Index: llvm/trunk/lib/Support/YAMLTraits.cpp
===================================================================
--- llvm/trunk/lib/Support/YAMLTraits.cpp
+++ llvm/trunk/lib/Support/YAMLTraits.cpp
@@ -660,11 +660,6 @@
     return;
   }
 
-  unsigned i = 0;
-  unsigned j = 0;
-  unsigned End = S.size();
-  const char *Base = S.data();
-
   const char *const Quote = MustQuote == QuotingType::Single ? "'" : "\"";
   output(Quote); // Starting quote.
 
@@ -672,11 +667,16 @@
   // present, and will be escaped using a variety of unicode-scalar and special short-form
   // escapes. This is handled in yaml::escape.
   if (MustQuote == QuotingType::Double) {
-    output(yaml::escape(Base, /* EscapePrintable= */ false));
+    output(yaml::escape(S, /* EscapePrintable= */ false));
     outputUpToEndOfLine(Quote);
     return;
   }
 
+  unsigned i = 0;
+  unsigned j = 0;
+  unsigned End = S.size();
+  const char *Base = S.data();
+
   // When using single-quoted strings, any single quote ' must be doubled to be escaped.
   while (j < End) {
     if (S[j] == '\'') {                    // Escape quotes.
Index: llvm/trunk/unittests/Support/YAMLIOTest.cpp
===================================================================
--- llvm/trunk/unittests/Support/YAMLIOTest.cpp
+++ llvm/trunk/unittests/Support/YAMLIOTest.cpp
@@ -688,6 +688,7 @@
   std::string stdstr10;
   std::string stdstr11;
   std::string stdstr12;
+  std::string stdstr13;
 };
 
 namespace llvm {
@@ -718,6 +719,7 @@
       io.mapRequired("stdstr10",  st.stdstr10);
       io.mapRequired("stdstr11",  st.stdstr11);
       io.mapRequired("stdstr12",  st.stdstr12);
+      io.mapRequired("stdstr13",  st.stdstr13);
     }
   };
 }
@@ -750,6 +752,7 @@
     map.stdstr10 = "0.2e20";
     map.stdstr11 = "0x30";
     map.stdstr12 = "- match";
+    map.stdstr13.assign("\0a\0b\0", 5);
 
     llvm::raw_string_ostream ostr(intermediate);
     Output yout(ostr);
@@ -775,6 +778,7 @@
   EXPECT_NE(std::string::npos, flowOut.find("'@hhh'"));
   EXPECT_NE(std::string::npos, flowOut.find("''\n"));
   EXPECT_NE(std::string::npos, flowOut.find("'0000000004000000'\n"));
+  EXPECT_NE(std::string::npos, flowOut.find("\"\\0a\\0b\\0\""));
 
   {
     Input yin(intermediate);
@@ -794,6 +798,7 @@
     EXPECT_TRUE(map.stdstr4 == "@hhh");
     EXPECT_TRUE(map.stdstr5 == "");
     EXPECT_TRUE(map.stdstr6 == "0000000004000000");
+    EXPECT_EQ(std::string("\0a\0b\0", 5), map.stdstr13);
   }
 }