diff --git a/compiler/optimizing/codegen_test.cc b/compiler/optimizing/codegen_test.cc
index ca85cf5..4fbb51d 100644
--- a/compiler/optimizing/codegen_test.cc
+++ b/compiler/optimizing/codegen_test.cc
@@ -192,7 +192,10 @@
                              std::function<void(HGraph*)> hook_before_codegen,
                              bool has_result,
                              Expected expected) {
-  graph->BuildDominatorTree();
+  // Tests may have already computed it.
+  if (graph->GetReversePostOrder().IsEmpty()) {
+    graph->BuildDominatorTree();
+  }
   SsaLivenessAnalysis liveness(graph, codegen);
   liveness.Analyze();