[C++/CLI] Direct access of unmanaged thread_local variable from managed code causes unexpected NullReferenceException

[LEI-Hongfaan]

Description

Consider the following code

// Compile with /clr

#pragma managed(push, off)

thread_local int thread_local_value;

int get_thread_local_value() {
    return thread_local_value;
}

void set_thread_local_value(int value) {
    thread_local_value = value;
}
#pragma managed(pop)

public ref class Test abstract sealed {

public:
    static int ManipulateThreadLocalValue(int value) {
        thread_local_value = value; // Throws a NullReferenceException at runtime
        return thread_local_value;
/* Jitted code
00007FFCF0FCFE70  push        rbp  
00007FFCF0FCFE71  sub         rsp,20h  
00007FFCF0FCFE75  lea         rbp,[rsp+20h]  
00007FFCF0FCFE7A  mov         dword ptr [rbp+10h],ecx  
00007FFCF0FCFE7D  mov         rcx,7FFCF1090020h  
00007FFCF0FCFE87  call        qword ptr [0]  ; <-- ???
00007FFCF0FCFE8E  mov         edx,dword ptr [rbp+10h]  
00007FFCF0FCFE91  mov         dword ptr [rax],edx  
00007FFCF0FCFE93  mov         eax,dword ptr [rbp+10h]  
00007FFCF0FCFE96  lea         rsp,[rbp]  
00007FFCF0FCFE9A  pop         rbp  
00007FFCF0FCFE9B  ret  
*/
    }

    static int ManipulateThreadLocalValue_Workaround(int value) {
        set_thread_local_value(value); // Ok
        return get_thread_local_value();
    }
};

Expected behavior

1. VC++ Compiler should warn about it if not supported. - Or -
2. VC++ Compiler should emit suitable metadata to identify thread_local fields and let the .NET runtime to deal with them. - Or -
3. VC++ Compiler should generate unmanaged stubs to manipulate thread_local fields for the managed code. - Or -
4. .NET jitter should correctly handle unmanaged thread_local field access.

I'm not very sure it's the right place to report the bug. It's more likely a problem of VC++ Compiler.

category:correctness
theme:managed-c++
skill-level:beginner
cost:medium
impact:small

[BruceForstall]

@LEI-Hongfaan What versions of VC++ and .NET are you using?

[BruceForstall]

@dotnet/jit-contrib Anyone familiar with the intersection of VC++ /clr, thread local variables, and JIT?

[jkotas]

This is related to CORINFO_HELP_GETSTATICFIELDADDR_TLS JIT helper:

runtime/src/coreclr/src/inc/jithelpers.h

Line 179 in 6072e4d

JITHELPER(CORINFO_HELP_GETSTATICFIELDADDR_TLS, NULL, CORINFO_HELP_SIG_CANNOT_USE_ALIGN_STUB)

It looks like this helper was not brought back as part of the adding managed C++ support back.

[BruceForstall]

@jkotas, is this a .NET 5 issue?

[jkotas]

.NET 3.1 shipped with this bug. It is not a .NET 3.1 -> 5.0 regression.

We should either fix in both 3.1 and 5.0 or neither.

[BruceForstall]

Marking as 6.0 for now.