diff --git a/llvm/lib/Support/Parallel.cpp b/llvm/lib/Support/Parallel.cpp
index c256d256be4fa1..f762e7abc83393 100644
--- a/llvm/lib/Support/Parallel.cpp
+++ b/llvm/lib/Support/Parallel.cpp
@@ -55,7 +55,10 @@ class ThreadPoolExecutor : public Executor {
     Threads.reserve(ThreadCount);
     Threads.resize(1);
     std::lock_guard<std::mutex> Lock(Mutex);
-    Threads[0] = std::thread([this, ThreadCount, S] {
+    // Use operator[] before creating the thread to avoid data race in .size()
+    // in “safe libc++” mode.
+    auto& Thread0 = Threads[0];
+    Thread0 = std::thread([this, ThreadCount, S] {
       for (unsigned I = 1; I < ThreadCount; ++I) {
         Threads.emplace_back([=] { work(S, I); });
         if (Stop)