Index: lib/Support/Windows/Path.inc
===================================================================
--- lib/Support/Windows/Path.inc
+++ lib/Support/Windows/Path.inc
@@ -775,49 +775,22 @@
   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: unittests/Support/Path.cpp
===================================================================
--- unittests/Support/Path.cpp
+++ unittests/Support/Path.cpp
@@ -351,6 +351,63 @@
   }
 }
 
+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());
+
+#ifdef LLVM_ON_WIN32
+  const wchar_t *OrigTmp = _wgetenv(L"TMP");
+  const wchar_t *OrigTemp = _wgetenv(L"TEMP");
+
+  _wputenv_s(L"TMP", L"C:\\OtherFolder");
+  path::system_temp_directory(true, TempDir);
+  EXPECT_EQ("C:\\OtherFolder", TempDir);
+
+  _wputenv_s(L"TMP", L"C:/Unix/Path/Seperators");
+  path::system_temp_directory(true, TempDir);
+  EXPECT_EQ("C:\\Unix\\Path\\Seperators", TempDir);
+
+  _wputenv_s(L"TMP", L"Local Path");
+  path::system_temp_directory(true, TempDir);
+  EXPECT_NE("Local Path", TempDir);
+  EXPECT_TRUE(path::is_absolute(TempDir));
+  EXPECT_EQ("Local Path", path::filename(TempDir));
+
+  SmallString<270> Expected{"C:\\Temp\\AB\\123456789"};
+  while (Expected.size() < 260)
+    Expected.append("\\DirNameWith19Charss");
+  ASSERT_EQ(260, Expected.size());
+
+  _putenv_s("TMP", Expected.c_str());
+  path::system_temp_directory(true, TempDir);
+  EXPECT_EQ(Expected, TempDir);
+
+  // Path longer than 260 is invalid
+  Expected.append("X");
+  ASSERT_EQ(261, Expected.size());
+  path::system_temp_directory(true, TempDir);
+  EXPECT_NE(Expected, TempDir);
+
+  _wputenv_s(L"TMP", L"");
+  _wputenv_s(L"TEMP", L"C:\\Valid\\Path");
+  path::system_temp_directory(true, TempDir);
+  EXPECT_EQ("C:\\Valid\\Path", TempDir);
+
+  // Test Unicode paths
+  _wputenv_s(L"TMP", L"C:\\2\x03C0r-\x00B5\x00B3\\\x2135\x2080");
+  path::system_temp_directory(true, TempDir);
+  EXPECT_EQ("C:\\2\xCF\x80r-\xC2\xB5\xC2\xB3\\\xE2\x84\xB5\xE2\x82\x80",
+            TempDir);
+
+  _wputenv_s(L"TEMP", OrigTemp);
+  _wputenv_s(L"TMP", OrigTmp);
+#endif
+}
+
 class FileSystemTest : public testing::Test {
 protected:
   /// Unique temporary directory in which all created filesystem entities must