Re: [boost] [xpressive] Performance Tuning?

1 Jul 2009

      Eric Niebler wrote:
...
Stewart, Robert wrote:
...
I was experimenting with Xpressive to see how it would
compare with some custom, string-based numeric input parsing.
The Xpressive code is over 175X slower than the custom code.
Have I done anything to make it really slow?  Can it be made
faster?
<snip>
Where is the code against which you are benchmarking?
See below.  As you'll see, it isn't nearly as maintainable or clear, but the wide performance difference certainly favors it.
...
From looking at the code, I can see a few areas for
improvement. With each call to parse(), you construct and
discard a match_results object. For optimal performance, you
should reuse the match_results object to avoid extra memory
allocation. Also, many of the quantifiers in your grammar do
not require backtracking, so you could use keep() to turn
backtracking off selectively.
The function is called in a one-off fashion that wouldn't permit keeping the match_results object, unfortunately.  (A function local static would suffice but then the function wouldn't be thread safe.)

I'm not certain I understand where to apply keep(), but I'll have a go at it.

_______________________
FYI: bar::to_number() calls strtod(), strtol(), etc.

template <class T>
T
foo::extract(char const * & _input, char const * const _description,
   std::string const & _value)
{
   T result;
   if (!bar::to_number(result, _input, &_input, 10))
   {
      raise_extract_failed(_description, _value);
   }
   return result;
}

inline int64_t
foo::round(const double _value)
{
   return static_cast<int64_t>(std::floor(_value + 0.5));
}

int64_t
foo::parse(std::string const & _value)
{
   if (_value.empty())
   {
      return 0;
   }
   size_t offset(_value.find_first_not_of("-+0123456789eE ./"));
   const bool invalid(std::string::npos != offset);
   if (invalid)
   {
      raise_parsing_failed(_value);
   }
   const char * const input(_value.c_str());
   const char * const last(input + _value.length());
   const char * end(input);
   int64_t result;
   (void)bar::to_number(result, end, &end, 10);
   const bool floating_point('.' == *end);
   if (floating_point)
   {
      end = input;
      const double value(extract<double>(end, "input", _value));
      const bool negative(0.0 > value);
      double multiple(value * 160000.0);
      if (negative)
      {
         multiple = -multiple;
      }
      result = round(multiple);
      if (negative)
      {
         result = -result;
      }
   }
   else // not floating point
   {
      result *= 160000;
      if (end != last)
      {
         offset = _value.find_first_of('/', end - input);
         const bool is_fraction(std::string::npos != offset);
         if (is_fraction)
         {
            const bool negative(0 > result);
            if (negative)
            {
               result = -result;
            }
            offset = _value.find_last_of(' ', offset);
            const bool is_mixed_number(std::string::npos != offset);
            if (is_mixed_number)
            {
               const unsigned numerator(
                  extract<unsigned>(end, "numerator", _value));
               end += 1; // skip the slash
               const unsigned denominator(
                  extract<unsigned>(end, "denominator", _value));
               const double fraction((160000.0 * numerator) / denominator);
               result = round(result + fraction);
            }
            else
            {
               end += 1; // skip the slash
               const unsigned denominator(
                  extract<unsigned>(end, "denominator", _value));
               const double fraction(result / denominator);
               result = round(fraction);
            }
            if (negative)
            {
               result = -result;
            }
         }
      }
   }
   return result;
}

_____
Rob Stewart                           robert.stewart@sig.com
Software Engineer, Core Software      using std::disclaimer;
Susquehanna International Group, LLP  http://www.sig.com

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