[Support/Endian] Add support for endian-specific enums

[labath]

Summary

Binary formats often include various enumerations or bitsets, but using
endian-specific types for accessing them is tricky because they
currently only support integral types. This is particularly true for
scoped enums (enum class), as these are not implicitly convertible to
integral types, and so one has to perform two casts just to read the
enum value.

This fixes that support by adding first-class support for enumeration
types to endian-specific types. The support for them was already almost
working -- all I needed to do was overload getSwappedBytes for
enumeration types (which casts the enum to its underlying type and performs the
conversion there). I also add some convenience template aliases to simplify
declaring endian-specific enums.

[Bigcheese]

I agree with Zach, this should be little_t and big_t. You should also add aligned_big_t and aligned_little_t.

[labath]

Rename the new types to big_t/little_t as they're not really enum-specific.

[labath]

Add aligned_{big/little}_t as well.

[Bigcheese]

LGTM

[labath]

Thank you for the quick review.

include/llvm/Support/Endian.h

 
 using unaligned_int16_t =
     detail::packed_endian_specific_integral<int16_t, native, unaligned>;
 using unaligned_int32_t =
     detail::packed_endian_specific_integral<int32_t, native, unaligned>;
 using unaligned_int64_t =
     detail::packed_endian_specific_integral<int64_t, native, unaligned>;
 
+template <typename T>
+using little_t =
+    detail::packed_endian_specific_integral<T, little, unaligned>;
+template <typename T>
+using big_t = detail::packed_endian_specific_integral<T, big, unaligned>;
+
 namespace endian {
 
 template <typename T> inline T read(const void *P, endianness E) {
   return read<T, unaligned>(P, E);
 }
 
 template <typename T, endianness E> inline T read(const void *P) {
   return *(const detail::packed_endian_specific_integral<T, E, unaligned> *)P;

include/llvm/Support/SwapByteOrder.h

union {
     uint64_t i;
     double d;
   } in, out;
   in.d = C;
   out.i = SwapByteOrder_64(in.i);
   return out.d;
 }
 
-template<typename T>
+template <typename T>
+inline typename std::enable_if<std::is_enum<T>::value, T>::type
+getSwappedBytes(T C) {
+  return static_cast<T>(
+      getSwappedBytes(static_cast<typename std::underlying_type<T>::type>(C)));
+}
+
+template<typename T>
 inline void swapByteOrder(T &Value) {
   Value = getSwappedBytes(Value);
 }
 
 } // end namespace sys
 } // end namespace llvm
 
 #endif

unittests/Support/EndianTest.cpp

TEST(Endian, PackedEndianSpecificIntegral) {
   big32_t    *big_val    =
     reinterpret_cast<big32_t *>(big + 1);
   little32_t *little_val =
     reinterpret_cast<little32_t *>(little + 1);
 
   EXPECT_EQ(*big_val, *little_val);
 }
 
+TEST(Endian, PacketEndianSpecificIntegralAsEnum) {
+  enum class Test : uint16_t { ONETWO = 0x0102, TWOONE = 0x0201 };
+  unsigned char bytes[] = {0x01, 0x02};
+  using LittleTest = little_t<Test>;
+  using BigTest = big_t<Test>;
+  EXPECT_EQ(Test::TWOONE, *reinterpret_cast<LittleTest *>(bytes));
+  EXPECT_EQ(Test::ONETWO, *reinterpret_cast<BigTest *>(bytes));
+}
+
 } // end anon namespace