Re: [boost] [proto] Looong compile times and other issues

Subject: Re: [boost] [proto] Looong compile times and other issues
From: Eric Niebler (eric_at_[hidden])
Date: 2011-09-11 23:42:54

On 9/11/2011 9:20 PM, Joel de Guzman wrote:
> On 9/12/2011 1:44 AM, Joel Falcou wrote:
>>
>> - fusion is also a big hitter : lots of PP and lots of forced instanciation instead of
>> lazy specialization.
> 
> PP: right, this can be fixed.
> 
> lots of forced instanciation: I don't know what you mean.
> Can you please be more specific?

I can't speak for Joel F. here, but consider the templates instantiated
simply to access the Nth element of a fusion vector. From a cursory
inspection of sequence/intrinsic/at.hpp, the following call:

  at_c<N>(v);

where v is a fusion vector instantiates:

  lazy_disable_if
  is_const
  result_of::at_c
  result_of::at
  mpl::int_
  detail::tag_of
  extension::at_impl
  extension::at_impl::apply
  mpl::at
  detail::ref_result
  add_reference

I believe that it also must compute the return type of the const
overload of at_c in order to do overload resolution, so that many of the
above templates must be instantiated twice: once for a const vector and
once for non-const, and throw in an additional add_const. (And come to
think of it, Proto probably suffers from this problem too!)

That's a lot of templates for a simple element access. I didn't chase
the template breadcrumbs into mpl, so there may be more.

In Proto, I roll my own poor-man's heterogeneous sequences so I can save
template instantiations. I even use my own typelists over mpl::vector
because it had a measurable impact. And yes, I measured.

I'll say this: Fusion's implementation is BEAUTIFUL compared to Proto's,
which is an ugly, hard-to-maintain mess of PP gunk. Proto also exposes
fewer customization points. These were tradeoffs I made in the interest
of compile times, and it's still not enough.

A complete rewrite using decltype, rvalue refs and variadic templates
would GREATLY improve things. That's probably true for both Fusion and
Proto.

-- 
Eric Niebler
BoostPro Computing
http://www.boostpro.com