Re: Reminder: Review for Zach's parsing library starts on Feb 19th

From: Peter Dimov (pdimov_at_[hidden])
Date: 2024-03-01 03:21:40

> Zach Laine wrote:
> > I'm shocked too.  That's really crazy.  Believe me, I'm not interested
> > in merging something with orders of magnitude of perf overhead into
> > Boost.  I will be getting to the bottom of this long before a merge could take
> place.
> 
> My profiler (VS2022) says that the top performance problem is the
> construction of a stringstream here
> 
> https://github.com/tzlaine/parser/blob/f99ae3b94ad0acef0cc92166d5108aa
> de41da4ea/include/boost/parser/detail/printing.hpp#L624
> 
> This constructs a std::locale, which is apparently very slow.
> 
> When I fix it
> 
> diff --git a/include/boost/parser/detail/printing.hpp
> b/include/boost/parser/detail/printing.hpp
> index 1e204796..6cbec059 100644
> --- a/include/boost/parser/detail/printing.hpp
> +++ b/include/boost/parser/detail/printing.hpp
> @@ -621,11 +621,19 @@ namespace boost { namespace parser { namespace
> detail {
>          flags f,
>          Attribute const & attr)
>      {
> -        std::stringstream oss;
>          if (detail::do_trace(f))
> +        {
> +            std::stringstream oss;
>              detail::print_parser(context, parser, oss);
> -        return scoped_trace_t<Iter, Sentinel, Context, Attribute>(
> -            first, last, context, f, attr, oss.str());
> +
> +            return scoped_trace_t<Iter, Sentinel, Context, Attribute>(
> +                first, last, context, f, attr, oss.str());
> +        }
> +        else
> +        {
> +            return scoped_trace_t<Iter, Sentinel, Context, Attribute>(
> +                first, last, context, f, attr, {});
> +        }
>      }
> 
>      template<typename Context, typename Attribute>

This change takes me from ~6130ms to ~520ms.

> the top function becomes the constructor of scoped_trace_t. (It's probably
> not getting inlined.)
> 
> Looks like the tracing functionality costs a lot even when off.

Commenting out the bodies of scoped_trace_t and ~scoped_trace_t takes
me to ~360ms.

The top two functions are now `skip`

https://github.com/tzlaine/parser/blob/f99ae3b94ad0acef0cc92166d5108aade41da4ea/include/boost/parser/parser.hpp#L1420

and `omit_parser::call`

https://github.com/tzlaine/parser/blob/f99ae3b94ad0acef0cc92166d5108aade41da4ea/include/boost/parser/parser.hpp#L4139

both with 5.5%.

The `SkipParser` of `skip` is

` boost::parser::rule<json::ws,boost::parser::detail::nope,boost::parser::detail::nope,boost::parser::detail::nope>`.