Index: docs/UsersManual.rst
===================================================================
--- docs/UsersManual.rst
+++ docs/UsersManual.rst
@@ -2849,6 +2849,7 @@
       /arch:<value>           Set architecture for code generation
       /Brepro-                Emit an object file which cannot be reproduced over time
       /Brepro                 Emit an object file which can be reproduced over time
+      /clang: <arg>           Pass <arg> to the clang driver
       /C                      Don't discard comments when preprocessing
       /c                      Compile only
       /d1PP                   Retain macro definitions in /E mode
@@ -3084,6 +3085,17 @@
       -W<warning>             Enable the specified warning
       -Xclang <arg>           Pass <arg> to the clang compiler
 
+The /clang: Option
+^^^^^^^^^^^^^^^^^^
+
+When clang-cl is run with a set of ``/clang:<arg>`` options, it will gather all
+of the ``<arg>`` arguments and process them as if they were passed to the clang
+driver. This mechanism allows you to pass flags that are not exposed in the
+clang-cl options or flags that have a different meaning when passed to the clang
+driver. Regardless of where they appear in the command line, the ``/clang:``
+arguments are treated as if they were passed at the end of the clang-cl command
+line.
+
 The /fallback Option
 ^^^^^^^^^^^^^^^^^^^^
 
Index: include/clang/Driver/CLCompatOptions.td
===================================================================
--- include/clang/Driver/CLCompatOptions.td
+++ include/clang/Driver/CLCompatOptions.td
@@ -321,6 +321,8 @@
 def _SLASH_volatile_ms  : Option<["/", "-"], "volatile:ms", KIND_FLAG>,
   Group<_SLASH_volatile_Group>, Flags<[CLOption, DriverOption]>,
   HelpText<"Volatile loads and stores have acquire and release semantics">;
+def _SLASH_clang : CLJoined<"clang:">,
+  HelpText<"Pass <arg> to the clang driver">, MetaVarName<"<arg>">;
 def _SLASH_Zl : CLFlag<"Zl">,
   HelpText<"Don't mention any default libraries in the object file">;
 
Index: include/clang/Driver/Driver.h
===================================================================
--- include/clang/Driver/Driver.h
+++ include/clang/Driver/Driver.h
@@ -362,6 +362,7 @@
   /// ParseArgStrings - Parse the given list of strings into an
   /// ArgList.
   llvm::opt::InputArgList ParseArgStrings(ArrayRef<const char *> Args,
+                                          bool IsClCompatMode,
                                           bool &ContainsError);
 
   /// BuildInputs - Construct the list of inputs and their types from
@@ -552,7 +553,7 @@
 
   /// Get bitmasks for which option flags to include and exclude based on
   /// the driver mode.
-  std::pair<unsigned, unsigned> getIncludeExcludeOptionFlagMasks() const;
+  std::pair<unsigned, unsigned> getIncludeExcludeOptionFlagMasks(bool IsClCompatMode) const;
 
   /// Helper used in BuildJobsForAction.  Doesn't use the cache when building
   /// jobs specifically for the given action, but will use the cache when
Index: lib/Driver/Driver.cpp
===================================================================
--- lib/Driver/Driver.cpp
+++ lib/Driver/Driver.cpp
@@ -166,6 +166,7 @@
 }
 
 InputArgList Driver::ParseArgStrings(ArrayRef<const char *> ArgStrings,
+                                     bool IsClCompatMode,
                                      bool &ContainsError) {
   llvm::PrettyStackTraceString CrashInfo("Command line argument parsing");
   ContainsError = false;
@@ -173,7 +174,7 @@
   unsigned IncludedFlagsBitmask;
   unsigned ExcludedFlagsBitmask;
   std::tie(IncludedFlagsBitmask, ExcludedFlagsBitmask) =
-      getIncludeExcludeOptionFlagMasks();
+      getIncludeExcludeOptionFlagMasks(IsClCompatMode);
 
   unsigned MissingArgIndex, MissingArgCount;
   InputArgList Args =
@@ -730,7 +731,7 @@
   ConfigFile = CfgFileName.str();
   bool ContainErrors;
   CfgOptions = llvm::make_unique<InputArgList>(
-      ParseArgStrings(NewCfgArgs, ContainErrors));
+      ParseArgStrings(NewCfgArgs, IsCLMode(), ContainErrors));
   if (ContainErrors) {
     CfgOptions.reset();
     return true;
@@ -924,7 +925,7 @@
   // Arguments specified in command line.
   bool ContainsError;
   CLOptions = llvm::make_unique<InputArgList>(
-      ParseArgStrings(ArgList.slice(1), ContainsError));
+      ParseArgStrings(ArgList.slice(1), IsCLMode(), ContainsError));
 
   // Try parsing configuration file.
   if (!ContainsError)
@@ -934,21 +935,47 @@
   // All arguments, from both config file and command line.
   InputArgList Args = std::move(HasConfigFile ? std::move(*CfgOptions)
                                               : std::move(*CLOptions));
-  if (HasConfigFile)
-    for (auto *Opt : *CLOptions) {
-      if (Opt->getOption().matches(options::OPT_config))
-        continue;
+
+  auto appendOneArg = [&Args](const Arg *Opt, const Arg *BaseArg) {
       unsigned Index = Args.MakeIndex(Opt->getSpelling());
-      const Arg *BaseArg = &Opt->getBaseArg();
-      if (BaseArg == Opt)
-        BaseArg = nullptr;
       Arg *Copy = new llvm::opt::Arg(Opt->getOption(), Opt->getSpelling(),
                                      Index, BaseArg);
       Copy->getValues() = Opt->getValues();
       if (Opt->isClaimed())
         Copy->claim();
       Args.append(Copy);
+  };
+
+  if (HasConfigFile)
+    for (auto *Opt : *CLOptions) {
+      if (Opt->getOption().matches(options::OPT_config))
+        continue;
+      const Arg *BaseArg = &Opt->getBaseArg();
+      if (BaseArg == Opt)
+        BaseArg = nullptr;
+      appendOneArg(Opt, BaseArg);
+    }
+
+  // In CL mode, look for any pass-through arguments
+  if (IsCLMode() && !ContainsError) {
+    SmallVector<const char *, 16> CLModePassThroughArgList;
+    for (const auto *A : Args.filtered(options::OPT__SLASH_clang)) {
+      A->claim();
+      CLModePassThroughArgList.push_back(A->getValue());
+    }
+
+    if (!CLModePassThroughArgList.empty()) {
+      // Parse any pass through args using default clang processing rather
+      // than clang-cl processing.
+      auto CLModePassThroughOptions = llvm::make_unique<InputArgList>(
+          ParseArgStrings(CLModePassThroughArgList, false, ContainsError));
+
+      if (!ContainsError)
+        for (auto *Opt : *CLModePassThroughOptions) {
+          appendOneArg(Opt, nullptr);
+        }
     }
+  }
 
   // FIXME: This stuff needs to go into the Compilation, not the driver.
   bool CCCPrintPhases;
@@ -1452,7 +1479,7 @@
   unsigned IncludedFlagsBitmask;
   unsigned ExcludedFlagsBitmask;
   std::tie(IncludedFlagsBitmask, ExcludedFlagsBitmask) =
-      getIncludeExcludeOptionFlagMasks();
+      getIncludeExcludeOptionFlagMasks(IsCLMode());
 
   ExcludedFlagsBitmask |= options::NoDriverOption;
   if (!ShowHidden)
@@ -4661,11 +4688,11 @@
   return false;
 }
 
-std::pair<unsigned, unsigned> Driver::getIncludeExcludeOptionFlagMasks() const {
+std::pair<unsigned, unsigned> Driver::getIncludeExcludeOptionFlagMasks(bool IsClCompatMode) const {
   unsigned IncludedFlagsBitmask = 0;
   unsigned ExcludedFlagsBitmask = options::NoDriverOption;
 
-  if (Mode == CLMode) {
+  if (IsClCompatMode) {
     // Include CL and Core options.
     IncludedFlagsBitmask |= options::CLOption;
     IncludedFlagsBitmask |= options::CoreOption;
Index: test/Driver/cl-options.c
===================================================================
--- test/Driver/cl-options.c
+++ test/Driver/cl-options.c
@@ -614,5 +614,17 @@
 // RUN:     --version \
 // RUN:     -Werror /Zs -- %s 2>&1
 
+// Accept clang options under the /clang: flag.
+
+// RUN: %clang_cl -O2 -### -- %s 2>&1 | FileCheck -check-prefix=NOCLANG %s
+// NOCLANG: "--dependent-lib=libcmt"
+// NOCLANG-SAME: "-vectorize-slp"
+// NOCLANG-NOT: "--dependent-lib=msvcrt"
+
+// RUN: %clang_cl -O2 -MD /clang:-fno-slp-vectorize /clang:-MD /clang:-MF /clang:my_dependency_file.dep -### -- %s 2>&1 | FileCheck -check-prefix=CLANG %s
+// CLANG: "--dependent-lib=msvcrt"
+// CLANG-SAME: "-dependency-file" "my_dependency_file.dep"
+// CLANG-NOT: "--dependent-lib=libcmt"
+// CLANG-NOT: "-vectorize-slp"
 
 void f() { }