diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -104,6 +104,10 @@
   simplify access to the many single purpose floating point options. The default
   setting is ``precise``.
 
+- The default module cache has moved from /tmp to a per-user cache directory.
+  By default, this is ~/.cache but on some platforms or installations, this
+  might be elsewhere. The -fmodules-cache-path=... flag continues to work.
+
 Deprecated Compiler Flags
 -------------------------
 
diff --git a/clang/include/clang/Driver/Driver.h b/clang/include/clang/Driver/Driver.h
--- a/clang/include/clang/Driver/Driver.h
+++ b/clang/include/clang/Driver/Driver.h
@@ -621,7 +621,8 @@
   static bool GetReleaseVersion(StringRef Str,
                                 MutableArrayRef<unsigned> Digits);
   /// Compute the default -fmodule-cache-path.
-  static void getDefaultModuleCachePath(SmallVectorImpl<char> &Result);
+  /// \return True if the system provides a default cache directory.
+  static bool getDefaultModuleCachePath(SmallVectorImpl<char> &Result);
 };
 
 /// \return True if the last defined optimization level is -Ofast.
diff --git a/clang/lib/Driver/ToolChains/Clang.cpp b/clang/lib/Driver/ToolChains/Clang.cpp
--- a/clang/lib/Driver/ToolChains/Clang.cpp
+++ b/clang/lib/Driver/ToolChains/Clang.cpp
@@ -721,38 +721,6 @@
   }
 }
 
-static void appendUserToPath(SmallVectorImpl<char> &Result) {
-#ifdef LLVM_ON_UNIX
-  const char *Username = getenv("LOGNAME");
-#else
-  const char *Username = getenv("USERNAME");
-#endif
-  if (Username) {
-    // Validate that LoginName can be used in a path, and get its length.
-    size_t Len = 0;
-    for (const char *P = Username; *P; ++P, ++Len) {
-      if (!clang::isAlphanumeric(*P) && *P != '_') {
-        Username = nullptr;
-        break;
-      }
-    }
-
-    if (Username && Len > 0) {
-      Result.append(Username, Username + Len);
-      return;
-    }
-  }
-
-// Fallback to user id.
-#ifdef LLVM_ON_UNIX
-  std::string UID = llvm::utostr(getuid());
-#else
-  // FIXME: Windows seems to have an 'SID' that might work.
-  std::string UID = "9999";
-#endif
-  Result.append(UID.begin(), UID.end());
-}
-
 static void addPGOAndCoverageFlags(const ToolChain &TC, Compilation &C,
                                    const Driver &D, const InputInfo &Output,
                                    const ArgList &Args,
@@ -3201,11 +3169,13 @@
     CmdArgs.push_back("-fno-math-builtin");
 }
 
-void Driver::getDefaultModuleCachePath(SmallVectorImpl<char> &Result) {
-  llvm::sys::path::system_temp_directory(/*erasedOnReboot=*/false, Result);
-  llvm::sys::path::append(Result, "org.llvm.clang.");
-  appendUserToPath(Result);
-  llvm::sys::path::append(Result, "ModuleCache");
+bool Driver::getDefaultModuleCachePath(SmallVectorImpl<char> &Result) {
+  if (llvm::sys::path::cache_directory(Result)) {
+    llvm::sys::path::append(Result, "clang");
+    llvm::sys::path::append(Result, "ModuleCache");
+    return true;
+  }
+  return false;
 }
 
 static void RenderModulesOptions(Compilation &C, const Driver &D,
@@ -3262,6 +3232,7 @@
     if (Arg *A = Args.getLastArg(options::OPT_fmodules_cache_path))
       Path = A->getValue();
 
+    bool HasPath = true;
     if (C.isForDiagnostics()) {
       // When generating crash reports, we want to emit the modules along with
       // the reproduction sources, so we ignore any provided module path.
@@ -3270,12 +3241,16 @@
       llvm::sys::path::append(Path, "modules");
     } else if (Path.empty()) {
       // No module path was provided: use the default.
-      Driver::getDefaultModuleCachePath(Path);
+      HasPath = Driver::getDefaultModuleCachePath(Path);
     }
 
-    const char Arg[] = "-fmodules-cache-path=";
-    Path.insert(Path.begin(), Arg, Arg + strlen(Arg));
-    CmdArgs.push_back(Args.MakeArgString(Path));
+    // `HasPath` will only be false if getDefaultModuleCachePath() fails.
+    // That being said, that failure is unlikely and not caching is harmless.
+    if (HasPath) {
+      const char Arg[] = "-fmodules-cache-path=";
+      Path.insert(Path.begin(), Arg, Arg + strlen(Arg));
+      CmdArgs.push_back(Args.MakeArgString(Path));
+    }
   }
 
   if (HaveModules) {
diff --git a/clang/test/Driver/modules-cache-path.m b/clang/test/Driver/modules-cache-path.m
--- a/clang/test/Driver/modules-cache-path.m
+++ b/clang/test/Driver/modules-cache-path.m
@@ -1,5 +1,2 @@
-// RUN: env USERNAME=asdf LOGNAME=asdf %clang -fmodules -### %s 2>&1 | FileCheck %s -check-prefix=CHECK-SET
-// CHECK-SET: -fmodules-cache-path={{.*}}org.llvm.clang.asdf{{[/\\]+}}ModuleCache
-
 // RUN: %clang -fmodules -### %s 2>&1 | FileCheck %s -check-prefix=CHECK-DEFAULT
-// CHECK-DEFAULT: -fmodules-cache-path={{.*}}org.llvm.clang.{{[A-Za-z0-9_]*[/\\]+}}ModuleCache
+// CHECK-DEFAULT: -fmodules-cache-path={{.*}}clang{{[/\\]+}}ModuleCache
diff --git a/clang/unittests/Driver/ModuleCacheTest.cpp b/clang/unittests/Driver/ModuleCacheTest.cpp
--- a/clang/unittests/Driver/ModuleCacheTest.cpp
+++ b/clang/unittests/Driver/ModuleCacheTest.cpp
@@ -21,7 +21,7 @@
   SmallString<128> Buf;
   Driver::getDefaultModuleCachePath(Buf);
   StringRef Path = Buf;
-  EXPECT_TRUE(Path.find("org.llvm.clang") != Path.npos);
+  EXPECT_TRUE(Path.find("clang") != Path.npos);
   EXPECT_TRUE(Path.endswith("ModuleCache"));  
 }
 } // end anonymous namespace.
diff --git a/llvm/lib/Support/Unix/Path.inc b/llvm/lib/Support/Unix/Path.inc
--- a/llvm/lib/Support/Unix/Path.inc
+++ b/llvm/lib/Support/Unix/Path.inc
@@ -1133,19 +1133,6 @@
   return true;
 }
 
-bool cache_directory(SmallVectorImpl<char> &result) {
-  if (const char *RequestedDir = getenv("XDG_CACHE_HOME")) {
-    result.clear();
-    result.append(RequestedDir, RequestedDir + strlen(RequestedDir));
-    return true;
-  }
-  if (!home_directory(result)) {
-    return false;
-  }
-  append(result, ".cache");
-  return true;
-}
-
 static bool getDarwinConfDir(bool TempDir, SmallVectorImpl<char> &Result) {
   #if defined(_CS_DARWIN_USER_TEMP_DIR) && defined(_CS_DARWIN_USER_CACHE_DIR)
   // On Darwin, use DARWIN_USER_TEMP_DIR or DARWIN_USER_CACHE_DIR.
@@ -1171,6 +1158,27 @@
   return false;
 }
 
+bool cache_directory(SmallVectorImpl<char> &result) {
+#ifdef __APPLE__
+  if (getDarwinConfDir(false/*tempDir*/, result)) {
+    return true;
+  }
+#else
+  // XDG_CACHE_HOME as defined in the XDG Base Directory Specification:
+  // http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html
+  if (const char *RequestedDir = getenv("XDG_CACHE_HOME")) {
+    result.clear();
+    result.append(RequestedDir, RequestedDir + strlen(RequestedDir));
+    return true;
+  }
+#endif
+  if (!home_directory(result)) {
+    return false;
+  }
+  append(result, ".cache");
+  return true;
+}
+
 static const char *getEnvTempDir() {
   // Check whether the temporary directory is specified by an environment
   // variable.