Index: cfe/trunk/test/Driver/cl-options.c
===================================================================
--- cfe/trunk/test/Driver/cl-options.c
+++ cfe/trunk/test/Driver/cl-options.c
@@ -379,6 +379,12 @@
 // RUN: %clang_cl -fmsc-version=1900 -TP -### -- %s 2>&1 | FileCheck -check-prefix=CXX14 %s
 // CXX14: -std=c++14
 
+// RUN: env CL="/Gy" %clang_cl -### -- %s 2>&1 | FileCheck -check-prefix=ENV-CL %s
+// ENV-CL: "-ffunction-sections"
+
+// RUN: env CL="/Gy" _CL_="/Gy- -- %s" %clang_cl -### 2>&1 | FileCheck -check-prefix=ENV-_CL_ %s
+// ENV-_CL_-NOT: "-ffunction-sections"
+
 // Accept "core" clang options.
 // (/Zs is for syntax-only, -Werror makes it fail hard on unknown options)
 // RUN: %clang_cl \
Index: cfe/trunk/tools/driver/driver.cpp
===================================================================
--- cfe/trunk/tools/driver/driver.cpp
+++ cfe/trunk/tools/driver/driver.cpp
@@ -300,6 +300,15 @@
   }
 }
 
+static void getCLEnvVarOptions(std::string &EnvValue, llvm::StringSaver &Saver,
+                               SmallVectorImpl<const char *> &Opts) {
+  llvm::cl::TokenizeWindowsCommandLine(EnvValue, Saver, Opts);
+  // The first instance of '#' should be replaced with '=' in each option.
+  for (const char *Opt : Opts)
+    if (char *NumberSignPtr = const_cast<char *>(::strchr(Opt, '#')))
+      *NumberSignPtr = '=';
+}
+
 static void SetBackdoorDriverOutputsFromEnvVars(Driver &TheDriver) {
   // Handle CC_PRINT_OPTIONS and CC_PRINT_OPTIONS_FILE.
   TheDriver.CCPrintOptions = !!::getenv("CC_PRINT_OPTIONS");
@@ -445,6 +454,29 @@
     }
   }
 
+  // Handle CL and _CL_ which permits additional command line options to be
+  // prepended or appended.
+  if (Tokenizer == &llvm::cl::TokenizeWindowsCommandLine) {
+    // Arguments in "CL" are prepended.
+    llvm::Optional<std::string> OptCL = llvm::sys::Process::GetEnv("CL");
+    if (OptCL.hasValue()) {
+      SmallVector<const char *, 8> PrependedOpts;
+      getCLEnvVarOptions(OptCL.getValue(), Saver, PrependedOpts);
+
+      // Insert right after the program name to prepend to the argument list.
+      argv.insert(argv.begin() + 1, PrependedOpts.begin(), PrependedOpts.end());
+    }
+    // Arguments in "_CL_" are appended.
+    llvm::Optional<std::string> Opt_CL_ = llvm::sys::Process::GetEnv("_CL_");
+    if (Opt_CL_.hasValue()) {
+      SmallVector<const char *, 8> AppendedOpts;
+      getCLEnvVarOptions(Opt_CL_.getValue(), Saver, AppendedOpts);
+
+      // Insert at the end of the argument list to append.
+      argv.append(AppendedOpts.begin(), AppendedOpts.end());
+    }
+  }
+
   std::set<std::string> SavedStrings;
   // Handle CCC_OVERRIDE_OPTIONS, used for editing a command line behind the
   // scenes.