Index: llvm/include/llvm/Support/ConvertUTF.h
===================================================================
--- llvm/include/llvm/Support/ConvertUTF.h
+++ llvm/include/llvm/Support/ConvertUTF.h
@@ -123,6 +123,9 @@
 #define UNI_UTF16_BYTE_ORDER_MARK_NATIVE  0xFEFF
 #define UNI_UTF16_BYTE_ORDER_MARK_SWAPPED 0xFFFE
 
+#define UNI_UTF32_BYTE_ORDER_MARK_NATIVE 0x0000FEFF
+#define UNI_UTF32_BYTE_ORDER_MARK_SWAPPED 0xFFFE0000
+
 typedef enum {
   conversionOK,           /* conversion successful */
   sourceExhausted,        /* partial character in source, but hit end */
@@ -278,6 +281,24 @@
 */
 bool convertUTF16ToUTF8String(ArrayRef<UTF16> Src, std::string &Out);
 
+/**
+ * Converts a stream of raw bytes assumed to be UTF32 into a UTF8 std::string.
+ *
+ * \param [in] SrcBytes A buffer of what is assumed to be UTF-32 encoded text.
+ * \param [out] Out Converted UTF-8 is stored here on success.
+ * \returns true on success
+ */
+bool convertUTF32ToUTF8String(ArrayRef<char> SrcBytes, std::string &Out);
+
+/**
+ * Converts a UTF32 string into a UTF8 std::string.
+ *
+ * \param [in] Src A buffer of UTF-32 encoded text.
+ * \param [out] Out Converted UTF-8 is stored here on success.
+ * \returns true on success
+ */
+bool convertUTF32ToUTF8String(ArrayRef<UTF32> Src, std::string &Out);
+
 /**
  * Converts a UTF-8 string into a UTF-16 string with native endianness.
  *
Index: llvm/lib/Support/ConvertUTFWrapper.cpp
===================================================================
--- llvm/lib/Support/ConvertUTFWrapper.cpp
+++ llvm/lib/Support/ConvertUTFWrapper.cpp
@@ -141,6 +141,60 @@
       Src.size() * sizeof(UTF16)), Out);
 }
 
+bool convertUTF32ToUTF8String(ArrayRef<char> SrcBytes, std::string &Out) {
+  assert(Out.empty());
+
+  // Avoid OOB by returning early on empty input.
+  if (SrcBytes.empty())
+    return true;
+
+  const UTF32 *Src = reinterpret_cast<const UTF32 *>(SrcBytes.begin());
+  const UTF32 *SrcEnd = reinterpret_cast<const UTF32 *>(SrcBytes.end());
+
+  assert((uintptr_t)Src % sizeof(UTF32) == 0);
+
+  // Byteswap if necessary.
+  std::vector<UTF32> ByteSwapped;
+  if (Src[0] == UNI_UTF32_BYTE_ORDER_MARK_SWAPPED) {
+    ByteSwapped.insert(ByteSwapped.end(), Src, SrcEnd);
+    for (unsigned I = 0, E = ByteSwapped.size(); I != E; ++I)
+      ByteSwapped[I] = llvm::ByteSwap_32(ByteSwapped[I]);
+    Src = &ByteSwapped[0];
+    SrcEnd = &ByteSwapped[ByteSwapped.size() - 1] + 1;
+  }
+
+  // Skip the BOM for conversion.
+  if (Src[0] == UNI_UTF32_BYTE_ORDER_MARK_NATIVE)
+    Src++;
+
+  // Just allocate enough space up front.  We'll shrink it later.  Allocate
+  // enough that we can fit a null terminator without reallocating.
+  Out.resize(SrcBytes.size() * UNI_MAX_UTF8_BYTES_PER_CODE_POINT + 1);
+  UTF8 *Dst = reinterpret_cast<UTF8 *>(&Out[0]);
+  UTF8 *DstEnd = Dst + Out.size();
+
+  ConversionResult CR =
+      ConvertUTF32toUTF8(&Src, SrcEnd, &Dst, DstEnd, strictConversion);
+  assert(CR != targetExhausted);
+
+  if (CR != conversionOK) {
+    Out.clear();
+    return false;
+  }
+
+  Out.resize(reinterpret_cast<char *>(Dst) - &Out[0]);
+  Out.push_back(0);
+  Out.pop_back();
+  return true;
+}
+
+bool convertUTF32ToUTF8String(ArrayRef<UTF32> Src, std::string &Out) {
+  return convertUTF32ToUTF8String(
+      llvm::ArrayRef<char>(reinterpret_cast<const char *>(Src.data()),
+                           Src.size() * sizeof(UTF32)),
+      Out);
+}
+
 bool convertUTF8ToUTF16String(StringRef SrcUTF8,
                               SmallVectorImpl<UTF16> &DstUTF16) {
   assert(DstUTF16.empty());
Index: llvm/unittests/Support/ConvertUTFTest.cpp
===================================================================
--- llvm/unittests/Support/ConvertUTFTest.cpp
+++ llvm/unittests/Support/ConvertUTFTest.cpp
@@ -25,6 +25,17 @@
   EXPECT_EQ(Expected, Result);
 }
 
+TEST(ConvertUTFTest, ConvertUTF32LittleEndianToUTF8String) {
+  // Src is a crystal ball.
+  alignas(UTF32) static const char Src[] = "\x2E\xF5\x01\x00";
+  ArrayRef<char> Ref(Src, sizeof(Src) - 1);
+  std::string Result;
+  bool Success = convertUTF32ToUTF8String(Ref, Result);
+  EXPECT_TRUE(Success);
+  std::string Expected("\xF0\x9F\x94\xAE");
+  EXPECT_EQ(Expected, Result);
+}
+
 TEST(ConvertUTFTest, ConvertUTF16BigEndianToUTF8String) {
   // Src is the look of disapproval.
   alignas(UTF16) static const char Src[] = "\xfe\xff\x0c\xa0\x00_\x0c\xa0";
@@ -36,6 +47,17 @@
   EXPECT_EQ(Expected, Result);
 }
 
+TEST(ConvertUTFTest, ConvertUTF32BigEndianToUTF8String) {
+  // Src is a crystal ball.
+  alignas(UTF32) static const char Src[] = "\x00\x00\xfe\xff\x00\x01\xF5\x2E";
+  ArrayRef<char> Ref(Src, sizeof(Src) - 1);
+  std::string Result;
+  bool Success = convertUTF32ToUTF8String(Ref, Result);
+  EXPECT_TRUE(Success);
+  std::string Expected("\xF0\x9F\x94\xAE");
+  EXPECT_EQ(Expected, Result);
+}
+
 TEST(ConvertUTFTest, ConvertUTF8ToUTF16String) {
   // Src is the look of disapproval.
   static const char Src[] = "\xe0\xb2\xa0_\xe0\xb2\xa0";