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Subject: Re: [boost] [xint] Third release is ready, requesting preliminary review
From: Stewart, Robert (Robert.Stewart_at_[hidden])
Date: 2010-05-05 11:01:18

Chad Nelson wrote:
> On 05/05/2010 07:56 AM, Stewart, Robert wrote:
> > However, there is an inherent
> > inefficiency here, though it may be miniscule relative to the
> > comparison code. A compare() function must return one of three
> > values: less than zero, zero, or greater than zero. That implies
> > additional logic that isn't necessary when computing <, ==, etc.
> > (Splitting those apart means duplicating portions of the logic in
> > compare(), of course.)
> It's quite minuscule. I'm no CPU hardware expert, but my
> educated guess
> would be two additional clock cycles per comparison at most. I'll take
> that cost over the cost of duplicating the logic in multiple
> functions, especially as it's not trivial.

I wasn't referring to the comparisons in the operator functions, but rather to the logic in compare() that must produce one of three results; I can't tell if we're talking about the same thing. When implementing the operators directly, there are shortcuts in the logic you can't take in compare():

// untested, written in e-mail editor
operator ==(base_integer const & _rhs) const
   if (_is_zero() != _rhs._is_zero()
      || _get_negative() != _rhs._get_negative())
      return false;
   size_t const length(_get_length());
   if (length != _rhs._get_length())
      return false;
   detail::digit_t * start(_get_digits());
   detail::digit_t * it(start + length);
   detail::digit_t * rhs_it(_rhs._get_digits() + length);
   while (it > start)
      if (*(--it) != *(--rhs_it))
         return false;
   return true;

That function is simpler than compare() plus a check for zero, so it could prove faster due to fewer branches and comparisons. Is it measurably faster in any meaningful use case? Perhaps.

You can reuse some of the algorithm by supplying a predicate and default result so multiple operators -- and compare() -- could use the same comparison loop. (The early returns probably can't be reused appropriately.)

Rob Stewart robert.stewart_at_[hidden]
Software Engineer, Core Software using std::disclaimer;
Susquehanna International Group, LLP

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