diff --git a/lld/Common/Threads.cpp b/lld/Common/Threads.cpp
--- a/lld/Common/Threads.cpp
+++ b/lld/Common/Threads.cpp
@@ -7,5 +7,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "lld/Common/Threads.h"
+#include <thread>
 
 bool lld::ThreadsEnabled = true;
+thread_local bool lld::ParallelForEachRunning;
diff --git a/lld/include/lld/Common/Threads.h b/lld/include/lld/Common/Threads.h
--- a/lld/include/lld/Common/Threads.h
+++ b/lld/include/lld/Common/Threads.h
@@ -64,20 +64,32 @@
 namespace lld {
 
 extern bool ThreadsEnabled;
+extern thread_local bool ParallelForEachRunning;
 
 template <typename R, class FuncTy> void parallelForEach(R &&Range, FuncTy Fn) {
+  // for_each is not reentrant, so guard against it.
+  assert(!ParallelForEachRunning);
+  ParallelForEachRunning = true;
+
   if (ThreadsEnabled)
     for_each(llvm::parallel::par, std::begin(Range), std::end(Range), Fn);
   else
     for_each(llvm::parallel::seq, std::begin(Range), std::end(Range), Fn);
+
+  ParallelForEachRunning = false;
 }
 
 inline void parallelForEachN(size_t Begin, size_t End,
                              llvm::function_ref<void(size_t)> Fn) {
+  assert(!ParallelForEachRunning);
+  ParallelForEachRunning = true;
+
   if (ThreadsEnabled)
     for_each_n(llvm::parallel::par, Begin, End, Fn);
   else
     for_each_n(llvm::parallel::seq, Begin, End, Fn);
+
+  ParallelForEachRunning = false;
 }
 
 template <typename R, class FuncTy> void parallelSort(R &&Range, FuncTy Fn) {