xcode - 为什么当 'official' clang 支持 C++11 thread_local 时，Apple clang 不允许它

Question

下面是一个简单的程序，它使用共享库中的非 POD 类型的 C++11 thread_local 变量进行测试。

如果我使用自制的 clang ，这很好用:

> /usr/local/Cellar/llvm/3.5.0_2/bin/clang --version                                          
clang version 3.5.0 (tags/RELEASE_350/final)
Target: x86_64-apple-darwin14.0.0
Thread model: posix

> cmake .. -G Ninja -DCMAKE_C_COMPILER=/usr/local/Cellar/llvm/3.5.0_2/bin/clang -DCMAKE_CXX_COMPILER=/usr/local/Cellar/llvm/3.5.0_2/bin/clang++
-- The C compiler identification is Clang 3.5.0
-- The CXX compiler identification is Clang 3.5.0
-- Check for working C compiler using: Ninja
-- Check for working C compiler using: Ninja -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working CXX compiler using: Ninja
-- Check for working CXX compiler using: Ninja -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Configuring done
-- Generating done
> ninja all
...                                                                                      

>  ./main                                                                                       
XXX LifeCycle::LifeCycle 0x7fedc0c04b90
X before: -17
XXX LifeCycle::LifeCycle 0x7fedc0c04c10
X before in thread: -17
X after in thread: 2
XXX LifeCycle::~LifeCycle 0x7fedc0c04c10
X after: 1
XXX LifeCycle::~LifeCycle 0x7fedc0c04b90

但是，如果我尝试使用 Apple Clang，我会收到一条错误消息，指出它不受支持:

> /usr/bin/clang --version
Apple LLVM version 6.0 (clang-600.0.56) (based on LLVM 3.5svn)
Target: x86_64-apple-darwin14.0.0
Thread model: posix
> cmake .. -G Ninja -DCMAKE_C_COMPILER=/usr/bin/clang -DCMAKE_CXX_COMPILER=/usr/bin/clang++
-- The C compiler identification is AppleClang 6.0.0.6000056
-- The CXX compiler identification is AppleClang 6.0.0.6000056
-- Check for working C compiler using: Ninja
-- Check for working C compiler using: Ninja -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working CXX compiler using: Ninja
-- Check for working CXX compiler using: Ninja -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Configuring done
-- Generating done
-- Build files have been written to:

> ninja all
[1/4] Building CXX object CMakeFiles/lib.dir/lib.cpp.o
FAILED: /usr/bin/clang++   -Dlib_EXPORTS -Wall -std=c++11 -mmacosx-version-min=10.7 -stdlib=libc++ -fPIC -MMD -MT CMakeFiles/lib.dir/lib.cpp.o -MF CMakeFiles/lib.dir/lib.cpp.o.d -o CMakeFiles/lib.dir/lib.cpp.o -c ../lib.cpp
../lib.cpp:23:5: error: thread-local storage is unsupported for the current target
    thread_local LifeCycle lc;
    ^
1 error generated.
ninja: build stopped: subcommand failed.

任何人都可以提供任何见解，尽管底层编译器支持它，并且生成的代码似乎可以工作，但为什么 Apple 的 clang 变体懦弱地拒绝尊重 thread_local？

库.h:

#pragma once

int doit(int) __attribute__((__visibility__("default")));

lib.cpp:

#include "lib.h"

#include <thread>
#include <cstdlib>
#include <cstdio>

namespace {

    class LifeCycle {
    public:
        LifeCycle()
            : x(-17) {
            printf("XXX LifeCycle::LifeCycle %p\n", this);
        }

        ~LifeCycle() {
            printf("XXX LifeCycle::~LifeCycle %p\n", this);
        }

        int x;
    };

    thread_local LifeCycle lc;
} // namespace

int doit(int arg) {
    printf("X before: %d\n", lc.x);
    lc.x = arg;
    std::thread xwriter([arg]() {
            if (lc.x == arg)
                abort();
            printf("X before in thread: %d\n", lc.x);
            lc.x = arg + 1;
            printf("X after in thread: %d\n", lc.x);
        });
    xwriter.join();
    printf("X after: %d\n", lc.x);
    return (lc.x == arg ? EXIT_SUCCESS : EXIT_FAILURE);
}

主.cpp:

#include "lib.h"

int main(int argc, char* argv[]) {
    return doit(argc);
}

CMakeLists.txt:

cmake_minimum_required(VERSION 3.1)

set(CMAKE_CXX_FLAGS "-Wall -std=c++11 -mmacosx-version-min=10.7 -stdlib=libc++")

add_library(lib SHARED lib.cpp)
add_executable(main main.cpp)
target_link_libraries(main lib)

Answer 1

Xcode 8 及更高版本中包含的 clang 编译器支持 C++11 thread_local关键词。此功能已添加到 Xcode 8 beta 中，如 the WWDC 2016 video "What's New in LLVM" 中所述。 , 从 the 5:50 mark 开始. ( external transcript )

问题中列出的示例程序在 OS X 10.11.6 下使用 Xcode 8 GM 编译和运行，并产生预期的输出。它随后在 macOS 10.13.4 下的 Xcode 9.3 和 macOS 10.14.4 下的 Xcode 10.2.1 进行了重新测试，并继续按预期运行。

关于iOS，我通过实验发现thread_local支持 iOS 9 及更高版本，但不支持 iOS 8.4 或更早版本。

对于 Xcode 7.x 及更早版本，以下是 2014 年 Apple 工程师在旧 Apple Developer Forum(不再可访问)上的回答:

We don't support the thread_local implementation from the open-source Clang because we believe we can provide a higher-performance implementation for our platforms using various features in the dynamic linker. Such an implementation would be ABI-incompatible with the implementation in the open-source Clang, so we won't support thread_local until we get an implementation we can live with for the foreseeable future.

随后的帖子证实 thread_local Xcode 6.3 中仍然不支持。