thread_local member variable construction

问题

I'm facing some strange behavior with thread_local and not sure whether I'm doing something wrong or it's a GCC bug. I have the following minimal repro scenario:

#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;
}

With the commented line above the output looks like this:

main
0

Ideone

With it uncommented, it becomes this:

main
foo
foo
42
42

Ideone

Am I just missing something stupid here?

$ 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)

Update:

This provides unexpected results as well:

#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;
}

Result:

main
0
far::foo
bar::foo
42

回答1:

This is too long for a comment, though I don't claim to fully understand it.

I have a shorter version you can run in Coliru

#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
}

I think the bug is in this gcc source file

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

At this point gcc is trying to decide if FOO, which is a static member of bar, needs a wrapper function to detect if it has been initialized... it decides no wrapper is needed, which is incorrect. It checks

Is it not an error_operand_p ? Yes, it is not. (I guess)
Is it thread_local (DECL_THREAD_LOCAL_P) ? Yes it is thread_local.
Is it not gnu __thread extension (DECL_GNU_TLS_P) ? Yes, it is not.
Is it not declared in function scope (DECL_FUNCTION_SCOPE_P) ? Yes, it is not.
Is the variable not defined in another translation unit (TU)? Yes, it is not. (bug?)
Does it not have a non-trivial destructor? Yes, it does not.
Does it have no initializer or a constant one? It has an initializer, but it is constant.
It doesn't need a wrapper

The flaw is either:

Concluding that if the initializer is constant then it isn't dynamically initialized, or
Failing to properly do the static initialization, or
Failing to notice that even though it is a member variable it could be externally defined

Since the initialization is done by the constructor, I think that is the source of the confusion, a constructor is called, but the value is a constant.

Here's the code

/* 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));
}

来源：https://stackoverflow.com/questions/22722941/thread-local-member-variable-construction

标签

c++

c++11

thread-local

construction

gcc4.8