diff --git a/llvm/unittests/ADT/StringExtrasTest.cpp b/llvm/unittests/ADT/StringExtrasTest.cpp
--- a/llvm/unittests/ADT/StringExtrasTest.cpp
+++ b/llvm/unittests/ADT/StringExtrasTest.cpp
@@ -182,3 +182,31 @@
   testConvertToCamelCase(true, "Op_Name", "Op_Name");
   testConvertToCamelCase(true, "opName", "OpName");
 }
+
+constexpr uint64_t MaxUint64 = std::numeric_limits<uint64_t>::max();
+constexpr int64_t MaxInt64 = std::numeric_limits<int64_t>::max();
+constexpr int64_t MinInt64 = std::numeric_limits<int64_t>::min();
+
+TEST(StringExtras, UToStr) {
+  EXPECT_EQ("0", utostr(0));
+  EXPECT_EQ("0", utostr(0, /*isNeg=*/false));
+  EXPECT_EQ("1", utostr(1));
+  EXPECT_EQ("1", utostr(1, /*isNeg=*/false));
+  EXPECT_EQ("18446744073709551615", utostr(MaxUint64));
+  EXPECT_EQ("18446744073709551615", utostr(MaxUint64, /*isNeg=*/false));
+
+  EXPECT_EQ("-0", utostr(0, /*isNeg=*/true));
+  EXPECT_EQ("-1", utostr(1, /*isNeg=*/true));
+  EXPECT_EQ("-9223372036854775807", utostr(MaxInt64, /*isNeg=*/true));
+  EXPECT_EQ("-9223372036854775808",
+            utostr(-static_cast<uint64_t>(MinInt64), /*isNeg=*/true));
+  EXPECT_EQ("-18446744073709551615", utostr(MaxUint64, /*isNeg=*/true));
+}
+
+TEST(StringExtras, IToStr) {
+  EXPECT_EQ("0", itostr(0));
+  EXPECT_EQ("1", itostr(1));
+  EXPECT_EQ("-1", itostr(-1));
+  EXPECT_EQ("-9223372036854775808", itostr(MinInt64));
+  EXPECT_EQ("9223372036854775807", itostr(MaxInt64));
+}