Sema, CodeGen: Ensure that an implicit copy ctor is always available under the Microsoft C++ ABI.

[pcc]

Summary

This is needed because whether the constructor is deleted can control whether
we pass structs by value directly.

To fix this properly we probably want a more direct way for CodeGen to ask
whether the constructor was deleted.

Fixes PR31049.

[rnk]

RIchard, thoughts on which quick fix is better? Forcing the implicit decls or only handling the 'definition_only' case by looking for nontrivial move ctors in MicrosoftCXXABI?

[pcc]

New approach

[rnk]

lgtm This seems like a reasonable fix for now. Thanks for going through all that!

clang/lib/CodeGen/MicrosoftCXXABI.cpp

case llvm::Triple::x86_64:
     // stack slot. Most objects with dtors are large-ish, so handle that early.
     // We can't call out all large objects as being indirect because there are
     // multiple x64 calling conventions and the C++ ABI code shouldn't dictate
     // how we pass large POD types.
     if (RD->hasNonTrivialDestructor() &&
         getContext().getTypeSize(RD->getTypeForDecl()) > 64)
       return RAA_Indirect;
 
+    // We need Sema to have determined whether the implicit copy constructor is
+    // deleted. FIXME: We should provide a more direct way for CodeGen to ask
+    // whether the constructor was deleted.
+    assert(!RD->needsImplicitCopyConstructor());
+
     // We have a trivial copy constructor or no copy constructors, but we have
     // to make sure it isn't deleted.
-    bool CopyDeleted = false;
     for (const CXXConstructorDecl *CD : RD->ctors()) {
       if (CD->isCopyConstructor()) {
         assert(CD->isTrivial());
         // We had at least one undeleted trivial copy ctor.  Return directly.
         if (!CD->isDeleted())
           return RAA_Default;
-        CopyDeleted = true;
       }
     }
 
     // The trivial copy constructor was deleted.  Return indirectly.
-    if (CopyDeleted)
-      return RAA_Indirect;
-
-    // There were no copy ctors.  Return in RAX.
-    return RAA_Default;
   }
 
   llvm_unreachable("invalid enum");
 }
 
 void MicrosoftCXXABI::emitVirtualObjectDelete(CodeGenFunction &CGF,
                                               const CXXDeleteExpr *DE,
                                               Address Ptr,

clang/lib/Sema/SemaDeclCXX.cpp

void Sema::AddImplicitlyDeclaredMembersToClass(CXXRecordDecl *ClassDecl) {
   }
 
   if (ClassDecl->needsImplicitCopyConstructor()) {
     ++ASTContext::NumImplicitCopyConstructors;
 
     // If the properties or semantics of the copy constructor couldn't be
     // determined while the class was being declared, force a declaration
     // of it now.
+    //
+    // We also need to declare the implicit copy constructor under the Microsoft
+    // C++ ABI because whether the constructor is deleted can control whether we
+    // pass the record by value directly (see MicrosoftCXXABI::getRecordArgABI).
+    // FIXME: We should provide a more direct way for CodeGen to ask whether the
+    // constructor was deleted.
     if (ClassDecl->needsOverloadResolutionForCopyConstructor() ||
-        ClassDecl->hasInheritedConstructor())
+        ClassDecl->hasInheritedConstructor() ||
+        Context.getTargetInfo().getCXXABI().isMicrosoft())
       DeclareImplicitCopyConstructor(ClassDecl);
   }
 
   if (getLangOpts().CPlusPlus11 && ClassDecl->needsImplicitMoveConstructor()) {
     ++ASTContext::NumImplicitMoveConstructors;
 
     if (ClassDecl->needsOverloadResolutionForMoveConstructor() ||
         ClassDecl->hasInheritedConstructor())

clang/test/CodeGenCXX/uncopyable-args.cpp

 // not clear whether we should pass by address or in registers.
 // CHECK-DISABLED-LABEL: define void @_ZN14two_copy_ctors3barEv()
 // CHECK-DISABLED: call void @_Z{{.*}}C1Ev(
 // CHECK-DISABLED: call void @_ZN14two_copy_ctors3fooENS_1BE(%"struct.two_copy_ctors::B"* %{{.*}})
 // CHECK-DISABLED-LABEL: declare void @_ZN14two_copy_ctors3fooENS_1BE(%"struct.two_copy_ctors::B"*)
 
 // WIN64-LABEL: declare void @"\01?foo@two_copy_ctors@@YAXUB@1@@Z"(%"struct.two_copy_ctors::B"*)
 }
+
+namespace definition_only {
+struct A {
+  A();
+  A(A &&o);
+  void *p;
+};
+void *foo(A a) { return a.p; }
+// WIN64-LABEL: define i8* @"\01?foo@definition_only@@YAPEAXUA@1@@Z"{{.*}}
+// WIN64-NEXT: :
+// WIN64-NEXT: getelementptr
+// WIN64-NEXT: load
+}
+
+namespace deleted_by_member {
+struct B {
+  B();
+  B(B &&o) : p(o.p) {}
+  void *p;
+};
+struct A {
+  A();
+  B b;
+};
+void *foo(A a) { return a.b.p; }
+// WIN64-LABEL: define i8* @"\01?foo@deleted_by_member@@YAPEAXUA@1@@Z"{{.*}}
+// WIN64-NEXT: :
+// WIN64-NEXT: getelementptr
+// WIN64-NEXT: getelementptr
+// WIN64-NEXT: load
+}