Index: cfe/trunk/CMakeLists.txt
===================================================================
--- cfe/trunk/CMakeLists.txt
+++ cfe/trunk/CMakeLists.txt
@@ -197,6 +197,15 @@
 set(DEFAULT_SYSROOT "" CACHE PATH
   "Default <path> to all compiler invocations for --sysroot=<path>." )
 
+set(CLANG_DEFAULT_CXX_STDLIB "" CACHE STRING
+  "Default C++ stdlib to use (empty for architecture default, \"libstdc++\" or \"libc++\"")
+if (NOT(CLANG_DEFAULT_CXX_STDLIB STREQUAL "" OR
+        CLANG_DEFAULT_CXX_STDLIB STREQUAL "libstdc++" OR
+        CLANG_DEFAULT_CXX_STDLIB STREQUAL "libc++"))
+  message(WARNING "Resetting default C++ stdlib to use architecture default")
+  set(CLANG_DEFAULT_CXX_STDLIB "")
+endif()
+
 set(CLANG_DEFAULT_OPENMP_RUNTIME "libomp" CACHE STRING
   "Default OpenMP runtime used by -fopenmp.")
 
Index: cfe/trunk/include/clang/Config/config.h.cmake
===================================================================
--- cfe/trunk/include/clang/Config/config.h.cmake
+++ cfe/trunk/include/clang/Config/config.h.cmake
@@ -8,6 +8,9 @@
 /* Bug report URL. */
 #define BUG_REPORT_URL "${BUG_REPORT_URL}"
 
+/* Default C++ stdlib to use. */
+#define CLANG_DEFAULT_CXX_STDLIB "${CLANG_DEFAULT_CXX_STDLIB}"
+
 /* Default OpenMP runtime used by -fopenmp. */
 #define CLANG_DEFAULT_OPENMP_RUNTIME "${CLANG_DEFAULT_OPENMP_RUNTIME}"
 
Index: cfe/trunk/include/clang/Config/config.h.in
===================================================================
--- cfe/trunk/include/clang/Config/config.h.in
+++ cfe/trunk/include/clang/Config/config.h.in
@@ -8,6 +8,9 @@
 /* Bug report URL. */
 #undef BUG_REPORT_URL
 
+/* Default C++ stdlib to use. */
+#undef CLANG_DEFAULT_CXX_STDLIB
+
 /* Default OpenMP runtime used by -fopenmp. */
 #undef CLANG_DEFAULT_OPENMP_RUNTIME
 
Index: cfe/trunk/include/clang/Driver/ToolChain.h
===================================================================
--- cfe/trunk/include/clang/Driver/ToolChain.h
+++ cfe/trunk/include/clang/Driver/ToolChain.h
@@ -256,6 +256,10 @@
     return ToolChain::RLT_Libgcc;
   }
 
+  virtual CXXStdlibType GetDefaultCXXStdlibType() const {
+    return ToolChain::CST_Libstdcxx;
+  }
+
   virtual std::string getCompilerRT(const llvm::opt::ArgList &Args,
                                     StringRef Component,
                                     bool Shared = false) const;
Index: cfe/trunk/lib/Driver/ToolChain.cpp
===================================================================
--- cfe/trunk/lib/Driver/ToolChain.cpp
+++ cfe/trunk/lib/Driver/ToolChain.cpp
@@ -9,6 +9,7 @@
 
 #include "Tools.h"
 #include "clang/Basic/ObjCRuntime.h"
+#include "clang/Config/config.h"
 #include "clang/Driver/Action.h"
 #include "clang/Driver/Driver.h"
 #include "clang/Driver/DriverDiagnostic.h"
@@ -533,18 +534,34 @@
   return GetDefaultRuntimeLibType();
 }
 
+static bool ParseCXXStdlibType(const StringRef& Name,
+                               ToolChain::CXXStdlibType& Type) {
+  if (Name == "libc++")
+    Type = ToolChain::CST_Libcxx;
+  else if (Name == "libstdc++")
+    Type = ToolChain::CST_Libstdcxx;
+  else
+    return false;
+
+  return true;
+}
+
 ToolChain::CXXStdlibType ToolChain::GetCXXStdlibType(const ArgList &Args) const{
-  if (Arg *A = Args.getLastArg(options::OPT_stdlib_EQ)) {
-    StringRef Value = A->getValue();
-    if (Value == "libc++")
-      return ToolChain::CST_Libcxx;
-    if (Value == "libstdc++")
-      return ToolChain::CST_Libstdcxx;
-    getDriver().Diag(diag::err_drv_invalid_stdlib_name)
-      << A->getAsString(Args);
+  ToolChain::CXXStdlibType Type;
+  bool HasValidType = false;
+
+  const Arg *A = Args.getLastArg(options::OPT_stdlib_EQ);
+  if (A) {
+    HasValidType = ParseCXXStdlibType(A->getValue(), Type);
+    if (!HasValidType)
+      getDriver().Diag(diag::err_drv_invalid_stdlib_name)
+        << A->getAsString(Args);
   }
 
-  return ToolChain::CST_Libstdcxx;
+  if (!HasValidType && !ParseCXXStdlibType(CLANG_DEFAULT_CXX_STDLIB, Type))
+    Type = GetDefaultCXXStdlibType();
+
+  return Type;
 }
 
 /// \brief Utility function to add a system include directory to CC1 arguments.
Index: cfe/trunk/lib/Driver/ToolChains.h
===================================================================
--- cfe/trunk/lib/Driver/ToolChains.h
+++ cfe/trunk/lib/Driver/ToolChains.h
@@ -512,6 +512,7 @@
   TranslateArgs(const llvm::opt::DerivedArgList &Args,
                 const char *BoundArch) const override;
 
+  CXXStdlibType GetDefaultCXXStdlibType() const override;
   ObjCRuntime getDefaultObjCRuntime(bool isNonFragile) const override;
   bool hasBlocksRuntime() const override;
 
@@ -699,7 +700,7 @@
   bool IsMathErrnoDefault() const override { return false; }
   bool IsObjCNonFragileABIDefault() const override { return true; }
 
-  CXXStdlibType GetCXXStdlibType(const llvm::opt::ArgList &Args) const override;
+  CXXStdlibType GetDefaultCXXStdlibType() const override;
   void AddClangCXXStdlibIncludeArgs(
       const llvm::opt::ArgList &DriverArgs,
       llvm::opt::ArgStringList &CC1Args) const override;
Index: cfe/trunk/lib/Driver/ToolChains.cpp
===================================================================
--- cfe/trunk/lib/Driver/ToolChains.cpp
+++ cfe/trunk/lib/Driver/ToolChains.cpp
@@ -65,6 +65,16 @@
 
 bool MachO::HasNativeLLVMSupport() const { return true; }
 
+ToolChain::CXXStdlibType Darwin::GetDefaultCXXStdlibType() const {
+  // Default to use libc++ on OS X 10.9+ and iOS 7+.
+  if ((isTargetMacOS() && !isMacosxVersionLT(10, 9)) ||
+       (isTargetIOSBased() && !isIPhoneOSVersionLT(7, 0)) ||
+       isTargetWatchOSBased())
+    return ToolChain::CST_Libcxx;
+
+  return ToolChain::CST_Libstdcxx;
+}
+
 /// Darwin provides an ARC runtime starting in MacOS X 10.7 and iOS 5.0.
 ObjCRuntime Darwin::getDefaultObjCRuntime(bool isNonFragile) const {
   if (isTargetWatchOSBased())
@@ -1020,7 +1030,6 @@
                                       const char *BoundArch) const {
   // First get the generic Apple args, before moving onto Darwin-specific ones.
   DerivedArgList *DAL = MachO::TranslateArgs(Args, BoundArch);
-  const OptTable &Opts = getDriver().getOpts();
 
   // If no architecture is bound, none of the translations here are relevant.
   if (!BoundArch)
@@ -1051,14 +1060,6 @@
     }
   }
 
-  // Default to use libc++ on OS X 10.9+ and iOS 7+.
-  if (((isTargetMacOS() && !isMacosxVersionLT(10, 9)) ||
-       (isTargetIOSBased() && !isIPhoneOSVersionLT(7, 0)) ||
-       isTargetWatchOSBased()) &&
-      !Args.getLastArg(options::OPT_stdlib_EQ))
-    DAL->AddJoinedArg(nullptr, Opts.getOption(options::OPT_stdlib_EQ),
-                      "libc++");
-
   // Validate the C++ standard library choice.
   CXXStdlibType Type = GetCXXStdlibType(*DAL);
   if (Type == ToolChain::CST_Libcxx) {
@@ -3027,16 +3028,7 @@
 
 Tool *Bitrig::buildLinker() const { return new tools::bitrig::Linker(*this); }
 
-ToolChain::CXXStdlibType Bitrig::GetCXXStdlibType(const ArgList &Args) const {
-  if (Arg *A = Args.getLastArg(options::OPT_stdlib_EQ)) {
-    StringRef Value = A->getValue();
-    if (Value == "libstdc++")
-      return ToolChain::CST_Libstdcxx;
-    if (Value == "libc++")
-      return ToolChain::CST_Libcxx;
-
-    getDriver().Diag(diag::err_drv_invalid_stdlib_name) << A->getAsString(Args);
-  }
+ToolChain::CXXStdlibType Bitrig::GetDefaultCXXStdlibType() const {
   return ToolChain::CST_Libcxx;
 }