntu-20.04.tar.xztar -xf clang+llvm-*.tar.xzsudo ln -s $(pwd)/clang+llvm-13.0.0-x86_64-linux-gnu-ubuntu-20.04 /usr/local/llvmcat <<-EOF >> ~/.bashrc# llvmexport LLVM_HOME=/usr/local/llvmexport PATH=\$LLVM_HOME/bin:\$PATHEOFllvm-config --versionclang --version

方式 2. 用 apt 进行安装： https://apt.llvm.org/

方式 3. 用已配好的工具链： LLVM toolchain for Bazel

配置 Clang 工具链

本文依照 Bazel Tutorial: Configure C++ Toolchains 步骤配置的 Clang 工具链，最后项目根目录会多如下文件：

• WORKSPACE
• .bazelrc
• toolchain/BUILD
• toolchain/cc_toolchain_config.bzl

WORKSPACE 表示 Bazel 工作区，内容空。

.bazelrc 允许 --config=clang_config 启用 Clang 工具链：

# Use our custom-configured c++ toolchain.build:clang_config --crosstool_top=//toolchain:clang_suite# Use --cpu as a differentiator.build:clang_config --cpu=linux_x86_64# Use the default Bazel C++ toolchain to build the tools used during the build.build:clang_config --host_crosstool_top=@bazel_tools//tools/cpp:toolchain

toolchain/BUILD 配置 Clang 工具链信息：

load(":cc_toolchain_config.bzl", "cc_toolchain_config")package(default_visibility = ["//visibility:public"])#filegroup(name = "clang_suite")cc_toolchain_suite(    name = "clang_suite",    toolchains = {        "linux_x86_64": ":linux_x86_64_toolchain",    },)filegroup(name = "empty")cc_toolchain(    name = "linux_x86_64_toolchain",    toolchain_identifier = "linux_x86_64-toolchain",    toolchain_config = ":linux_x86_64_toolchain_config",    all_files = ":empty",    compiler_files = ":empty",    dwp_files = ":empty",    linker_files = ":empty",    objcopy_files = ":empty",    strip_files = ":empty",    supports_param_files = 0,)#filegroup(name = "linux_x86_64_toolchain_config")cc_toolchain_config(name = "linux_x86_64_toolchain_config")

toolchain/cc_toolchain_config.bzl 配置 Clang 工具链规则：

# C++ Toolchain Configuration#  https://bazel.build/docs/cc-toolchain-config-reference#  https://github.com/bazelbuild/bazel/blob/master/tools/build_defs/cc/action_names.bzlload("@bazel_tools//tools/build_defs/cc:action_names.bzl", "ACTION_NAMES")load(    "@bazel_tools//tools/cpp:cc_toolchain_config_lib.bzl",    "feature",    "flag_group",    "flag_set",    "tool_path",)all_compile_actions = [    ACTION_NAMES.c_compile,    ACTION_NAMES.cpp_compile,    ACTION_NAMES.linkstamp_compile,    ACTION_NAMES.assemble,    ACTION_NAMES.preprocess_assemble,    ACTION_NAMES.cpp_header_parsing,    ACTION_NAMES.cpp_module_compile,    ACTION_NAMES.cpp_module_codegen,]all_link_actions = [    ACTION_NAMES.cpp_link_executable,    ACTION_NAMES.cpp_link_dynamic_library,    ACTION_NAMES.cpp_link_nodeps_dynamic_library,]def _impl(ctx):    llvm_version = "14.0.6"    llvm_prefix = "/home/john/Apps/llvm-{}".format(llvm_version)    llvm_bindir = llvm_prefix + "/bin"    tool_paths = [        tool_path(            name = "gcc",            path = llvm_bindir + "/clang",        ),        tool_path(            name = "ld",            path = llvm_bindir + "/ld.lld",        ),        tool_path(            name = "ar",            path = llvm_bindir + "/llvm-ar",        ),        tool_path(            name = "cpp",            path = llvm_bindir + "/clang-cpp",        ),        tool_path(            name = "gcov",            path = llvm_bindir + "/llvm-cov",        ),        tool_path(            name = "nm",            path = llvm_bindir + "/llvm-nm",        ),        tool_path(            name = "objdump",            path = llvm_bindir + "/llvm-objdump",        ),        tool_path(            name = "strip",            path = llvm_bindir + "/llvm-strip",        ),    ]    features = [        feature(            name = "default_compiler_flags",            enabled = True,            flag_sets = [                flag_set(                    actions = all_compile_actions,                    flag_groups = ([                        flag_group(                            flags = [                                "-O2", "-DNDEBUG",                                "-Wall", "-Wextra", "-Wpedantic", "-fPIC",                                "-std=c++20", "-stdlib=libc++",                            ],                        ),                    ]),                ),            ],        ),        feature(            name = "default_linker_flags",            enabled = True,            flag_sets = [                flag_set(                    actions = all_link_actions,                    flag_groups = ([                        flag_group(                            flags = [                                "-lc++", "-lc++abi",                                "-lm", "-ldl", "-lpthread",                            ],                        ),                    ]),                ),            ],        ),    ]    return cc_common.create_cc_toolchain_config_info(        ctx = ctx,        features = features,        cxx_builtin_include_directories = [            llvm_prefix + "/lib/clang/{}/include".format(llvm_version),            llvm_prefix + "/include/x86_64-unknown-linux-gnu/c++/v1",            llvm_prefix + "/include/c++/v1",            "/usr/local/include",            "/usr/include/x86_64-linux-gnu",            "/usr/include",        ],        toolchain_identifier = "local",        host_system_name = "local",        target_system_name = "local",        target_cpu = "linux_x86_64",        target_libc = "unknown",        compiler = "clang",        abi_version = "unknown",        abi_libc_version = "unknown",        tool_paths = tool_paths,    )cc_toolchain_config = rule(    implementation = _impl,    attrs = {},    provides = [CcToolchainConfigInfo],)

llvm_prefix 给到自己的 LLVM 安装路径。

构建 C++20 库与应用

本文示例的 code/00/ 路径下准备了 C++20 的库与应用：

code/00/├── BUILD├── greet│   ├── BUILD│   ├── greet.cc│   └── greet.h└── main.cc

编写 binary

main.cc:

#include <format>#include <iostream>#include <string>#include <string_view>#include "greet/greet.h"template <typename... Args>std::string dyna_print(std::string_view rt_fmt_str, Args&&... args) {  return std::vformat(rt_fmt_str, std::make_format_args(args...));}int main() {  std::cout << greet::hello("world") << std::endl;  std::string fmt;  for (int i{}; i != 3; ++i) {    fmt += "{} ";  // constructs the formatting string    std::cout << fmt << " : ";    std::cout << dyna_print(fmt, "alpha", 'Z', 3.14, "unused");    std::cout << '\n';  }}

BUILD:

load("@rules_cc//cc:defs.bzl", "cc_binary")cc_binary(    name = "main",    srcs = ["main.cc"],    deps = [        "//code/00/greet:greet",    ],)

编写 library

greet.h:

#pragma once#include <string>#include <string_view>namespace greet {std::string hello(std::string_view who);}  // namespace greet

greet.cc:

#include "greet.h"#include <format>#include <utility>namespace greet {std::string hello(std::string_view who) {  return std::format("Hello {}!", std::move(who));}}  // namespace greet

BUILD:

load("@rules_cc//cc:defs.bzl", "cc_library")package(default_visibility = ["//visibility:public"])cc_library(    name = "greet",    srcs = [        "greet.cc",    ],    hdrs = [        "greet.h",    ],)

Bazel 构建

bazel build --config=clang_config //code/00:main

运行测试

$ bazel-bin/code/00/mainHello world!{}  : alpha{} {}  : alpha Z{} {} {}  : alpha Z 3.14

查看依赖

sudo apt update && sudo apt install graphviz xdot -y# viewxdot <(bazel query --notool_deps --noimplicit_deps "deps(//code/00:main)" --output graph)# to svgdot -Tsvg <(bazel query --notool_deps --noimplicit_deps "deps(//code/00:main)" --output graph) -o 00_main.svg

更多参考

• Bazel Tutorial
  • Configure C++ Toolchains
  • Build a C++ Project
• Bazel Issue
  • Support C++20 modules #4005
• Project Example
  • How to Use C++20 Modules with Bazel and Clang
  • bazel-cpp20: Template for bazel with C++20
  • Clang toolchain

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