Index: llvm/trunk/include/llvm/Support/YAMLParser.h
===================================================================
--- llvm/trunk/include/llvm/Support/YAMLParser.h
+++ llvm/trunk/include/llvm/Support/YAMLParser.h
@@ -73,8 +73,11 @@
 /// \returns true if there was an error, false otherwise.
 bool scanTokens(StringRef Input);
 
-/// \brief Escape \a Input for a double quoted scalar.
-std::string escape(StringRef Input);
+/// \brief Escape \a Input for a double quoted scalar; if \p EscapePrintable
+/// is true, all UTF8 sequences will be escaped, if \p EscapePrintable is
+/// false, those UTF8 sequences encoding printable unicode scalars will not be
+/// escaped, but emitted verbatim.
+std::string escape(StringRef Input, bool EscapePrintable = true);
 
 /// \brief This class represents a YAML stream potentially containing multiple
 ///        documents.
Index: llvm/trunk/lib/Support/YAMLParser.cpp
===================================================================
--- llvm/trunk/lib/Support/YAMLParser.cpp
+++ llvm/trunk/lib/Support/YAMLParser.cpp
@@ -26,6 +26,7 @@
 #include "llvm/Support/MemoryBuffer.h"
 #include "llvm/Support/SMLoc.h"
 #include "llvm/Support/SourceMgr.h"
+#include "llvm/Support/Unicode.h"
 #include "llvm/Support/raw_ostream.h"
 #include <algorithm>
 #include <cassert>
@@ -687,7 +688,7 @@
   return true;
 }
 
-std::string yaml::escape(StringRef Input) {
+std::string yaml::escape(StringRef Input, bool EscapePrintable) {
   std::string EscapedInput;
   for (StringRef::iterator i = Input.begin(), e = Input.end(); i != e; ++i) {
     if (*i == '\\')
@@ -734,6 +735,9 @@
         EscapedInput += "\\L";
       else if (UnicodeScalarValue.first == 0x2029)
         EscapedInput += "\\P";
+      else if (!EscapePrintable &&
+               sys::unicode::isPrintable(UnicodeScalarValue.first))
+        EscapedInput += StringRef(i, UnicodeScalarValue.second);
       else {
         std::string HexStr = utohexstr(UnicodeScalarValue.first);
         if (HexStr.size() <= 2)
Index: llvm/trunk/lib/Support/YAMLTraits.cpp
===================================================================
--- llvm/trunk/lib/Support/YAMLTraits.cpp
+++ llvm/trunk/lib/Support/YAMLTraits.cpp
@@ -638,39 +638,22 @@
   const char *Base = S.data();
 
   const char *const Quote = MustQuote == QuotingType::Single ? "'" : "\"";
-  const char QuoteChar = MustQuote == QuotingType::Single ? '\'' : '"';
-
   output(Quote); // Starting quote.
 
-  // When using single-quoted strings, any single quote ' must be doubled to be
-  // escaped.
-  // When using double-quoted strings, print \x + hex for non-printable ASCII
-  // characters, and escape double quotes.
-  while (j < End) {
-    if (S[j] == QuoteChar) {                  // Escape quotes.
-      output(StringRef(&Base[i], j - i));     // "flush".
-      if (MustQuote == QuotingType::Double) { // Print it as \"
-        output(StringLiteral("\\"));
-        output(StringRef(Quote, 1));
-      } else {                       // Single
-        output(StringLiteral("''")); // Print it as ''
-      }
-      i = j + 1;
-    } else if (MustQuote == QuotingType::Double &&
-               !sys::unicode::isPrintable(S[j]) && (S[j] & 0x80) == 0) {
-      // If we're double quoting non-printable characters, we prefer printing
-      // them as "\x" + their hex representation. Note that special casing is
-      // needed for UTF-8, where a byte may be part of a UTF-8 sequence and
-      // appear as non-printable, in which case we want to print the correct
-      // unicode character and not its hex representation.
-      output(StringRef(&Base[i], j - i)); // "flush"
-      output(StringLiteral("\\x"));
-
-      // Output the byte 0x0F as \x0f.
-      auto FormattedHex = format_hex_no_prefix(S[j], 2);
-      Out << FormattedHex;
-      Column += 4; // one for the '\', one for the 'x', and two for the hex
+  // When using double-quoted strings (and only in that case), non-printable characters may be
+  // 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));
+    this->outputUpToEndOfLine(Quote);
+    return;
+  }
 
+  // When using single-quoted strings, any single quote ' must be doubled to be escaped.
+  while (j < End) {
+    if (S[j] == '\'') {                    // Escape quotes.
+      output(StringRef(&Base[i], j - i));  // "flush".
+      output(StringLiteral("''"));         // Print it as ''
       i = j + 1;
     }
     ++j;
Index: llvm/trunk/unittests/Support/YAMLIOTest.cpp
===================================================================
--- llvm/trunk/unittests/Support/YAMLIOTest.cpp
+++ llvm/trunk/unittests/Support/YAMLIOTest.cpp
@@ -2464,7 +2464,10 @@
   yamlize(xout, Input, true, Ctx);
 
   ostr.flush();
-  EXPECT_EQ(Expected, out);
+
+  // Make a separate StringRef so we get nice byte-by-byte output.
+  llvm::StringRef Got(out);
+  EXPECT_EQ(Expected, Got);
 }
 
 TEST(YAMLIO, TestEscaped) {
@@ -2485,4 +2488,17 @@
   // UTF8 with single quote inside double quote
   TestEscaped("parameter 'параметр' is unused",
               "\"parameter 'параметр' is unused\"");
+
+  // String with embedded non-printable multibyte UTF-8 sequence (U+200B
+  // zero-width space). The thing to test here is that we emit a
+  // unicode-scalar level escape like \uNNNN (at the YAML level), and don't
+  // just pass the UTF-8 byte sequence through as with quoted printables.
+  TestEscaped("foo\u200Bbar", "\"foo\\u200Bbar\"");
+  {
+    const unsigned char foobar[10] = {'f', 'o', 'o',
+                                      0xE2, 0x80, 0x8B, // UTF-8 of U+200B
+                                      'b', 'a', 'r',
+                                      0x0};
+    TestEscaped((char const *)foobar, "\"foo\\u200Bbar\"");
+  }
 }