MS ABI: Don't assert when generating virtual function call with virtual bases and -flto (PR30731)

[hans]

Summary

getClassAtVTableLocation() was calling ASTRecordLayout::getBaseClassOffset() on a virtual base, causing an assert.

[pcc]

LGTM

[majnemer]

Why did this only fail with -flto? Seems strange.

[hans]

In D25779#574630, @majnemer wrote:

Why did this only fail with -flto? Seems strange.

I was asking myself the same thing :-) It's because of this code in MicrosoftCXXABI::getVirtualFunctionPointer:

if (CGM.getCodeGenOpts().PrepareForLTO) // XXX: Aha!
  CGF.EmitTypeMetadataCodeForVCall(
      getClassAtVTableLocation(getContext(), GD, ML), VTable, Loc);

lib/CodeGen/MicrosoftCXXABI.cpp

static const CXXRecordDecl *getClassAtVTableLocation(ASTContext &Ctx,
   if (Offset.isZero())
     return RD;
 
   const ASTRecordLayout &Layout = Ctx.getASTRecordLayout(RD);
   const CXXRecordDecl *MaxBase = nullptr;
   CharUnits MaxBaseOffset;
   for (auto &&B : RD->bases()) {
     const CXXRecordDecl *Base = B.getType()->getAsCXXRecordDecl();
-    CharUnits BaseOffset = Layout.getBaseClassOffset(Base);
-    if (BaseOffset <= Offset && BaseOffset >= MaxBaseOffset) {
-      MaxBase = Base;
-      MaxBaseOffset = BaseOffset;
-    }
-  }
-  for (auto &&B : RD->vbases()) {
-    const CXXRecordDecl *Base = B.getType()->getAsCXXRecordDecl();
-    CharUnits BaseOffset = Layout.getVBaseClassOffset(Base);
+    CharUnits BaseOffset = B.isVirtual() ? Layout.getVBaseClassOffset(Base)
+                                         : Layout.getBaseClassOffset(Base);
     if (BaseOffset <= Offset && BaseOffset >= MaxBaseOffset) {
       MaxBase = Base;
       MaxBaseOffset = BaseOffset;
     }
   }
   assert(MaxBase);
   return getClassAtVTableLocation(Ctx, MaxBase, Offset - MaxBaseOffset);
 }

test/CodeGenCXX/pr30731.cpp

+// RUN: %clang_cc1 -triple i386-pc-win32 -emit-llvm -flto -std=c++11 -o - %s | FileCheck %s
+
+struct A {
+  virtual ~A();
+};
+
+struct B {};
+
+struct C {
+  virtual void f();
+};
+
+struct S : A, virtual B, C {
+  void f() override;
+};
+
+void f(S* s) { s->f(); }
+
+// CHECK-LABEL: define void @"\01?f@@YAXPAUS@@@Z"
+// CHECK: call
+// CHECK: ret void