[libc++] std::cmp_less and other integer comparison functions could be improved

Author: kisuhorikka

libcxx/include/__utility/cmp.h

@@ -28,7 +28,13 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 
 template <__signed_or_unsigned_integer _Tp, __signed_or_unsigned_integer _Up>
 _LIBCPP_HIDE_FROM_ABI constexpr bool cmp_equal(_Tp __t, _Up __u) noexcept {
-  if constexpr (is_signed_v<_Tp> == is_signed_v<_Up>)
+  if constexpr (sizeof(_Tp) < sizeof(int) && sizeof(_Up) < sizeof(int)) {
+    __builtin_assume(__t < numeric_limits<int>::max() && __u < numeric_limits<int>::max());
+    return static_cast<int>(__t) == static_cast<int>(__u);
+  } else if constexpr (sizeof(_Tp) < sizeof(long long) && sizeof(_Up) < sizeof(long long)) {
+    __builtin_assume(__t < numeric_limits<long long>::max() && __u < numeric_limits<long long>::max());
+    return static_cast<long long>(__t) == static_cast<long long>(__u);
+  } else if constexpr (is_signed_v<_Tp> == is_signed_v<_Up>)
     return __t == __u;
   else if constexpr (is_signed_v<_Tp>)
     return __t < 0 ? false : make_unsigned_t<_Tp>(__t) == __u;
@@ -43,7 +49,13 @@ _LIBCPP_HIDE_FROM_ABI constexpr bool cmp_not_equal(_Tp __t, _Up __u) noexcept {
 
 template <__signed_or_unsigned_integer _Tp, __signed_or_unsigned_integer _Up>
 _LIBCPP_HIDE_FROM_ABI constexpr bool cmp_less(_Tp __t, _Up __u) noexcept {
-  if constexpr (is_signed_v<_Tp> == is_signed_v<_Up>)
+  if constexpr (sizeof(_Tp) < sizeof(int) && sizeof(_Up) < sizeof(int)) {
+    __builtin_assume(__t < numeric_limits<int>::max() && __u < numeric_limits<int>::max());
+    return static_cast<int>(__t) < static_cast<int>(__u);
+  } else if constexpr (sizeof(_Tp) < sizeof(long long) && sizeof(_Up) < sizeof(long long)) {
+    __builtin_assume(__t < numeric_limits<long long>::max() && __u < numeric_limits<long long>::max());
+    return static_cast<long long>(__t) < static_cast<long long>(__u);
+  } else if constexpr (is_signed_v<_Tp> == is_signed_v<_Up>)
     return __t < __u;
   else if constexpr (is_signed_v<_Tp>)
     return __t < 0 ? true : make_unsigned_t<_Tp>(__t) < __u;