Another vtable layout fix, making us match gcc better.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@99812 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/CGVtable.cpp

@@ -1401,8 +1401,15 @@ void VtableBuilder::ComputeThisAdjustments() {
                                             MethodInfo.BaseOffset), MD);
     
     // Check if we need an adjustment at all.
-    if (MethodInfo.BaseOffsetInLayoutClass == Overrider.Offset)
-      continue;
+    if (MethodInfo.BaseOffsetInLayoutClass == Overrider.Offset) {
+      // When a return thunk is needed by a derived class that overrides a
+      // virtual base, gcc uses a virtual 'this' adjustment as well. 
+      // While the thunk itself might be needed by vtables in subclasses or
+      // in construction vtables, there doesn't seem to be a reason for using
+      // the thunk in this vtable. Still, we do so to match gcc.
+      if (VTableThunks.lookup(VtableIndex).Return.isEmpty())
+        continue;
+    }
 
     ThisAdjustment ThisAdjustment =
       ComputeThisAdjustment(MD, MethodInfo.BaseOffsetInLayoutClass, Overrider);

test/CodeGenCXX/vtable-layout.cpp

@@ -1273,3 +1273,33 @@ struct E : D {
 void E::e() { }
 
 }
+
+namespace Test29 {
+
+// Test that the covariant return thunk for B::f will have a virtual 'this' adjustment,
+// matching gcc.
+
+struct V1 { };
+struct V2 : virtual V1 { };
+
+struct A {
+  virtual V1 *f();
+};
+
+// CHECK:      Vtable for 'Test29::B' (6 entries).
+// CHECK-NEXT:    0 | vbase_offset (0)
+// CHECK-NEXT:    1 | vcall_offset (0)
+// CHECK-NEXT:    2 | offset_to_top (0)
+// CHECK-NEXT:    3 | Test29::B RTTI
+// CHECK-NEXT:        -- (Test29::A, 0) vtable address --
+// CHECK-NEXT:        -- (Test29::B, 0) vtable address --
+// CHECK-NEXT:    4 | Test29::V2 *Test29::B::f()
+// CHECK-NEXT:        [return adjustment: 0 non-virtual, -24 vbase offset offset]
+// CHECK-NEXT:        [this adjustment: 0 non-virtual, -24 vcall offset offset]
+// CHECK-NEXT:    5 | Test29::V2 *Test29::B::f()
+struct B : virtual A {
+  virtual V2 *f();
+};
+V2 *B::f() { return 0; }
+
+}