Index: cfe/trunk/include/clang/Driver/Driver.h
===================================================================
--- cfe/trunk/include/clang/Driver/Driver.h
+++ cfe/trunk/include/clang/Driver/Driver.h
@@ -155,6 +155,9 @@
   /// Whether the driver is just the preprocessor.
   bool CCCIsCPP() const { return Mode == CPPMode; }
 
+  /// Whether the driver should follow gcc like behavior.
+  bool CCCIsCC() const { return Mode == GCCMode; }
+
   /// Whether the driver should follow cl.exe like behavior.
   bool IsCLMode() const { return Mode == CLMode; }
 
@@ -291,7 +294,7 @@
   /// @{
 
   /// ParseDriverMode - Look for and handle the driver mode option in Args.
-  void ParseDriverMode(ArrayRef<const char *> Args);
+  void ParseDriverMode(StringRef ProgramName, ArrayRef<const char *> Args);
 
   /// ParseArgStrings - Parse the given list of strings into an
   /// ArgList.
@@ -440,6 +443,10 @@
   LTOKind getLTOMode() const { return LTOMode; }
 
 private:
+  /// Set the driver mode (cl, gcc, etc) from an option string of the form
+  /// --driver-mode=<mode>.
+  void setDriverModeFromOption(StringRef Opt);
+
   /// Parse the \p Args list for LTO options and record the type of LTO
   /// compilation based on which -f(no-)?lto(=.*)? option occurs last.
   void setLTOMode(const llvm::opt::ArgList &Args);
Index: cfe/trunk/lib/Driver/Driver.cpp
===================================================================
--- cfe/trunk/lib/Driver/Driver.cpp
+++ cfe/trunk/lib/Driver/Driver.cpp
@@ -88,31 +88,39 @@
   llvm::DeleteContainerSeconds(ToolChains);
 }
 
-void Driver::ParseDriverMode(ArrayRef<const char *> Args) {
-  const std::string OptName =
-      getOpts().getOption(options::OPT_driver_mode).getPrefixedName();
+void Driver::ParseDriverMode(StringRef ProgramName,
+                             ArrayRef<const char *> Args) {
+  auto Default = ToolChain::getTargetAndModeFromProgramName(ProgramName);
+  StringRef DefaultMode(Default.second);
+  setDriverModeFromOption(DefaultMode);
 
   for (const char *ArgPtr : Args) {
     // Ingore nullptrs, they are response file's EOL markers
     if (ArgPtr == nullptr)
       continue;
     const StringRef Arg = ArgPtr;
-    if (!Arg.startswith(OptName))
-      continue;
+    setDriverModeFromOption(Arg);
+  }
+}
+
+void Driver::setDriverModeFromOption(StringRef Opt) {
+  const std::string OptName =
+      getOpts().getOption(options::OPT_driver_mode).getPrefixedName();
+  if (!Opt.startswith(OptName))
+    return;
+  StringRef Value = Opt.drop_front(OptName.size());
 
-    const StringRef Value = Arg.drop_front(OptName.size());
-    const unsigned M = llvm::StringSwitch<unsigned>(Value)
-                           .Case("gcc", GCCMode)
-                           .Case("g++", GXXMode)
-                           .Case("cpp", CPPMode)
-                           .Case("cl", CLMode)
-                           .Default(~0U);
+  const unsigned M = llvm::StringSwitch<unsigned>(Value)
+                         .Case("gcc", GCCMode)
+                         .Case("g++", GXXMode)
+                         .Case("cpp", CPPMode)
+                         .Case("cl", CLMode)
+                         .Default(~0U);
 
-    if (M != ~0U)
-      Mode = static_cast<DriverMode>(M);
-    else
-      Diag(diag::err_drv_unsupported_option_argument) << OptName << Value;
-  }
+  if (M != ~0U)
+    Mode = static_cast<DriverMode>(M);
+  else
+    Diag(diag::err_drv_unsupported_option_argument) << OptName << Value;
 }
 
 InputArgList Driver::ParseArgStrings(ArrayRef<const char *> ArgStrings) {
@@ -468,7 +476,7 @@
 
   // We look for the driver mode option early, because the mode can affect
   // how other options are parsed.
-  ParseDriverMode(ArgList.slice(1));
+  ParseDriverMode(ClangExecutable, ArgList.slice(1));
 
   // FIXME: What are we going to do with -V and -b?
 
Index: cfe/trunk/unittests/Driver/ToolChainTest.cpp
===================================================================
--- cfe/trunk/unittests/Driver/ToolChainTest.cpp
+++ cfe/trunk/unittests/Driver/ToolChainTest.cpp
@@ -117,4 +117,29 @@
             S);
 }
 
+TEST(ToolChainTest, DefaultDriverMode) {
+  IntrusiveRefCntPtr<DiagnosticOptions> DiagOpts = new DiagnosticOptions();
+
+  IntrusiveRefCntPtr<DiagnosticIDs> DiagID(new DiagnosticIDs());
+  struct TestDiagnosticConsumer : public DiagnosticConsumer {};
+  DiagnosticsEngine Diags(DiagID, &*DiagOpts, new TestDiagnosticConsumer);
+  IntrusiveRefCntPtr<vfs::InMemoryFileSystem> InMemoryFileSystem(
+      new vfs::InMemoryFileSystem);
+
+  Driver CCDriver("/home/test/bin/clang", "arm-linux-gnueabi", Diags,
+                  InMemoryFileSystem);
+  Driver CXXDriver("/home/test/bin/clang++", "arm-linux-gnueabi", Diags,
+                   InMemoryFileSystem);
+  Driver CLDriver("/home/test/bin/clang-cl", "arm-linux-gnueabi", Diags,
+                  InMemoryFileSystem);
+
+  std::unique_ptr<Compilation> CC(CCDriver.BuildCompilation({"foo.cpp"}));
+  std::unique_ptr<Compilation> CXX(CXXDriver.BuildCompilation({"foo.cpp"}));
+  std::unique_ptr<Compilation> CL(CLDriver.BuildCompilation({"foo.cpp"}));
+
+  EXPECT_TRUE(CCDriver.CCCIsCC());
+  EXPECT_TRUE(CXXDriver.CCCIsCXX());
+  EXPECT_TRUE(CLDriver.IsCLMode());
+}
+
 } // end anonymous namespace