Re: [boost] [fusion] improving compile times

Subject: Re: [boost] [fusion] improving compile times
From: Eric Niebler (eric_at_[hidden])
Date: 2009-06-02 01:58:02

Larry Evans wrote:
> On 06/01/09 20:26, Eric Niebler wrote:
>> I'm attaching a simple patch to vector_n_chooser.hpp that replaces 
>> some template metaprogramming with preprocessor metaprogramming in the 
>> interest of improving compile times. I found this hotspot through 
>> profiling, 
> [snip]
> Eric,
> 
> Could you post your benchmark code that showed the improvement in
> compile speed?  I'd like to eventually try it with a variadic template
> compiler version of fusion vector.

I confess that I'm not actually benchmarking compile speed; rather, I'm 
benchmarking the number of template instantiations as reported by 
Steven's template profiler. I'm profiling TMP-heavy code like some of 
Proto's and xpressive's tests and cherry-picking the worst offenders. 
The Fusion vector_n_chooser patch knocked off 100's of template 
instantiations, for instance.

> Somewhat off topic:
> 
> What I'd really like to see is someone explain how metaprogramming
> improves compile speed. Steven said earlier that the slowdown
> depends on the template:
> 
> http://article.gmane.org/gmane.comp.lib.boost.devel/186051

Interesting.

> I would have guessed that the more template instances that,
> are created, then the slower the compile time.  However,
> Steven's remark made me wonder.  What I'm guessing
> is that if the template metaprogram produces a lot of
> intermediate results, then it might be better to
> use preprocessor metaprogramming to just produce the
> final result.  Is that about right or is it more
> complicated?

My experience matches yours: more instantiations --> longer compiles. I 
wonder what Steven's experience is. Steven?

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