Improve performance of Enum's generic IsDefined / GetName / GetNames (#44355)

Eliminates the boxing in IsDefined/GetName/GetValues, and in GetNames avoids having to go through RuntimeType's GetEnumNames override.

Author: stephentoub

src/libraries/System.Private.CoreLib/src/System/Enum.cs

@@ -258,11 +258,26 @@ internal static ulong ToUInt64(object value)
  return result;
  }
 
+ private static ulong ToUInt64<TEnum>(TEnum value) where TEnum : struct, Enum =>
+ Type.GetTypeCode(typeof(TEnum)) switch
+ {
+ TypeCode.SByte => (ulong)Unsafe.As<TEnum, sbyte>(ref value),
+ TypeCode.Byte => Unsafe.As<TEnum, byte>(ref value),
+ TypeCode.Boolean => Convert.ToByte(Unsafe.As<TEnum, bool>(ref value)),
+ TypeCode.Int16 => (ulong)Unsafe.As<TEnum, short>(ref value),
+ TypeCode.UInt16 => Unsafe.As<TEnum, ushort>(ref value),
+ TypeCode.Char => Unsafe.As<TEnum, char>(ref value),
+ TypeCode.UInt32 => Unsafe.As<TEnum, uint>(ref value),
+ TypeCode.Int32 => (ulong)Unsafe.As<TEnum, int>(ref value),
+ TypeCode.UInt64 => Unsafe.As<TEnum, ulong>(ref value),
+ TypeCode.Int64 => (ulong)Unsafe.As<TEnum, long>(ref value),
+ _ => throw new InvalidOperationException(SR.InvalidOperation_UnknownEnumType),
+ };
  #endregion
 
  #region Public Static Methods
  public static string? GetName<TEnum>(TEnum value) where TEnum : struct, Enum
- => GetName(typeof(TEnum), value);
+ => GetEnumName((RuntimeType)typeof(TEnum), ToUInt64(value));
 
  public static string? GetName(Type enumType, object value)
  {
@@ -273,7 +288,7 @@ internal static ulong ToUInt64(object value)
  }
 
  public static string[] GetNames<TEnum>() where TEnum : struct, Enum
- => GetNames(typeof(TEnum));
+ => new ReadOnlySpan<string>(InternalGetNames((RuntimeType)typeof(TEnum))).ToArray();
 
  public static string[] GetNames(Type enumType)
  {