Index: llvm/trunk/lib/Support/CommandLine.cpp
===================================================================
--- llvm/trunk/lib/Support/CommandLine.cpp
+++ llvm/trunk/lib/Support/CommandLine.cpp
@@ -874,6 +874,13 @@
     // QUOTED state means that it's reading a token quoted by double quotes.
     if (State == QUOTED) {
       if (C == '"') {
+        if (I < (E - 1) && Src[I + 1] == '"') {
+          // Consecutive double-quotes inside a quoted string implies one
+          // double-quote.
+          Token.push_back('"');
+          I = I + 1;
+          continue;
+        }
         State = UNQUOTED;
         continue;
       }
Index: llvm/trunk/unittests/Support/CommandLineTest.cpp
===================================================================
--- llvm/trunk/unittests/Support/CommandLineTest.cpp
+++ llvm/trunk/unittests/Support/CommandLineTest.cpp
@@ -185,7 +185,7 @@
                            array_lengthof(Output));
 }
 
-TEST(CommandLineTest, TokenizeWindowsCommandLine) {
+TEST(CommandLineTest, TokenizeWindowsCommandLine1) {
   const char Input[] = "a\\b c\\\\d e\\\\\"f g\" h\\\"i j\\\\\\\"k \"lmn\" o pqr "
                       "\"st \\\"u\" \\v";
   const char *const Output[] = { "a\\b", "c\\\\d", "e\\f g", "h\"i", "j\\\"k",
@@ -194,6 +194,13 @@
                            array_lengthof(Output));
 }
 
+TEST(CommandLineTest, TokenizeWindowsCommandLine2) {
+  const char Input[] = "clang -c -DFOO=\"\"\"ABC\"\"\" x.cpp";
+  const char *const Output[] = { "clang", "-c", "-DFOO=\"ABC\"", "x.cpp"};
+  testCommandLineTokenizer(cl::TokenizeWindowsCommandLine, Input, Output,
+                           array_lengthof(Output));
+}
+
 TEST(CommandLineTest, TokenizeConfigFile1) {
   const char *Input = "\\";
   const char *const Output[] = { "\\" };