[flang] [llvm] [Flang] Introduce *Value classes with unittests (PR #216958)

[Michael Kruse via llvm-commits]

================
@@ -305,4 +305,69 @@ template <int BITS>
 using HostSignedIntType = typename HostSignedIntTypeHelper<BITS>::type;
 
 } // namespace Fortran::common
+
+namespace std {
+// Specializing std::numeric_limits is an intended extension point for
+// user-defined type.
+template <> class numeric_limits<Fortran::common::UnsignedInt128> {
+public:
+  using T = Fortran::common::UnsignedInt128;
+
+  static constexpr bool is_specialized{true};
+  static constexpr bool is_signed{false};
+  static constexpr bool is_integer{true};
+
+  static constexpr T min() { return T{0, 0}; }
+  static constexpr T max() { return T{UINT64_MAX, UINT64_MAX}; }
+};
+
+template <> class numeric_limits<Fortran::common::SignedInt128> {
+public:
+  using T = Fortran::common::SignedInt128;
+
+  static constexpr bool is_specialized{true};
+  static constexpr bool is_signed{true};
+  static constexpr bool is_integer{true};
+
+  static constexpr T min() {
+    return T{static_cast<std::uint64_t>(INT64_MIN), 0};
+  }
+  static constexpr T max() {
+    return T{static_cast<std::uint64_t>(INT64_MAX), UINT64_MAX};
+  }
+};
+
+#if defined(__SIZEOF_INT128__) && defined(_MSVC_STL_VERSION)
+// clang-cl knows __int128 and will be used for (u)int128_t, but the MSVC STL
+// does not define stl::numeric_limits for it.
+
+template <> class numeric_limits<unsigned __int128> {
+public:
+  using T = unsigned __int128;
+
+  static constexpr bool is_specialized{true};
+  static constexpr bool is_signed{false};
+  static constexpr bool is_integer{true};
+
+  static constexpr T min() { return static_cast<T>(0); }
+  static constexpr T max() { return ~static_cast<T>(0); }
+};
+
+template <> class numeric_limits<__int128> {
+public:
+  using T = __int128;
+
+  static constexpr bool is_specialized{true};
+  static constexpr bool is_signed{true};
+  static constexpr bool is_integer{true};
+
+  static constexpr T min() {
+    return static_cast<T>(static_cast<unsigned __int128>(1) << 127u);
+  }
+  static constexpr T max() {
+    return static_cast<T>(~(static_cast<unsigned __int128>(1) << 127u));
+  }
+};
+#endif
----------------
Meinersbur wrote:

This means changing the `std::numeric_limits` to `Flang::common::numeric_limits` in several places. I considered specifying the  `_MSVC_STL_VERSION` version that eventually defines in the `#if` once there is an MSVC STL that defines it the lesser evil.

I will make that change.

https://github.com/llvm/llvm-project/pull/216958