Index: llvm/trunk/lib/Support/Windows/Path.inc
===================================================================
--- llvm/trunk/lib/Support/Windows/Path.inc
+++ llvm/trunk/lib/Support/Windows/Path.inc
@@ -773,49 +773,22 @@
   return getKnownFolderPath(FOLDERID_Profile, result);
 }
 
-static bool getTempDirEnvVar(const char *Var, SmallVectorImpl<char> &Res) {
-  SmallVector<wchar_t, 128> NameUTF16;
-  if (windows::UTF8ToUTF16(Var, NameUTF16))
-    return false;
-
-  SmallVector<wchar_t, 1024> Buf;
-  size_t Size = 1024;
-  do {
-    Buf.reserve(Size);
-    Size =
-        GetEnvironmentVariableW(NameUTF16.data(), Buf.data(), Buf.capacity());
-    if (Size == 0)
-      return false;
-
-    // Try again with larger buffer.
-  } while (Size > Buf.capacity());
-  Buf.set_size(Size);
-
-  if (windows::UTF16ToUTF8(Buf.data(), Size, Res))
-    return false;
-  return true;
-}
-
-static bool getTempDirEnvVar(SmallVectorImpl<char> &Res) {
-  const char *EnvironmentVariables[] = {"TMP", "TEMP", "USERPROFILE"};
-  for (const char *Env : EnvironmentVariables) {
-    if (getTempDirEnvVar(Env, Res))
-      return true;
-  }
-  return false;
-}
-
 void system_temp_directory(bool ErasedOnReboot, SmallVectorImpl<char> &Result) {
   (void)ErasedOnReboot;
-  Result.clear();
 
-  // Check whether the temporary directory is specified by an environment
-  // variable.
-  if (getTempDirEnvVar(Result))
-    return;
+  wchar_t Path[MAX_PATH + 2]; // GetTempPath can return MAX_PATH + 1 + null
+  if (auto PathLength = ::GetTempPathW(sizeof(Path) / sizeof(wchar_t), Path)) {
+    assert(PathLength > 0 && PathLength <= (MAX_PATH + 1) &&
+           "GetTempPath returned undocumented result");
+    if (Path[PathLength - 1] == L'\\')
+      --PathLength; // skip trailing "\" added by GetTempPath
+    if (!UTF16ToUTF8(Path, PathLength, Result))
+      return;
+  }
 
   // Fall back to a system default.
   const char *DefaultResult = "C:\\TEMP";
+  Result.clear();
   Result.append(DefaultResult, DefaultResult + strlen(DefaultResult));
 }
 } // end namespace path
Index: llvm/trunk/unittests/Support/Path.cpp
===================================================================
--- llvm/trunk/unittests/Support/Path.cpp
+++ llvm/trunk/unittests/Support/Path.cpp
@@ -351,6 +351,76 @@
   }
 }
 
+TEST(Support, TempDirectory) {
+  SmallString<32> TempDir;
+  path::system_temp_directory(false, TempDir);
+  EXPECT_TRUE(!TempDir.empty());
+  TempDir.clear();
+  path::system_temp_directory(true, TempDir);
+  EXPECT_TRUE(!TempDir.empty());
+}
+
+static std::string path2regex(std::string Path) {
+  size_t Pos = 0;
+  while ((Pos = Path.find('\\', Pos)) != std::string::npos) {
+    Path.replace(Pos, 1, "\\\\");
+    Pos += 2;
+  }
+  return Path;
+}
+
+/// Helper for running temp dir test in separated process. See below.
+#define EXPECT_TEMP_DIR(prepare, expected)                                     \
+  EXPECT_EXIT(                                                                 \
+      {                                                                        \
+        prepare;                                                               \
+        SmallString<300> TempDir;                                              \
+        path::system_temp_directory(true, TempDir);                            \
+        raw_os_ostream(std::cerr) << TempDir;                                  \
+        std::exit(0);                                                          \
+      },                                                                       \
+      ::testing::ExitedWithCode(0), path2regex(expected))
+
+#ifdef LLVM_ON_WIN32
+TEST(SupportDeathTest, TempDirectoryOnWindows) {
+  // In this test we want to check how system_temp_directory responds to
+  // different values of specific env vars. To prevent corrupting env vars of
+  // the current process all checks are done in separated processes.
+  EXPECT_TEMP_DIR(_wputenv_s(L"TMP", L"C:\\OtherFolder"), "C:\\OtherFolder");
+  EXPECT_TEMP_DIR(_wputenv_s(L"TMP", L"C:/Unix/Path/Seperators"),
+                  "C:\\Unix\\Path\\Seperators");
+  EXPECT_TEMP_DIR(_wputenv_s(L"TMP", L"Local Path"), ".+\\Local Path$");
+  EXPECT_TEMP_DIR(
+      _wputenv_s(L"TMP", L"C:\\2\x03C0r-\x00B5\x00B3\\\x2135\x2080"),
+      "C:\\2\xCF\x80r-\xC2\xB5\xC2\xB3\\\xE2\x84\xB5\xE2\x82\x80");
+
+  // Test $TMP empty, $TEMP set.
+  EXPECT_TEMP_DIR(
+      {
+        _wputenv_s(L"TMP", L"");
+        _wputenv_s(L"TEMP", L"C:\\Valid\\Path");
+      },
+      "C:\\Valid\\Path");
+
+  // Test evn var / path with 260 chars.
+  SmallString<270> Expected{"C:\\Temp\\AB\\123456789"};
+  while (Expected.size() < 260)
+    Expected.append("\\DirNameWith19Charss");
+  ASSERT_EQ(260, Expected.size());
+  EXPECT_TEMP_DIR(_putenv_s("TMP", Expected.c_str()), Expected.c_str());
+
+  // Test evn var 261 chars.
+  Expected.append("X");
+  ASSERT_EQ(261, Expected.size());
+  EXPECT_TEMP_DIR(
+      {
+        _putenv_s("TMP", Expected.c_str());
+        _wputenv_s(L"TEMP", L"C:\\Short\\Path");
+      },
+      "C:\\Short\\Path");
+}
+#endif
+
 class FileSystemTest : public testing::Test {
 protected:
   /// Unique temporary directory in which all created filesystem entities must