Index: clang/include/clang/Driver/ToolChain.h
===================================================================
--- clang/include/clang/Driver/ToolChain.h
+++ clang/include/clang/Driver/ToolChain.h
@@ -166,6 +166,10 @@
     EffectiveTriple = std::move(ET);
   }
 
+  mutable llvm::Optional<CXXStdlibType> cxxStdlibType;
+  mutable llvm::Optional<RuntimeLibType> runtimeLibType;
+  mutable llvm::Optional<UnwindLibType> unwindLibType;
+
 protected:
   MultilibSet Multilibs;
   Multilib SelectedMultilib;
Index: clang/lib/Driver/ToolChain.cpp
===================================================================
--- clang/lib/Driver/ToolChain.cpp
+++ clang/lib/Driver/ToolChain.cpp
@@ -884,66 +884,86 @@
 
 ToolChain::RuntimeLibType ToolChain::GetRuntimeLibType(
     const ArgList &Args) const {
+  if (runtimeLibType)
+    return *runtimeLibType;
+
   const Arg* A = Args.getLastArg(options::OPT_rtlib_EQ);
   StringRef LibName = A ? A->getValue() : CLANG_DEFAULT_RTLIB;
 
   // Only use "platform" in tests to override CLANG_DEFAULT_RTLIB!
   if (LibName == "compiler-rt")
-    return ToolChain::RLT_CompilerRT;
+    runtimeLibType = ToolChain::RLT_CompilerRT;
   else if (LibName == "libgcc")
-    return ToolChain::RLT_Libgcc;
+    runtimeLibType = ToolChain::RLT_Libgcc;
   else if (LibName == "platform")
-    return GetDefaultRuntimeLibType();
+    runtimeLibType = GetDefaultRuntimeLibType();
+  else {
+    if (A)
+      getDriver().Diag(diag::err_drv_invalid_rtlib_name)
+          << A->getAsString(Args);
 
-  if (A)
-    getDriver().Diag(diag::err_drv_invalid_rtlib_name) << A->getAsString(Args);
+    runtimeLibType = GetDefaultRuntimeLibType();
+  }
 
-  return GetDefaultRuntimeLibType();
+  return *runtimeLibType;
 }
 
 ToolChain::UnwindLibType ToolChain::GetUnwindLibType(
     const ArgList &Args) const {
+  if (unwindLibType)
+    return *unwindLibType;
+
   const Arg *A = Args.getLastArg(options::OPT_unwindlib_EQ);
   StringRef LibName = A ? A->getValue() : CLANG_DEFAULT_UNWINDLIB;
 
   if (LibName == "none")
-    return ToolChain::UNW_None;
+    unwindLibType = ToolChain::UNW_None;
   else if (LibName == "platform" || LibName == "") {
     ToolChain::RuntimeLibType RtLibType = GetRuntimeLibType(Args);
     if (RtLibType == ToolChain::RLT_CompilerRT)
-      return ToolChain::UNW_None;
+      unwindLibType = ToolChain::UNW_None;
     else if (RtLibType == ToolChain::RLT_Libgcc)
-      return ToolChain::UNW_Libgcc;
+      unwindLibType = ToolChain::UNW_Libgcc;
   } else if (LibName == "libunwind") {
     if (GetRuntimeLibType(Args) == RLT_Libgcc)
       getDriver().Diag(diag::err_drv_incompatible_unwindlib);
-    return ToolChain::UNW_CompilerRT;
+    unwindLibType = ToolChain::UNW_CompilerRT;
   } else if (LibName == "libgcc")
-    return ToolChain::UNW_Libgcc;
+    unwindLibType = ToolChain::UNW_Libgcc;
+  else {
+    if (A)
+      getDriver().Diag(diag::err_drv_invalid_unwindlib_name)
+          << A->getAsString(Args);
 
-  if (A)
-    getDriver().Diag(diag::err_drv_invalid_unwindlib_name)
-        << A->getAsString(Args);
+    unwindLibType = GetDefaultUnwindLibType();
+  }
 
-  return GetDefaultUnwindLibType();
+  return *unwindLibType;
 }
 
 ToolChain::CXXStdlibType ToolChain::GetCXXStdlibType(const ArgList &Args) const{
+  if (cxxStdlibType)
+    return *cxxStdlibType;
+
   const Arg *A = Args.getLastArg(options::OPT_stdlib_EQ);
   StringRef LibName = A ? A->getValue() : CLANG_DEFAULT_CXX_STDLIB;
 
   // Only use "platform" in tests to override CLANG_DEFAULT_CXX_STDLIB!
   if (LibName == "libc++")
-    return ToolChain::CST_Libcxx;
+    cxxStdlibType = ToolChain::CST_Libcxx;
   else if (LibName == "libstdc++")
-    return ToolChain::CST_Libstdcxx;
+    cxxStdlibType = ToolChain::CST_Libstdcxx;
   else if (LibName == "platform")
-    return GetDefaultCXXStdlibType();
+    cxxStdlibType = GetDefaultCXXStdlibType();
+  else {
+    if (A)
+      getDriver().Diag(diag::err_drv_invalid_stdlib_name)
+          << A->getAsString(Args);
 
-  if (A)
-    getDriver().Diag(diag::err_drv_invalid_stdlib_name) << A->getAsString(Args);
+    cxxStdlibType = GetDefaultCXXStdlibType();
+  }
 
-  return GetDefaultCXXStdlibType();
+  return *cxxStdlibType;
 }
 
 /// Utility function to add a system include directory to CC1 arguments.
Index: clang/test/Driver/undefined-libs.cpp
===================================================================
--- /dev/null
+++ clang/test/Driver/undefined-libs.cpp
@@ -0,0 +1,15 @@
+
+// Check that all the following options print a warning when given a non-existent
+// value. But only one warning.
+
+// RUN: not %clangxx -stdlib=nostdlib %s 2>&1 | FileCheck --check-prefix=STDLIB %s
+// STDLIB: error: invalid library name in argument '-stdlib=nostdlib'
+// STDLIB-EMPTY:
+//
+// RUN: not %clangxx -rtlib=nortlib %s 2>&1 | FileCheck --check-prefix=RTLIB %s
+// RTLIB: error: invalid runtime library name in argument '-rtlib=nortlib'
+// RTLIB-EMPTY:
+//
+// RUN: not %clangxx -unwindlib=nounwindlib %s 2>&1 | FileCheck --check-prefix=UNWINDLIB %s
+// UNWINDLIB: error: invalid unwind library name in argument '-unwindlib=nounwindlib'
+// UNWINDLIB-EMPTY: