Index: docs/TestingGuide.rst
===================================================================
--- docs/TestingGuide.rst
+++ docs/TestingGuide.rst
@@ -598,3 +598,10 @@
 
 For more information on the ``test-suite`` Makefile setup, please see
 the :doc:`Test Suite Makefile Guide <TestSuiteMakefileGuide>`.
+
+Capturing Diagnostic Information
+--------------------------------
+
+The compiler may crash while running a test. In this situation, it may emit
+useful diagnostic information in the system temp directory. To specify an
+alternate temp directory, set the ``LLVM_TEMP_DIR`` environment variable.
Index: lib/Support/Path.cpp
===================================================================
--- lib/Support/Path.cpp
+++ lib/Support/Path.cpp
@@ -157,6 +157,20 @@
   FS_Name
 };
 
+template<unsigned N>
+static void getTempDirectory(bool ErasedOnReboot, SmallString<N> &TDir) {
+  Optional<std::string> TempDir = sys::Process::GetEnv("LLVM_TEMP_DIR");
+  if (TempDir.hasValue()) {
+    std::string &Dir = TempDir.getValue();
+    if (sys::fs::is_directory(Dir)) {
+      TDir.assign(Dir.begin(), Dir.end());
+      return;
+    }
+  }
+
+  sys::path::system_temp_directory(ErasedOnReboot, TDir);
+}
+
 static std::error_code createUniqueEntity(const Twine &Model, int &ResultFD,
                                           SmallVectorImpl<char> &ResultPath,
                                           bool MakeAbsolute, unsigned Mode,
@@ -168,7 +182,7 @@
     // Make model absolute by prepending a temp directory if it's not already.
     if (!sys::path::is_absolute(Twine(ModelStorage))) {
       SmallString<128> TDir;
-      sys::path::system_temp_directory(true, TDir);
+      getTempDirectory(true, TDir);
       sys::path::append(TDir, Twine(ModelStorage));
       ModelStorage.swap(TDir);
     }