C++20 以 Bazel & Clang 开始

技术分享 2年前 (2022-07-12) 0 999+

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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-amd64 chmod a+x bazelisk-*  sudo ln -s $(pwd)/bazelisk-1.12.0-linux-amd64 /usr/local/bin/bazel  touch 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/220 cat <<-EOF > .bazeliskrc BAZELISK_BASE_URL=https://github.com/bazelbuild/bazel/releases/download USE_BAZEL_VERSION=5.2.0 EOF  bazel 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/buildifier sudo 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：

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

git clone -b llvmorg-14.0.6 --depth 1 https://github.com/llvm/llvm-project.git  cd llvm-project mkdir _build cd _build  # llvm install path, such as /usr/local/llvm LLVM_PREFIX=$HOME/Apps/llvm-14.0.6  cmake -DCMAKE_BUILD_TYPE=Release  -DCMAKE_INSTALL_PREFIX=$LLVM_PREFIX  -DLLVM_ENABLE_PROJECTS=clang  -DLLVM_ENABLE_RUNTIMES="libcxx;libcxxabi"  -DLIBCXX_ENABLE_INCOMPLETE_FEATURES=ON  ../llvm  make -j`nproc` make install  sudo ln -s $LLVM_PREFIX /usr/local/llvm  cat <<-EOF >> ~/.bashrc # llvm export LLVM_HOME=/usr/local/llvm export PATH=$LLVM_HOME/bin:$PATH export LD_LIBRARY_PATH=$LLVM_HOME/lib/x86_64-unknown-linux-gnu:$LD_LIBRARY_PATH EOF  llvm-config --version clang --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; } EOF  clang++ -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.xz tar -xf clang+llvm-*.tar.xz  sudo ln -s $(pwd)/clang+llvm-13.0.0-x86_64-linux-gnu-ubuntu-20.04 /usr/local/llvm  cat <<-EOF >> ~/.bashrc # llvm export LLVM_HOME=/usr/local/llvm export PATH=$LLVM_HOME/bin:$PATH EOF  llvm-config --version clang --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 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.bzl load("@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/main Hello world! {}  : alpha {} {}  : alpha Z {} {} {}  : alpha Z 3.14

查看依赖

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