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

Author: kisuhorikka

libcxx/include/__utility/cmp.h

@@ -9,6 +9,7 @@
 #ifndef _LIBCPP___UTILITY_CMP_H
 #define _LIBCPP___UTILITY_CMP_H
 
+#include "__assert"
 #include <__config>
 #include <__type_traits/integer_traits.h>
 #include <__type_traits/is_signed.h>
@@ -26,16 +27,18 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 
 #if _LIBCPP_STD_VER >= 20
 
+template <__signed_or_unsigned_integer _Tp, __signed_integer _Ip>
+inline constexpr bool __comparison_can_promote_to =
+    __signed_integer<_Tp> ? sizeof(_Tp) <= sizeof(_Ip) : sizeof(_Tp) < sizeof(_Ip);
+
 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 (sizeof(_Tp) < sizeof(int) && sizeof(_Up) < sizeof(int)) {
-    __builtin_assume(__t < numeric_limits<int>::max() && __u < numeric_limits<int>::max());
+  if constexpr (is_signed_v<_Tp> == is_signed_v<_Up>)
+    return __t == __u;
+  else if constexpr (__comparison_can_promote_to<_Tp, int> && __comparison_can_promote_to<_Up, int>)
     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());
+  else if constexpr (__comparison_can_promote_to<_Tp, long long> && __comparison_can_promote_to<_Up, long long>)
     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;
   else
@@ -49,14 +52,12 @@ _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 (sizeof(_Tp) < sizeof(int) && sizeof(_Up) < sizeof(int)) {
-    __builtin_assume(__t < numeric_limits<int>::max() && __u < numeric_limits<int>::max());
+  if constexpr (is_signed_v<_Tp> == is_signed_v<_Up>)
+    return __t < __u;
+  else if constexpr (__comparison_can_promote_to<_Tp, int> && __comparison_can_promote_to<_Up, int>)
     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());
+  else if constexpr (__comparison_can_promote_to<_Tp, long long> && __comparison_can_promote_to<_Up, long long>)
     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;
   else