[ItaniumCXXABI] Don't mark an extern_weak init function as dso_local on windows

[mstorsjo]

Summary

Since 6bf108d77a3c, we try to not mark extern_weak symbols as dso_local, to allow using COFF stubs for references to those symbols (as the symbol may be missing, resolving to an absolute address zero, outside of the current DSO).

[rnk]

lgtm

TLS thread wrappers seem to reimplement a lot of the logic that would happen naturally for a regular function declaration. I wonder if there is some way to restructure the code to have less duplication.

clang/lib/CodeGen/ItaniumCXXABI.cpp

if (!usesThreadWrapperFunction(VD)) {
                                     InitFnName.str(), &CGM.getModule());
       const CGFunctionInfo &FI = CGM.getTypes().arrangeNullaryFunction();
       CGM.SetLLVMFunctionAttributes(GlobalDecl(), FI,
                                     cast<llvm::Function>(Init));
     }
 
     if (Init) {
       Init->setVisibility(Var->getVisibility());
+      // Don't mark an extern_weak function DSO local on windows.
+      if (!CGM.getTriple().isOSWindows() || !Init->hasExternalWeakLinkage())
         Init->setDSOLocal(Var->isDSOLocal());
     }
 
     llvm::LLVMContext &Context = CGM.getModule().getContext();
     llvm::BasicBlock *Entry = llvm::BasicBlock::Create(Context, "", Wrapper);
     CGBuilderTy Builder(CGM, Entry);
     if (HasConstantInitialization) {
       // No dynamic initialization to invoke.
     } else if (InitIsInitFunc) {

clang/test/CodeGenCXX/mingw-thread-local.cpp

+// RUN: %clang_cc1 -std=c++11 -emit-llvm %s -o - -triple x86_64-w64-mingw32 | FileCheck %s
+
+extern thread_local int var;
+
+int get() {
+  return var;
+}
+
+// CHECK: declare extern_weak void @_ZTH3var()