Make the spliceFrom case where one graph is completely empty be constant time.

llvm-svn: 20825

1 files changed, 6 insertions, 2 deletions

diff --git a/llvm/lib/Analysis/DataStructure/DataStructure.cpp b/llvm/lib/Analysis/DataStructure/DataStructure.cpp
index 1133743b5d3..6460287ac71 100644
--- a/llvm/lib/Analysis/DataStructure/DataStructure.cpp
+++ b/llvm/lib/Analysis/DataStructure/DataStructure.cpp
@@ -1321,8 +1321,12 @@ void DSGraph::spliceFrom(DSGraph &RHS) {
   AuxFunctionCalls.splice(AuxFunctionCalls.end(), RHS.AuxFunctionCalls);
 
   // Take all of the return nodes.
-  ReturnNodes.insert(RHS.ReturnNodes.begin(), RHS.ReturnNodes.end());
-  RHS.ReturnNodes.clear();
+  if (ReturnNodes.empty()) {
+    ReturnNodes.swap(RHS.ReturnNodes);
+  } else {
+    ReturnNodes.insert(RHS.ReturnNodes.begin(), RHS.ReturnNodes.end());
+    RHS.ReturnNodes.clear();
+  }
 
   // Merge the scalar map in.
   ScalarMap.spliceFrom(RHS.ScalarMap);