为什么Apple clang在'官方'clang支持时不允许使用C++ 11 thread_local

Question

下面是一个简单的程序,它在共享库中使用非POD类型的C++ 11 thread_local变量进行测试.

如果我使用自制软件,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洞察到为什么Apple的铿锵变种懦弱拒绝尊重thread_local,尽管底层编译器支持它,并且生成的代码似乎有效？

lib.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);
}

main.cpp中:

#include "lib.h"

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

的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和9中包含的clang编译器支持C++ 11 thread_local关键字.此功能已添加到Xcode 8测试版中,如WWDC 2016视频"LLVM中的新功能"所述,从5:50开始.(外部成绩单)

问题中列出的示例程序在OS X 10.11.6下与Xcode 8 GM一起编译并运行,以及在macOS 10.13.4下与Xcode 9.3一起运行,并在每种情况下生成预期输出.

关于iOS,我通过实验发现thread_localiOS 9及更高版本支持,但iOS 8.4或更早版本不支持.

对于Xcode 7.x及更早版本,以下是2014年苹果工程师在旧Apple开发者论坛上的答案(不再可访问):

我们不支持开源Clang的thread_local实现,因为我们相信我们可以使用动态链接器中的各种功能为我们的平台提供更高性能的实现.这样的实现将与开源Clang中的实现ABI不兼容,因此我们将不会支持thread_local,直到我们得到一个我们可以在可预见的未来实现的实现.

后续帖子确认thread_localXcode 6.3中仍然不支持.