Extract endianness checks into Portability.h

Summary: [Folly] Extract endianness checks into Portability.h.

Reviewed By: fugalh

Differential Revision: D2857924

fb-gh-sync-id: 23ecd2a3cad661024acb62769cd85df394786c59

folly/FBString.h

@@ -289,13 +289,6 @@ private:
  * to extract capacity/category.
  */
 template <class Char> class fbstring_core {
-protected:
-  static constexpr bool kIsLittleEndian =
-    __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__;
-  static constexpr bool kIsBigEndian =
-    __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__;
-  static_assert(
-      kIsLittleEndian || kIsBigEndian, "unable to identify endianness");
 public:
   fbstring_core() noexcept { reset(); }
 

folly/Portability.h

@@ -305,6 +305,21 @@ typedef SSIZE_T ssize_t;
 
 #endif
 
+// Endianness
+namespace folly {
+#ifdef _MSC_VER
+// It's MSVC, so we just have to guess ... and allow an override
+#ifdef FOLLY_ENDIAN_BE
+constexpr auto kIsLittleEndian = false;
+#else
+constexpr auto kIsLittleEndian = true;
+#endif
+#else
+constexpr auto kIsLittleEndian = __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__;
+#endif
+constexpr auto kIsBigEndian = !kIsLittleEndian;
+}
+
 #ifndef FOLLY_SSE
 # if defined(__SSE4_2__)
 #  define FOLLY_SSE 4