[CodeGen] Use the non-virtual alignment when emitting the base class subobject constructor

[ahatanak]

Summary

This patch fixes what looks like a bug in IRGen's member initialization where it uses the alignment of a complete object instead of its non-virtual alignment when emitting the base constructor.

For example, when emitting the base constructor for B, IRGen uses a 16-byte alignment to initialize B::x because B's alignment is 16-byte (which is because A is 16-byte aligned). This is not correct since A is a virtual base of B and therefore we cannot guarantee that the pointer to B that is passed to the base constructor is always 16-byte aligned.

struct A {
  __attribute__((aligned(16))) double data1;
};

struct B : public virtual A {
  B() : x(123) {}
  double a;
  int x;
};

struct C : public virtual B {};

void test() { B b; C c; }

To fix the bug, this patch calls CGF.MakeNaturalAlignPointeeAddrLValue in EmitMemberInitializer to create an LValue that uses the non-virtual alignment of the class.

rdar://problem/36382481

[rjmccall]

LGTM.

lib/CodeGen/CGClass.cpp

static void EmitMemberInitializer(CodeGenFunction &CGF,
   assert(MemberInit->getInit() && "Must have initializer!");
 
   // non-static data member initializers.
   FieldDecl *Field = MemberInit->getAnyMember();
   QualType FieldType = Field->getType();
 
   llvm::Value *ThisPtr = CGF.LoadCXXThis();
   QualType RecordTy = CGF.getContext().getTypeDeclType(ClassDecl);
-  LValue LHS = CGF.MakeNaturalAlignAddrLValue(ThisPtr, RecordTy);
+  LValue LHS;
+
+  // If a base constructor is being emitted, create an LValue that has the
+  // non-virtual alignment.
+  if (CGF.CurGD.getCtorType() == Ctor_Base)
+    LHS = CGF.MakeNaturalAlignPointeeAddrLValue(ThisPtr, RecordTy);
+  else
+    LHS = CGF.MakeNaturalAlignAddrLValue(ThisPtr, RecordTy);
 
   EmitLValueForAnyFieldInitialization(CGF, MemberInit, LHS);
 
   // Special case: if we are in a copy or move constructor, and we are copying
   // an array of PODs or classes with trivial copy constructors, ignore the
   // AST and perform the copy we know is equivalent.
   // FIXME: This is hacky at best... if we had a bit more explicit information
   // in the AST, we could generalize it more easily.

test/CodeGenCXX/virtual-bases.cpp

 
 // CHECK-LABEL: define void @_ZN6PR62511DC1Ev(%"struct.PR6251::D"* %this) unnamed_addr
 // CHECK: call void @_ZN6PR62511AIcEC2Ev
 // CHECK-NOT: call void @_ZN6PR62511AIcEC2Ev
 // CHECK: ret void
 D::D() { }
 
 }
+
+namespace virtualBaseAlignment {
+
+// Check that the store to B::x in the base constructor has an 8-byte alignment.
+
+// CHECK: define linkonce_odr void @_ZN20virtualBaseAlignment1BC1Ev(%[[STRUCT_B:.*]]* %[[THIS:.*]])
+// CHECK: %[[THIS_ADDR:.*]] = alloca %[[STRUCT_B]]*, align 8
+// CHECK: store %[[STRUCT_B]]* %[[THIS]], %[[STRUCT_B]]** %[[THIS_ADDR]], align 8
+// CHECK: %[[THIS1:.*]] = load %[[STRUCT_B]]*, %[[STRUCT_B]]** %[[THIS_ADDR]], align 8
+// CHECK: %[[X:.*]] = getelementptr inbounds %[[STRUCT_B]], %[[STRUCT_B]]* %[[THIS1]], i32 0, i32 2
+// CHECK: store i32 123, i32* %[[X]], align 16
+
+// CHECK: define linkonce_odr void @_ZN20virtualBaseAlignment1BC2Ev(%[[STRUCT_B]]* %[[THIS:.*]], i8** %{{.*}})
+// CHECK: %[[THIS_ADDR:.*]] = alloca %[[STRUCT_B]]*, align 8
+// CHECK: store %[[STRUCT_B]]* %[[THIS]], %[[STRUCT_B]]** %[[THIS_ADDR]], align 8
+// CHECK: %[[THIS1:.*]] = load %[[STRUCT_B]]*, %[[STRUCT_B]]** %[[THIS_ADDR]], align 8
+// CHECK: %[[X:.*]] = getelementptr inbounds %[[STRUCT_B]], %[[STRUCT_B]]* %[[THIS1]], i32 0, i32 2
+// CHECK: store i32 123, i32* %[[X]], align 8
+
+struct A {
+  __attribute__((aligned(16))) double data1;
+};
+
+struct B : public virtual A {
+  B() : x(123) {}
+  double a;
+  int x;
+};
+
+struct C : public virtual B {};
+
+void test() { B b; C c; }
+
+}