Index: lib/CodeGen/CodeGenAction.cpp
===================================================================
--- lib/CodeGen/CodeGenAction.cpp
+++ lib/CodeGen/CodeGenAction.cpp
@@ -434,7 +434,13 @@
   FileManager &FileMgr = SourceMgr.getFileManager();
   StringRef Filename;
   unsigned Line, Column;
-  D.getLocation(&Filename, &Line, &Column);
+  if (D.isLocationAvailable())
+    D.getLocation(&Filename, &Line, &Column);
+  else {
+    Filename = "<unknown>";
+    Line = 0;
+    Column = 0;
+  }
   SourceLocation DILoc;
   const FileEntry *FE = FileMgr.getFile(Filename);
   if (FE && Line > 0) {
Index: test/Misc/backend-optimization-failure-nodbg.cpp
===================================================================
--- /dev/null
+++ test/Misc/backend-optimization-failure-nodbg.cpp
@@ -0,0 +1,21 @@
+// RUN: %clang_cc1 -triple x86_64-unknown-unknown %s -O3 -emit-llvm -S -verify -o /dev/null
+// REQUIRES: x86-registered-target
+
+// Test verifies optimization failures generated by the backend are handled
+// correctly by clang. LLVM tests verify all of the failure conditions.
+
+void test_switch(int *A, int *B, int Length) {
+#pragma clang loop vectorize(enable) unroll(disable)
+  for (int i = 0; i < Length; i++) {
+    switch (A[i]) {
+    case 0:
+      B[i] = 1;
+      break;
+    case 1:
+      B[i] = 2;
+      break;
+    default:
+      B[i] = 3;
+    }
+  }
+/* expected-note {{could not determine the original source location for <unknown>:0:0}} expected-warning {{loop not vectorized: failed explicitly specified loop vectorization}} */ }