Re: [boost] [fusion] improving compile times

Subject: Re: [boost] [fusion] improving compile times
From: Eric Niebler (eric_at_[hidden])
Date: 2009-06-03 16:36:33

David Abrahams wrote:
> on Tue Jun 02 2009, Eric Niebler <eric-AT-boostpro.com> wrote:
>> I read form the Wikipedia entry that Clang's C++ support is 2-3 years
>> from being usable, though.
> 
> I wouldn't bet against Doug Gregor when he's firing on all cylinders :-)

I didn't know Doug was involved. That changes everything! :-)

>> From the user time, my recent changes make this test compile twice as fast for gcc-3.4
>> (cygwin). For MSVC, the wins are less dramatic.
>>
>> Your point is taken, though ... instantiation count is merely a rule
>> of thumb and the real measure is clock time. It is, in my experience
>> and with compilers actually in use today, a very good rule of thumb,
>> though.
> 
> Well, it's great to get the instantiation count down, but consider that
> what you're replacing it with may not be any faster :-)

See the comment above about a measured 2x speed-up.

> If you *are* getting a win from PP metaprogramming, there's a good
> chance that you could improve the speed a lot more, e.g. by using the
> "z" parameter as described in
> http://www.boostpro.com/tmpbook/preprocessor.html#horizontal-repetition

Yep, I know about that, but generally avoid nested horizontal 
repetition. Nevertheless, I appreciate the suggestions.

For anybody concerned about the nature of my changes, here is an example 
of the fat in Fusion I'm trimming:

https://svn.boost.org/trac/boost/changeset/53566/trunk/boost/fusion/container/vector/detail/vector_n_chooser.hpp

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