[ADT] Add an in-place version of toHex()

[hans]

Summary

and use that to simplify MD5's hex string code which was previously using a string stream, as well as Clang's CGDebugInfo::computeChecksum().

NFC.

[hans]

Trying to upload the right diff this time.

[hans]

(avoid toStringRef -> arrayRefFromStringRef by reshuffling the toHex overloads)

[dexonsmith]

Mostly LGTM, just a couple of comments on the drive-by changes to hexdigit() (I might split that (and the new call in toHex()) into a separate commit, but if you keep them here please mention them in the commit message).

llvm/include/llvm/ADT/StringExtras.h

38–41

Should this be X < 16?

39

Can you use constexpr StringLiteral here?

[hans]

In D116960#3232461, @dexonsmith wrote:

Mostly LGTM, just a couple of comments on the drive-by changes to hexdigit() (I might split that (and the new call in toHex()) into a separate commit, but if you keep them here please mention them in the commit message).

Thanks! I'll commit the hexdigit() change separately.

llvm/include/llvm/ADT/StringExtras.h

38–41

D'oh, yes of course. Thanks for spotting that!

39

Yes that would work, but I think it's simpler to just use the built-in types here.
Actually, there's no need for this to be a pointer, it should just be an array. I'll fix that.

llvm/include/llvm/ADT/StringExtras.h

 namespace llvm {
 
 template<typename T> class SmallVectorImpl;
 class raw_ostream;
 
 /// hexdigit - Return the hexadecimal character for the
 /// given number \p X (which should be less than 16).
 inline char hexdigit(unsigned X, bool LowerCase = false) {
-  const char HexChar = LowerCase ? 'a' : 'A';
-  return X < 10 ? '0' + X : HexChar + X - 10;
+  assert(X < 16);
+  static const char LUT[] = "0123456789ABCDEF";
+  const uint8_t Offset = LowerCase ? 32 : 0;
+  return LUT[X] | Offset;
 }
 
 /// Given an array of c-style strings terminated by a null pointer, construct
 /// a vector of StringRefs representing the same strings without the terminating
 /// null string.
 inline std::vector<StringRef> toStringRefArray(const char *const *Strings) {
   std::vector<StringRef> Result;
   while (*Strings)
inline std::string utohexstr(uint64_t X, bool LowerCase = false) {
   }
 
   return std::string(BufPtr, std::end(Buffer));
 }
 
 /// Convert buffer \p Input to its hexadecimal representation.
 /// The returned string is double the size of \p Input.
 inline std::string toHex(StringRef Input, bool LowerCase = false) {
-  static const char *const LUT = "0123456789ABCDEF";
-  const uint8_t Offset = LowerCase ? 32 : 0;
   size_t Length = Input.size();
 
   std::string Output;
   Output.reserve(2 * Length);
   for (size_t i = 0; i < Length; ++i) {
     const unsigned char c = Input[i];
-    Output.push_back(LUT[c >> 4] | Offset);
-    Output.push_back(LUT[c & 15] | Offset);
+    Output.push_back(hexdigit(c >> 4, LowerCase));
+    Output.push_back(hexdigit(c & 15, LowerCase));
   }
   return Output;
 }
 
 inline std::string toHex(ArrayRef<uint8_t> Input, bool LowerCase = false) {
   return toHex(toStringRef(Input), LowerCase);
 }