1 files changed, 23 insertions, 6 deletions
diff --git a/clang/lib/CodeGen/CGExprAgg.cpp b/clang/lib/CodeGen/CGExprAgg.cpp
index 3b1c5bf876e..a41b420801f 100644
--- a/clang/lib/CodeGen/CGExprAgg.cpp
+++ b/clang/lib/CodeGen/CGExprAgg.cpp
@@ -1846,15 +1846,32 @@ LValue CodeGenFunction::EmitAggExprToLValue(const Expr *E) {
   return LV;
 }
 
+AggValueSlot::Overlap_t
+CodeGenFunction::overlapForFieldInit(const FieldDecl *FD) {
+  if (!FD->hasAttr<NoUniqueAddressAttr>() || !FD->getType()->isRecordType())
+    return AggValueSlot::DoesNotOverlap;
+
+  // If the field lies entirely within the enclosing class's nvsize, its tail
+  // padding cannot overlap any already-initialized object. (The only subobjects
+  // with greater addresses that might already be initialized are vbases.)
+  const RecordDecl *ClassRD = FD->getParent();
+  const ASTRecordLayout &Layout = getContext().getASTRecordLayout(ClassRD);
+  if (Layout.getFieldOffset(FD->getFieldIndex()) +
+          getContext().getTypeSize(FD->getType()) <=
+      (uint64_t)getContext().toBits(Layout.getNonVirtualSize()))
+    return AggValueSlot::DoesNotOverlap;
+
+  // The tail padding may contain values we need to preserve.
+  return AggValueSlot::MayOverlap;
+}
+
 AggValueSlot::Overlap_t CodeGenFunction::overlapForBaseInit(
     const CXXRecordDecl *RD, const CXXRecordDecl *BaseRD, bool IsVirtual) {
-  // Virtual bases are initialized first, in address order, so there's never
-  // any overlap during their initialization.
-  //
-  // FIXME: Under P0840, this is no longer true: the tail padding of a vbase
-  // of a field could be reused by a vbase of a containing class.
+  // If the most-derived object is a field declared with [[no_unique_address]],
+  // the tail padding of any virtual base could be reused for other subobjects
+  // of that field's class.
   if (IsVirtual)
-    return AggValueSlot::DoesNotOverlap;
+    return AggValueSlot::MayOverlap;
 
   // If the base class is laid out entirely within the nvsize of the derived
   // class, its tail padding cannot yet be initialized, so we can issue