Fast path for {U}Int128.operator % when the modulus is within UInt64 #106915

public static UInt64 ModMul_1(UInt64 first, UInt64 second, UInt64 modulus)
{
    return unchecked((UInt64)((Math.BigMul(first, second) % modulus));
}

public static UInt64 ModMul_2(UInt64 first, UInt64 second, UInt64 modulus)
{
    return unchecked((UInt64)(((UInt128)first * second) % modulus));
}

public static UInt128 operator %(UInt128 left, UInt128 right)
{
    if (X86Base.X64.IsSupported)
    {
        if (right._upper == 0)
        {
            return X86Base.X64.DivRem(left._lower, left._upper, right._lower).Remainder;
        }
    }
        
    return left % right;
}

public class ModMulModule
{
    [Benchmark]
    [Arguments(0x123456789ABCDEF0, 0x123456789ABCDEF0, 0x123456789ABCDEF0)]
    public UInt64 ModMul_Slow(UInt64 first, UInt64 second, UInt64 modulus)
    {
        return unchecked((UInt64)(((UInt128)first * second) % modulus));
    }

    [Benchmark]
    [Arguments(0x123456789ABCDEF0, 0x123456789ABCDEF0, 0x123456789ABCDEF0)]
    public UInt64 ModMul_Fast(UInt64 first, UInt64 second, UInt64 modulus)
    {
        return unchecked((UInt64)Mod((UInt128)first * second, modulus));
    }

    public static UInt128 Mod(UInt128 left, UInt128 right)
    {
        if (GetUpper(in right) == 0)
        {
            if (X86Base.X64.IsSupported)
            {
                return X86Base.X64.DivRem(GetLower(in left), GetUpper(in left), GetLower(in right)).Remainder;
            }
        }

        return left % right;
    }

    [UnsafeAccessor(UnsafeAccessorKind.Field, Name = "_upper")]
    static extern ref UInt64 GetUpper(ref readonly UInt128 value);

    [UnsafeAccessor(UnsafeAccessorKind.Field, Name = "_lower")]
    static extern ref UInt64 GetLower(ref readonly UInt128 value);
}