diff --git a/src/llvm-project/llvm/unittests/ADT/TwineTest.cpp b/src/llvm-project/llvm/unittests/ADT/TwineTest.cpp
new file mode 100644
index 0000000..950eda2
--- /dev/null
+++ b/src/llvm-project/llvm/unittests/ADT/TwineTest.cpp
@@ -0,0 +1,123 @@
+//===- TwineTest.cpp - Twine unit tests -----------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/ADT/Twine.h"
+#include "llvm/ADT/SmallString.h"
+#include "llvm/Support/FormatAdapters.h"
+#include "llvm/Support/FormatVariadic.h"
+#include "llvm/Support/raw_ostream.h"
+#include "gtest/gtest.h"
+using namespace llvm;
+
+namespace {
+
+std::string repr(const Twine &Value) {
+  std::string res;
+  llvm::raw_string_ostream OS(res);
+  Value.printRepr(OS);
+  return OS.str();
+}
+
+TEST(TwineTest, Construction) {
+  EXPECT_EQ("", Twine().str());
+  EXPECT_EQ("hi", Twine("hi").str());
+  EXPECT_EQ("hi", Twine(std::string("hi")).str());
+  EXPECT_EQ("hi", Twine(StringRef("hi")).str());
+  EXPECT_EQ("hi", Twine(StringRef(std::string("hi"))).str());
+  EXPECT_EQ("hi", Twine(StringRef("hithere", 2)).str());
+  EXPECT_EQ("hi", Twine(SmallString<4>("hi")).str());
+  EXPECT_EQ("hi", Twine(formatv("{0}", "hi")).str());
+}
+
+TEST(TwineTest, Numbers) {
+  EXPECT_EQ("123", Twine(123U).str());
+  EXPECT_EQ("123", Twine(123).str());
+  EXPECT_EQ("-123", Twine(-123).str());
+  EXPECT_EQ("123", Twine(123).str());
+  EXPECT_EQ("-123", Twine(-123).str());
+
+  EXPECT_EQ("7b", Twine::utohexstr(123).str());
+}
+
+TEST(TwineTest, Characters) {
+  EXPECT_EQ("x", Twine('x').str());
+  EXPECT_EQ("x", Twine(static_cast<unsigned char>('x')).str());
+  EXPECT_EQ("x", Twine(static_cast<signed char>('x')).str());
+}
+
+TEST(TwineTest, Concat) {
+  // Check verse repr, since we care about the actual representation not just
+  // the result.
+
+  // Concat with null.
+  EXPECT_EQ("(Twine null empty)", 
+            repr(Twine("hi").concat(Twine::createNull())));
+  EXPECT_EQ("(Twine null empty)", 
+            repr(Twine::createNull().concat(Twine("hi"))));
+  
+  // Concat with empty.
+  EXPECT_EQ("(Twine cstring:\"hi\" empty)", 
+            repr(Twine("hi").concat(Twine())));
+  EXPECT_EQ("(Twine cstring:\"hi\" empty)", 
+            repr(Twine().concat(Twine("hi"))));
+  EXPECT_EQ("(Twine smallstring:\"hi\" empty)", 
+            repr(Twine().concat(Twine(SmallString<5>("hi")))));
+  EXPECT_EQ("(Twine formatv:\"howdy\" empty)",
+            repr(Twine(formatv("howdy")).concat(Twine())));
+  EXPECT_EQ("(Twine formatv:\"howdy\" empty)",
+            repr(Twine().concat(Twine(formatv("howdy")))));
+  EXPECT_EQ("(Twine smallstring:\"hey\" cstring:\"there\")", 
+            repr(Twine(SmallString<7>("hey")).concat(Twine("there"))));
+
+  // Concatenation of unary ropes.
+  EXPECT_EQ("(Twine cstring:\"a\" cstring:\"b\")", 
+            repr(Twine("a").concat(Twine("b"))));
+
+  // Concatenation of other ropes.
+  EXPECT_EQ("(Twine rope:(Twine cstring:\"a\" cstring:\"b\") cstring:\"c\")", 
+            repr(Twine("a").concat(Twine("b")).concat(Twine("c"))));
+  EXPECT_EQ("(Twine cstring:\"a\" rope:(Twine cstring:\"b\" cstring:\"c\"))",
+            repr(Twine("a").concat(Twine("b").concat(Twine("c")))));
+  EXPECT_EQ("(Twine cstring:\"a\" rope:(Twine smallstring:\"b\" cstring:\"c\"))",
+            repr(Twine("a").concat(Twine(SmallString<3>("b")).concat(Twine("c")))));
+}
+
+TEST(TwineTest, toNullTerminatedStringRef) {
+  SmallString<8> storage;
+  EXPECT_EQ(0, *Twine("hello").toNullTerminatedStringRef(storage).end());
+  EXPECT_EQ(0,
+           *Twine(StringRef("hello")).toNullTerminatedStringRef(storage).end());
+  EXPECT_EQ(0, *Twine(SmallString<11>("hello"))
+                    .toNullTerminatedStringRef(storage)
+                    .end());
+  EXPECT_EQ(0, *Twine(formatv("{0}{1}", "how", "dy"))
+                    .toNullTerminatedStringRef(storage)
+                    .end());
+}
+
+TEST(TwineTest, LazyEvaluation) {
+  struct formatter : FormatAdapter<int> {
+    explicit formatter(int &Count) : FormatAdapter(0), Count(Count) {}
+    int &Count;
+
+    void format(raw_ostream &OS, StringRef Style) { ++Count; }
+  };
+
+  int Count = 0;
+  formatter Formatter(Count);
+  (void)Twine(formatv("{0}", Formatter));
+  EXPECT_EQ(0, Count);
+  (void)Twine(formatv("{0}", Formatter)).str();
+  EXPECT_EQ(1, Count);
+}
+
+  // I suppose linking in the entire code generator to add a unit test to check
+  // the code size of the concat operation is overkill... :)
+
+} // end anonymous namespace