std::for_each 与 std::execution::par_unseq 不适用于 GCC 但适用于 MSVC

Question

我想并行化一个 for 循环并发现 aboutstd::for_each以及它的execution policies. 令人惊讶的是，它在使用时没有并行化GCC：

#include <iostream>
#include <algorithm>
#include <execution>
#include <chrono>
#include <thread>
#include <random>

int main() {
    std::vector<int> foo;
    foo.reserve(1000);
    for (int i = 0; i < 1000; i++) {
        foo.push_back(i);
    }

    std::for_each(std::execution::par_unseq,
                  foo.begin(), foo.end(),
                  [](auto &&item) {
                      std::cout << item << std::endl;
                      std::random_device dev;
                      std::mt19937 rng(dev());
                      std::uniform_int_distribution<std::mt19937::result_type> dist6(10, 100);
                      std::this_thread::sleep_for(std::chrono::milliseconds(dist6(rng)));
                      std::cout << "Thread ID: " << std::this_thread::get_id() << std::endl;
                  });
}

此代码仍按顺序运行。

使用MSVC代码是并行的，并且完成得更快。

GCC：

$ gcc --version
gcc (Ubuntu 10.1.0-2ubuntu1~18.04) 10.1.0
Copyright (C) 2020 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

MSVC：

>cl.exe
Microsoft (R) C/C++ Optimizing Compiler Version 19.27.29112 for x86
Copyright (C) Microsoft Corporation.  All rights reserved.

usage: cl [ option... ] filename... [ /link linkoption... ]

CMakeLists.txt：

cmake_minimum_required(VERSION 3.17)
project(ParallelTesting)

set(CMAKE_CXX_STANDARD 20)

add_executable(ParallelTesting main.cpp)

我还需要做些什么来实现并行化GCC？

ldd 我的二进制文件的输出：

$ ldd my_binary
    linux-vdso.so.1 (0x00007ffe9e6b9000)
    libdl.so.2 => /lib/x86_64-linux-gnu/libdl.so.2 (0x00007f79efaa0000)
    libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0 (0x00007f79ef881000)
    libstdc++.so.6 => /usr/lib/x86_64-linux-gnu/libstdc++.so.6 (0x00007f79ef4ad000)
    libgcc_s.so.1 => /lib/x86_64-linux-gnu/libgcc_s.so.1 (0x00007f79ef295000)
    libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f79eeea4000)
    /lib64/ld-linux-x86-64.so.2 (0x00007f79f041a000)
    libm.so.6 => /lib/x86_64-linux-gnu/libm.so.6 (0x00007f79eeb06000)

二进制文件的debug和release版本总体具有相同的ldd输出。

Answer 1

我通过首先将我的WSL Ubuntu发行版从版本升级18.04到来解决这个问题20.04，因为运行sudo apt install gcc libtbb-dev安装后TBB我仍然收到以下错误： #error Intel(R) Threading Building Blocks 2018 is required; older versions are not supported. 这是由于TBB太旧而引起的。

现在TBB 2002.1-2安装完毕，它可以按预期工作：

$ sudo apt install libtbb-dev
[sudo] password for ubuntu:
Reading package lists... Done
Building dependency tree
Reading state information... Done
libtbb-dev is already the newest version (2020.1-2).
0 upgraded, 0 newly installed, 0 to remove and 10 not upgraded.

这个答案很好地描述了所有细节。

由于我正在使用，CMake我还必须将以下行添加到我的CMakeLists.txt：

# Link against the dependency of Intel TBB (for parallel C++17 algorithms)
target_link_libraries(${PROJECT_NAME} tbb)