Add some support for classes without TypeInfos (#2765)

For -betterC and/or a minimal object.d without TypeInfo:

* Skip TypeInfo emission for classes and interfaces.
* Emit null as first vtable member.

Makes dmd-testsuite's {compilable,runnable}/minimal2.d work.

gen/classes.cpp

@@ -65,11 +65,6 @@ void DtoResolveClass(ClassDeclaration *cd) {
     getIrField(vd, true);
   }
 
-  // emit the interfaceInfosZ symbol if necessary
-  if (cd->vtblInterfaces && cd->vtblInterfaces->dim > 0) {
-    irAggr->getInterfaceArraySymbol(); // initializer is applied when it's built
-  }
-
   // interface only emit typeinfo and classinfo
   if (cd->isInterfaceDeclaration()) {
     irAggr->initializeInterface();

gen/declarations.cpp

@@ -89,14 +89,13 @@ class CodegenVisitor : public Visitor {
       }
 
       // Emit TypeInfo.
-      DtoTypeInfoOf(decl->type, /*base=*/false);
-
-      // Declare __InterfaceZ.
-      IrAggr *ir = getIrAggr(decl);
-      llvm::GlobalVariable *interfaceZ = ir->getClassInfoSymbol();
-      // Only define if not speculative.
-      if (!isSpeculativeType(decl->type)) {
-        defineGlobal(interfaceZ, ir->getClassInfoInit(), decl);
+      if (global.params.useTypeInfo && Type::dtypeinfo) {
+        IrAggr *ir = getIrAggr(decl);
+        llvm::GlobalVariable *interfaceZ = ir->getClassInfoSymbol();
+        // Only define if not speculative.
+        if (!isSpeculativeType(decl->type)) {
+          defineGlobal(interfaceZ, ir->getClassInfoInit(), decl);
+        }
       }
     }
   }
@@ -196,9 +195,13 @@ class CodegenVisitor : public Visitor {
 
       ir->defineInterfaceVtbls();
 
-      llvm::GlobalVariable *classZ = ir->getClassInfoSymbol();
-      if (!isSpeculativeType(decl->type)) {
-        defineGlobal(classZ, ir->getClassInfoInit(), decl);
+      // Emit TypeInfo.
+      if (global.params.useTypeInfo && Type::dtypeinfo) {
+        llvm::GlobalVariable *classZ = ir->getClassInfoSymbol();
+        // Only define if not speculative.
+        if (!isSpeculativeType(decl->type)) {
+          defineGlobal(classZ, ir->getClassInfoInit(), decl);
+        }
       }
     }
   }

ir/irclass.cpp

@@ -164,8 +164,13 @@ LLConstant *IrAggr::getVtblInit() {
   // start with the classinfo
   llvm::Constant *c;
   if (!cd->isCPPclass()) {
-    c = getClassInfoSymbol();
-    c = DtoBitCast(c, voidPtrType);
+    if (global.params.useTypeInfo && Type::dtypeinfo) {
+      c = getClassInfoSymbol();
+      c = DtoBitCast(c, voidPtrType);
+    } else {
+      // use null if there are no TypeInfos
+      c = llvm::Constant::getNullValue(voidPtrType);
+    }
     constants.push_back(c);
   }
 
@@ -312,16 +317,21 @@ void IrAggr::defineInterfaceVtbl(BaseClass *b, bool new_instance,
   const auto voidPtrTy = getVoidPtrType();
 
   if (!b->sym->isCPPinterface()) { // skip interface info for CPP interfaces
-    // index into the interfaces array
-    llvm::Constant *idxs[2] = {DtoConstSize_t(0),
-                               DtoConstSize_t(interfaces_index)};
+    if (global.params.useTypeInfo && Type::dtypeinfo) {
+      // index into the interfaces array
+      llvm::Constant *idxs[2] = {DtoConstSize_t(0),
+                                 DtoConstSize_t(interfaces_index)};
 
-    llvm::GlobalVariable *interfaceInfosZ = getInterfaceArraySymbol();
-    llvm::Constant *c = llvm::ConstantExpr::getGetElementPtr(
-        isaPointer(interfaceInfosZ)->getElementType(),
-        interfaceInfosZ, idxs, true);
+      llvm::GlobalVariable *interfaceInfosZ = getInterfaceArraySymbol();
+      llvm::Constant *c = llvm::ConstantExpr::getGetElementPtr(
+          isaPointer(interfaceInfosZ)->getElementType(), interfaceInfosZ, idxs,
+          true);
 
-    constants.push_back(DtoBitCast(c, voidPtrTy));
+      constants.push_back(DtoBitCast(c, voidPtrTy));
+    } else {
+      // use null if there are no TypeInfos
+      constants.push_back(llvm::Constant::getNullValue(voidPtrTy));
+    }
   }
 
   // add virtual function pointers

tests/baremetal/classes.d

@@ -0,0 +1,29 @@
+// Use classes with virtual functions.
+
+// -betterC for C assert.
+// RUN: %ldc -betterC %baremetal_args -run %s
+
+class A
+{
+    int x;
+    bool isB() { return false; }
+}
+
+class B : A
+{
+    override bool isB() { return true; }
+}
+
+__gshared A a = new A();
+__gshared B b = new B();
+
+extern(C) int main()
+{
+    A obj = a;
+    assert(!obj.isB());
+
+    obj = b;
+    assert(obj.isB());
+
+    return 0;
+}