diff --git a/llvm/include/llvm/ADT/APInt.h b/llvm/include/llvm/ADT/APInt.h
--- a/llvm/include/llvm/ADT/APInt.h
+++ b/llvm/include/llvm/ADT/APInt.h
@@ -1635,9 +1635,10 @@
   void print(raw_ostream &OS, bool isSigned) const;
 
   /// Converts an APInt to a string and append it to Str.  Str is commonly a
-  /// SmallString.
+  /// SmallString. If Radix > 10, UpperCase determine the case of letter
+  /// digits.
   void toString(SmallVectorImpl<char> &Str, unsigned Radix, bool Signed,
-                bool formatAsCLiteral = false) const;
+                bool formatAsCLiteral = false, bool UpperCase = true) const;
 
   /// Considers the APInt to be unsigned and converts it into a string in the
   /// radix given. The radix can be 2, 8, 10 16, or 36.
diff --git a/llvm/lib/Support/APInt.cpp b/llvm/lib/Support/APInt.cpp
--- a/llvm/lib/Support/APInt.cpp
+++ b/llvm/lib/Support/APInt.cpp
@@ -2136,8 +2136,8 @@
     this->negate();
 }
 
-void APInt::toString(SmallVectorImpl<char> &Str, unsigned Radix,
-                     bool Signed, bool formatAsCLiteral) const {
+void APInt::toString(SmallVectorImpl<char> &Str, unsigned Radix, bool Signed,
+                     bool formatAsCLiteral, bool UpperCase) const {
   assert((Radix == 10 || Radix == 8 || Radix == 16 || Radix == 2 ||
           Radix == 36) &&
          "Radix should be 2, 8, 10, 16, or 36!");
@@ -2173,7 +2173,9 @@
     return;
   }
 
-  static const char Digits[] = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ";
+  static const char LowerCaseDigits[] = "0123456789abcdefghijklmnopqrstuvwxyz";
+  static const char UpperCaseDigits[] = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ";
+  StringRef Digits = UpperCase ? UpperCaseDigits : LowerCaseDigits;
 
   if (isSingleWord()) {
     char Buffer[65];
diff --git a/llvm/unittests/ADT/APIntTest.cpp b/llvm/unittests/ADT/APIntTest.cpp
--- a/llvm/unittests/ADT/APIntTest.cpp
+++ b/llvm/unittests/ADT/APIntTest.cpp
@@ -1388,6 +1388,9 @@
   APInt(8, 255, isSigned).toString(S, 10, isSigned, true);
   EXPECT_EQ(std::string(S), "255");
   S.clear();
+  APInt(8, 255, isSigned).toString(S, 16, isSigned, true, /*UpperCase=*/false);
+  EXPECT_EQ(std::string(S), "0xff");
+  S.clear();
   APInt(8, 255, isSigned).toString(S, 16, isSigned, true);
   EXPECT_EQ(std::string(S), "0xFF");
   S.clear();