Re: [boost] [fusion] improving compile times

Subject: Re: [boost] [fusion] improving compile times
From: Steven Watanabe (watanabesj_at_[hidden])
Date: 2009-06-02 10:06:09

AMDG

Eric Niebler wrote:
> 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?

For macro optimizations that substantially reduce the number
of template instantiations, that's a pretty safe assumption.  I
definitely don't trust it for micro optimizations, though.
Also, be wary of the total instantiation count produced by my
tool.  It's easy to change it by changing the #includes for instance.

In Christ,
Steven Watanabe