diff --git a/llvm/lib/CodeGen/LexicalScopes.cpp b/llvm/lib/CodeGen/LexicalScopes.cpp
--- a/llvm/lib/CodeGen/LexicalScopes.cpp
+++ b/llvm/lib/CodeGen/LexicalScopes.cpp
@@ -232,21 +232,20 @@
 /// constructScopeNest
 void LexicalScopes::constructScopeNest(LexicalScope *Scope) {
   assert(Scope && "Unable to calculate scope dominance graph!");
-  SmallVector<LexicalScope *, 4> WorkStack;
-  WorkStack.push_back(Scope);
+  SmallVector<std::pair<LexicalScope *, size_t>, 4> WorkStack;
+  WorkStack.push_back(std::make_pair(Scope, 0));
   unsigned Counter = 0;
   while (!WorkStack.empty()) {
-    LexicalScope *WS = WorkStack.back();
+    auto &p = WorkStack.back();
+    LexicalScope *WS = p.first;
+    size_t i = p.second++;
     const SmallVectorImpl<LexicalScope *> &Children = WS->getChildren();
-    bool visitedChildren = false;
-    for (auto &ChildScope : Children)
-      if (!ChildScope->getDFSOut()) {
-        WorkStack.push_back(ChildScope);
-        visitedChildren = true;
-        ChildScope->setDFSIn(++Counter);
-        break;
-      }
-    if (!visitedChildren) {
+    if (i < Children.size()) {
+      auto &ChildScope = Children[i];
+      // Because of tree structure, !ChildScope->getDFSIn().
+      WorkStack.push_back(std::make_pair(ChildScope, 0));
+      ChildScope->setDFSIn(++Counter);
+    } else {
       WorkStack.pop_back();
       WS->setDFSOut(++Counter);
     }