UBSan: Fix alignment checks emitted in downcasts.

Summary:
UBSan was checking for alignment of the derived class on the pointer to
the base class, before converting. With some class hierarchies, this could
generate false positives.

Added test-case.

llvm-svn: 187948

2 files changed, 13 insertions, 10 deletions

diff --git a/clang/lib/CodeGen/CGExpr.cpp b/clang/lib/CodeGen/CGExpr.cpp
index 73824afc17e..0e87d8311c1 100644
--- a/clang/lib/CodeGen/CGExpr.cpp
+++ b/clang/lib/CodeGen/CGExpr.cpp
@@ -2764,18 +2764,18 @@ LValue CodeGenFunction::EmitCastLValue(const CastExpr *E) {
 
     LValue LV = EmitLValue(E->getSubExpr());
 
-    // C++11 [expr.static.cast]p2: Behavior is undefined if a downcast is
-    // performed and the object is not of the derived type.
-    if (SanitizePerformTypeCheck)
-      EmitTypeCheck(TCK_DowncastReference, E->getExprLoc(),
-                    LV.getAddress(), E->getType());
-
     // Perform the base-to-derived conversion
     llvm::Value *Derived =
       GetAddressOfDerivedClass(LV.getAddress(), DerivedClassDecl,
                                E->path_begin(), E->path_end(),
                                /*NullCheckValue=*/false);
 
+    // C++11 [expr.static.cast]p2: Behavior is undefined if a downcast is
+    // performed and the object is not of the derived type.
+    if (SanitizePerformTypeCheck)
+      EmitTypeCheck(TCK_DowncastReference, E->getExprLoc(),
+                    Derived, E->getType());
+
     return MakeAddrLValue(Derived, E->getType());
   }
   case CK_LValueBitCast: {
diff --git a/clang/lib/CodeGen/CGExprScalar.cpp b/clang/lib/CodeGen/CGExprScalar.cpp
index b6bda284717..287dc6c0231 100644
--- a/clang/lib/CodeGen/CGExprScalar.cpp
+++ b/clang/lib/CodeGen/CGExprScalar.cpp
@@ -1234,15 +1234,18 @@ Value *ScalarExprEmitter::VisitCastExpr(CastExpr *CE) {
 
     llvm::Value *V = Visit(E);
 
+    llvm::Value *Derived =
+      CGF.GetAddressOfDerivedClass(V, DerivedClassDecl,
+                                   CE->path_begin(), CE->path_end(),
+                                   ShouldNullCheckClassCastValue(CE));
+
     // C++11 [expr.static.cast]p11: Behavior is undefined if a downcast is
     // performed and the object is not of the derived type.
     if (CGF.SanitizePerformTypeCheck)
       CGF.EmitTypeCheck(CodeGenFunction::TCK_DowncastPointer, CE->getExprLoc(),
-                        V, DestTy->getPointeeType());
+                        Derived, DestTy->getPointeeType());
 
-    return CGF.GetAddressOfDerivedClass(V, DerivedClassDecl,
-                                        CE->path_begin(), CE->path_end(),
-                                        ShouldNullCheckClassCastValue(CE));
+    return Derived;
   }
   case CK_UncheckedDerivedToBase:
   case CK_DerivedToBase: {