Checked build JIT assertion (`pfnHelper != NULL`) when accessing `__declspec(thread)` TLS from IJW/C++CLI managed entry points

[jkoritzinsky]

Description

When a mixed-mode (IJW/C++/CLI) assembly declares a native, file-scope __declspec(thread) global (wrapped in #pragma unmanaged/#pragma managed) and that global is accessed from a public ref class static method, calling into that method from managed code under a Checked CoreCLR build crashes with a JIT assertion:

Assert failure(PID ####): pfnHelper != (PCODE)NULL

CORECLR! CEECodeGenInfo::getHelperFtn + 0x4E7
CLRJIT! Compiler::compGetHelperFtn + 0x35
CLRJIT! Lowering::LowerDirectCall + 0xA5
CLRJIT! Lowering::LowerCall + 0x429
CLRJIT! Lowering::LowerNode + 0x32C
CLRJIT! Lowering::DoPhase + 0x13B
CLRJIT! Phase::Run + 0x76
CLRJIT! Compiler::compCompile + 0x1AD2
CLRJIT! Compiler::compCompileHelper + 0xC51
CLRJIT! Compiler::compCompileAfterInit + 0x7B1
    File: src\coreclr\vm\jitinterface.cpp:11353

The assertion fires in CEECodeGenInfo::getHelperFtn (src/coreclr/vm/jitinterface.cpp:11404, in a FEATURE_PORTABLE_ENTRYPOINTS code path), where the JIT's Lowering::LowerDirectCall requests a JIT helper function pointer and the helper table entry is unpopulated.

Repro

Minimal repro (Windows, x64, Checked build):

test.cpp (compiled as an IJW/C++CLI assembly, e.g. via /clr):

#pragma unmanaged
__declspec(thread) int s_tlsFieldData = 51966;
#pragma managed

public ref class TlsTest
{
public:
    static int Test()
    {
        int value = s_tlsFieldData;
        s_tlsFieldData = value;
        s_tlsFieldData = 100;
        return s_tlsFieldData;
    }
};

Managed caller (any invocation mechanism — both plain reflection MethodInfo.Invoke and UnsafeAccessor reproduce identically):

Assembly a = Assembly.Load("TestTLSNative");
int result = (int)a.GetType("TlsTest").GetMethod("Test").Invoke(null, null);

Running this against a Checked CoreCLR build crashes with the assertion above. The same code runs successfully against Debug and Release builds. Switching the native storage from __declspec(thread) to a managed [System::ThreadStatic] field on the ref class avoids the crash entirely.

Analysis so far

• The crash is independent of the managed→native call mechanism: it reproduces identically whether the native method is invoked via reflection (MethodInfo.Invoke) or via [UnsafeAccessor]/[UnsafeAccessorType].
• The crash is tied specifically to __declspec(thread) TLS storage accessed through the IJW-hosted managed entry point; using [System::ThreadStatic] instead does not reproduce the crash.
• This was discovered while converting src/tests regression tests that originally used IL .data tls declarations to C++/CLI (see Convert .data tls IL tests to C++/CLI #133533), where a reviewer requested using true native __declspec(thread) TLS (rather than [System::ThreadStatic]) "to actually be testing the right feature." Reintroducing __declspec(thread) reproduced this crash in all four converted tests.

Configuration

• OS: Windows x64
• Build: Checked
• Repro requires /clr (IJW/C++CLI) mixed-mode assembly

Note

This issue was filed with assistance from GitHub Copilot.

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[MichalStrehovsky]

This is likely a dup of #42187

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