diff --git a/llvm/lib/LTO/LTO.cpp b/llvm/lib/LTO/LTO.cpp
--- a/llvm/lib/LTO/LTO.cpp
+++ b/llvm/lib/LTO/LTO.cpp
@@ -933,6 +933,11 @@
                          }))
     return Err;
 
+  LLVM_DEBUG(
+      dbgs() << "Module "
+             << ThinLTO.CombinedIndex.getModuleId(BM.getModuleIdentifier())
+             << ": " << BM.getModuleIdentifier() << "\n");
+
   for (const InputFile::Symbol &Sym : Syms) {
     assert(ResI != ResE);
     SymbolResolution Res = *ResI++;