[dllimport] Honor always_inline when deciding whether a dllimport function should be available for inlining (PR48925)

[hans]

Summary

Normally, Clang will not make dllimport functions available for inlining if they reference non-imported symbols, as this can lead to confusing link errors. But if the function is marked always_inline, the user presumably knows what they're doing and the attribute should be honored.

[rnk]

lgtm

clang/lib/CodeGen/CodeGenModule.cpp

 
 bool CodeGenModule::shouldEmitFunction(GlobalDecl GD) {
   if (getFunctionLinkage(GD) != llvm::Function::AvailableExternallyLinkage)
     return true;
   const auto *F = cast<FunctionDecl>(GD.getDecl());
   if (CodeGenOpts.OptimizationLevel == 0 && !F->hasAttr<AlwaysInlineAttr>())
     return false;
 
-  if (F->hasAttr<DLLImportAttr>()) {
+  if (F->hasAttr<DLLImportAttr>() && !F->hasAttr<AlwaysInlineAttr>()) {
     // Check whether it would be safe to inline this dllimport function.
     DLLImportFunctionVisitor Visitor;
     Visitor.TraverseFunctionDecl(const_cast<FunctionDecl*>(F));
     if (!Visitor.SafeToInline)
       return false;
 
     if (const CXXDestructorDecl *Dtor = dyn_cast<CXXDestructorDecl>(F)) {
       // Implicit destructor invocations aren't captured in the AST, so the

clang/test/CodeGenCXX/dllimport.cpp

 // A dllimport function with a TLS variable must not be available_externally.
 __declspec(dllimport) inline void FunctionWithTLSVar() { static __thread int x = 42; }
 // MO1-DAG: declare dllimport void @"?FunctionWithTLSVar@@YAXXZ"
 __declspec(dllimport) inline void FunctionWithNormalVar() { static int x = 42; }
 // MO1-DAG: define available_externally dllimport void @"?FunctionWithNormalVar@@YAXXZ"
 USE(FunctionWithTLSVar)
 USE(FunctionWithNormalVar)
 
+// always_inline and __force_inline take precedence.
+__declspec(dllimport) inline int ReferencingNonImportedFuncAlwaysInline() __attribute__((always_inline)) { return NonImportedFunc(); }
+USE(ReferencingNonImportedFuncAlwaysInline)
+// MO1-DAG: define available_externally dllimport i32 @"?ReferencingNonImportedFuncAlwaysInline@@YAHXZ"
+__declspec(dllimport) __forceinline int ReferencingNonImportedFuncForceInline() { return NonImportedFunc(); }
+USE(ReferencingNonImportedFuncForceInline)
+// MO1-DAG: define available_externally dllimport i32 @"?ReferencingNonImportedFuncForceInline@@YAHXZ"
 
 //===----------------------------------------------------------------------===//
 // Function templates
 //===----------------------------------------------------------------------===//
 
 // Import function template declaration.
 // MSC-DAG: declare dllimport void @"??$funcTmplDecl@UImplicitInst_Imported@@@@YAXXZ"()
 // GNU-DAG: declare dllimport void @_Z12funcTmplDeclI21ImplicitInst_ImportedEvv()