Re: [boost] [xpressive] Performance Tuning?

Subject: Re: [boost] [xpressive] Performance Tuning?
From: Stewart, Robert (Robert.Stewart_at_[hidden])
Date: 2009-07-28 09:33:46

OvermindDL1 wrote:
> On Tue, Jul 28, 2009 at 6:44 AM, Stewart,
> Robert<Robert.Stewart_at_[hidden]> wrote:
> >> P.S.  I would be quite happy if anyone could get rid of that
> >> freakishly long double->int64_t cast warning in the
> >> xpressive code, I like clean builds.  :)
> >
> > I don't get the warning, so you'll have to be more specific.
>
> Here are the warnings I get, there are two, the first is
> plain and basic:
> 1>r:\programming_projects\spirit_price\price_parsing\main.cpp(789) :
> warning C4244: '=' : conversion from 'const boost::int64_t' to
> 'double', possible loss of data
>
> Although this is with the above ejg file set I posted so the line
> number might be wrong, the line of code is:        value = _result /
> denominator;

Hmmm, the only warnings I ever got were from double to int64_t.  There is, of course, a precision issue between the two types, but they don't apply to this context, so simply change the line to:

   value = static_cast<double>(_result / denominator);

> The other warning is apparently a single xpressive warning about a
> conversion from int64_t to double.  The line of code in main.hpp it
> references is:
>   const sregex price =
>      *blank
>      >> !sign
>      >> (real | mixed_number | fraction | integer)
>      >> *space;
>
> And the long error is (this outdoes even the sizable spirit error
> message ;-)  How I wish we were getting concepts in C++1x, I still do
> not understand why they took it out):

There was still a lot of experimentation, discussion, and twiddling going on.  Read http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2009/n2906.pdf.  Better to get everything else out the door sooner than later.

> Hmm, apparently too big to post on this list (over 800kb, yes, for one
> error message from xpressive).  I put it in a text file, zipped it up,
> and attached it, now 6kb.

I'm not certain what is causing that warning.  I have three suggestions:

1. In xpressive_parsing::to_price_impl::operator ()(Value const &, Value const &), change "lcm.as<double>() * numerator / denominator" to "lcm.as<double>() * static_cast<double>(numerator) / static_cast<double>(denominator)" (Perhaps only one cast is actually needed)

2. In xpressive_parsing::to_price_impl::operator ()(Value const &, double), change "lcm.as<double>() * whole + _fraction" to "lcm.as<double>() * static_cast<double>(whole) + static_cast<double>(_fraction)" (Perhaps only one cast is actually needed)

3. Simplify the regex to isolate the source.  Remove ">> (real | mixed_number | fraction | integer)" first.  If that eliminates the warning, then put ">> real" back in.  If still no warning, change it to ">> (real | mixed_number)," etc.

_____
Rob Stewart                           robert.stewart_at_[hidden]
Software Engineer, Core Software      using std::disclaimer;
Susquehanna International Group, LLP  http://www.sig.com

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