Index: lib/CodeGen/BackendUtil.cpp
===================================================================
--- lib/CodeGen/BackendUtil.cpp
+++ lib/CodeGen/BackendUtil.cpp
@@ -36,6 +36,7 @@
 #include "llvm/Object/ModuleSummaryIndexObjectFile.h"
 #include "llvm/Passes/PassBuilder.h"
 #include "llvm/Support/CommandLine.h"
+#include "llvm/Support/FileSystem.h"
 #include "llvm/Support/MemoryBuffer.h"
 #include "llvm/Support/PrettyStackTrace.h"
 #include "llvm/Support/TargetRegistry.h"
@@ -57,6 +58,12 @@
 using namespace clang;
 using namespace llvm;
 
+static llvm::cl::opt<bool> IgnoreEmptyThinLTOIndexFile(
+    "ignore-empty-index-file", llvm::cl::ZeroOrMore,
+    llvm::cl::desc(
+        "Ignore an empty index file and perform non-ThinLTO compilation"),
+    llvm::cl::init(false));
+
 namespace {
 
 class EmitAssemblyHelper {
@@ -935,8 +942,16 @@
                               Module *M, BackendAction Action,
                               std::unique_ptr<raw_pwrite_stream> OS) {
   if (!CGOpts.ThinLTOIndexFile.empty()) {
-    runThinLTOBackend(CGOpts, M, std::move(OS));
-    return;
+    bool DoThinLTOBackend = true;
+    if (IgnoreEmptyThinLTOIndexFile) {
+      uint64_t IndexFileSize;
+      llvm::sys::fs::file_size(CGOpts.ThinLTOIndexFile, IndexFileSize);
+      DoThinLTOBackend = IndexFileSize > 0;
+    }
+    if (DoThinLTOBackend) {
+      runThinLTOBackend(CGOpts, M, std::move(OS));
+      return;
+    }
   }
 
   EmitAssemblyHelper AsmHelper(Diags, CGOpts, TOpts, LOpts, M);
Index: test/CodeGen/thinlto_backend.ll
===================================================================
--- test/CodeGen/thinlto_backend.ll
+++ test/CodeGen/thinlto_backend.ll
@@ -12,6 +12,11 @@
 ; RUN: %clang -O2 -o %t4.o -x ir %t1.o -c -fthinlto-index=bad.thinlto.bc 2>&1 | FileCheck %s -check-prefix=CHECK-ERROR1
 ; CHECK-ERROR1: Error loading index file 'bad.thinlto.bc'
 
+; Ensure we ignore empty index file under -ignore-empty-index-file
+; RUN: touch %t4.thinlto.bc
+; RUN: %clang -O2 -o %t4.o -x ir %t1.o -c -fthinlto-index=%t4.thinlto.bc -ignore-empty-index-file 2>&1 | FileCheck %s -check-prefix=CHECK-NOERROR
+; CHECK-NOERROR-NOT: Error loading index file 'bad.thinlto.bc'
+
 ; Ensure f2 was imported
 ; RUN: %clang -target x86_64-unknown-linux-gnu -O2 -o %t3.o -x ir %t1.o -c -fthinlto-index=%t.thinlto.bc
 ; RUN: llvm-nm %t3.o | FileCheck --check-prefix=CHECK-OBJ %s