diff --git a/llvm/lib/Support/TimeProfiler.cpp b/llvm/lib/Support/TimeProfiler.cpp
--- a/llvm/lib/Support/TimeProfiler.cpp
+++ b/llvm/lib/Support/TimeProfiler.cpp
@@ -89,6 +89,7 @@
            "All profiler sections should be ended when calling Write");
 
     json::Array Events;
+    Events.reserve(Entries.size() + CountAndTotalPerName.size() + 1);
 
     // Emit all events for the main flame graph.
     for (const auto &E : Entries) {
@@ -149,6 +150,9 @@
         {"args", json::Object{{"name", "clang"}}},
     });
 
+    assert(
+        (Entries.size() + CountAndTotalPerName.size() + 1 == Events.size()) &&
+        "Events reserved size should be enough!");
     OS << formatv("{0:2}", json::Value(json::Object(
                                {{"traceEvents", std::move(Events)}})));
   }