thread_local成员变量构造

Question

我在使用thread_local时遇到了一些奇怪的行为,并且不确定我是做错了什么还是GCC错误.我有以下最小的repro场景:

#include <iostream>

using namespace std;

struct bar {
    struct foo {
        foo () {
            cerr << "foo" << endl;
        }
        int i = 42;
    };

    static thread_local foo FOO;
};

static thread_local bar::foo FREE_FOO;
thread_local bar::foo bar::FOO;

int main() {
    bar b;
    cerr << "main" << endl;
    // cerr << FREE_FOO.i << endl;
    cerr << b.FOO.i << endl;
    return 0;
}

使用上面的注释行,输出如下所示:

main
0

Ideone

随着它取消注释,它变成了这样:

main
foo
foo
42
42

Ideone

我只是在这里错过了一些愚蠢的东西吗？

$ gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/4.8/lto-wrapper
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu/Linaro 4.8.1-10ubuntu9' --with-bugurl=file:///usr/share/doc/gcc-4.8/README.Bugs --enable-languages=c,c++,java,go,d,fortran,objc,obj-c++ --prefix=/usr --program-suffix=-4.8 --enable-shared --enable-linker-build-id --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --with-gxx-include-dir=/usr/include/c++/4.8 --libdir=/usr/lib --enable-nls --with-sysroot=/ --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --enable-gnu-unique-object --enable-plugin --with-system-zlib --disable-browser-plugin --enable-java-awt=gtk --enable-gtk-cairo --with-java-home=/usr/lib/jvm/java-1.5.0-gcj-4.8-amd64/jre --enable-java-home --with-jvm-root-dir=/usr/lib/jvm/java-1.5.0-gcj-4.8-amd64 --with-jvm-jar-dir=/usr/lib/jvm-exports/java-1.5.0-gcj-4.8-amd64 --with-arch-directory=amd64 --with-ecj-jar=/usr/share/java/eclipse-ecj.jar --enable-objc-gc --enable-multiarch --disable-werror --with-arch-32=i686 --with-abi=m64 --with-multilib-list=m32,m64,mx32 --with-tune=generic --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
gcc version 4.8.1 (Ubuntu/Linaro 4.8.1-10ubuntu9) 

更新:

这也提供了意想不到的结果:

#include <iostream>

using namespace std;

template<class T>
struct bar {
    struct foo {
        foo () {
            cerr << "bar::foo" << endl;
        }
        int i = 42;
    };

    void baz() {
        cerr << bar::FOO.i << endl;
    }

    static thread_local foo FOO;
};

struct far {
    struct foo {
        foo () {
            cerr << "far::foo" << endl;
        }
        int i = 42;
    };

    void baz() {
        cerr << far::FOO.i << endl;
    }

    static thread_local foo FOO;
};

template<class T> thread_local typename bar<T>::foo bar<T>::FOO;
thread_local typename far::foo far::FOO;

int main() {
    cerr << "main" << endl;
    bar<int> b;
    b.baz();

    far f;
    f.baz();
    return 0;
}

结果:

main
0
far::foo
bar::foo
42

Answer 1

这对于评论来说太长了,尽管我没有声称完全理解它.

我有一个较短的版本,你可以在Coliru运行

#include <iostream>
using namespace std;

struct foo {
    int i;
    foo() : i{42} {}
};

struct bar {
    static thread_local foo FOO;
};

thread_local foo bar::FOO;

int main() {
    //cerr << string((bar::FOO.i == 42) ? "Ok" : "Bug") << endl; //Ok
    cerr << string((bar().FOO.i == 42) ? "Ok" : "Bug") << endl;  //Bug
}

我认为这个bug存在于这个gcc源文件中

https://chromium.googlesource.com/native_client/nacl-gcc/+/upstream/master/gcc/cp/decl2.c

此时GCC正试图决定是否FOO,这是一个静态成员bar,需要的包装功能,以检测它是否已被初始化......它决定不需要包装,这是不正确.它检查

1. 它不是error_operand_p吗？是的,事实并非如此.(我猜)
2. 是thread_local(DECL_THREAD_LOCAL_P)吗？是的,它是thread_local.
3. 是不是gnu __thread扩展名(DECL_GNU_TLS_P)？是的,事实并非如此.
4. 它是否未在函数范围内声明(DECL_FUNCTION_SCOPE_P)？是的,事实并非如此.
5. 变量是否未在另一个翻译单元(TU)中定义？是的,事实并非如此.(错误？)
6. 它没有非平凡的析构函数吗？是的,它没有.
7. 它没有初始化器或常量器吗？它有一个初始化程序,但它是不变的.
8. 它不需要包装器

缺陷是:

1. 得出结论,如果初始化程序是常量,那么它不会动态初始化,或者
2. 无法正确执行静态初始化,或
3. 没注意到即使它是一个成员变量,它也可以在外部定义

由于初始化是由构造函数完成的,我认为这是混淆的来源,调用构造函数,但值是常量.

这是代码

/* Returns true iff we can tell that VAR does not have a dynamic
   initializer.  */

static bool
var_defined_without_dynamic_init (tree var)
{
    /* If it's defined in another TU, we can't tell.  */
    if (DECL_EXTERNAL (var))
        return false;
    /* If it has a non-trivial destructor, registering the destructor
        counts as dynamic initialization.  */
    if (TYPE_HAS_NONTRIVIAL_DESTRUCTOR (TREE_TYPE (var)))
        return false;
    /* If it's in this TU, its initializer has been processed.  */
        gcc_assert (DECL_INITIALIZED_P (var));
    /* If it has no initializer or a constant one, it's not dynamic.  */
        return (!DECL_NONTRIVIALLY_INITIALIZED_P (var)
          || DECL_INITIALIZED_BY_CONSTANT_EXPRESSION_P (var));
}

/* Returns true iff VAR is a variable that needs uses to be
   wrapped for possible dynamic initialization.  */

static bool
var_needs_tls_wrapper (tree var)
{
    return (!error_operand_p (var)
          && DECL_THREAD_LOCAL_P (var)
          && !DECL_GNU_TLS_P (var)
          && !DECL_FUNCTION_SCOPE_P (var)
          && !var_defined_without_dynamic_init (var));
}