Re: [boost] [fusion] improving compile times

Subject: Re: [boost] [fusion] improving compile times
From: David Abrahams (dave_at_[hidden])
Date: 2009-06-03 12:28:16

on Tue Jun 02 2009, Eric Niebler <eric-AT-boostpro.com> wrote:

> Joel de Guzman wrote:
>> Sebastian Redl wrote:
>>> Eric Niebler wrote:
>>>> 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.
>>>
>>> That's not necessarily a good benchmark, especially if you replace it by
>>> preprocessor metaprogramming which leads to more non-template code. GCC
>>> is extremely slow at instantiating templates, but this is not
>>> necessarily true for other compilers - I believe, for example, that
>>> Clang will be faster at instantiating templates than parsing raw code.
>>> (No benchmarks - but I know the code.)
>
> Cool! I wonder how that's possible. I have it from Walter Bright (Zortech, Symantec,
> Digital Mars) that instantiating a template is inherently expensive, and certain
> features of the C++ language (ADL, partial specialization, etc.) force that to be the
> case. If Clang has found a way to solve these problems, that's good
> news indeed. 

It may be "inherently expensive" by some measure, but most compilers
were implemented by people for whom template instantiation speed was way
down the list of priorities, and most got their template implementations
before "interesting TMP" was even available for them to test against.
In some cases they do *really* dumb things.

> 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 :-)

>> Agreed 100%
>
> OK. When compiling Fusion's vector_make.cpp test ...
>
> Before ...
>
>> $ time g++ -I ../../../.. -c vector_make.cpp
>>
>> real    0m1.670s
>> user    0m1.216s
>> sys     0m0.325s
>
> After ...
>
>> $ time g++ -I ../../../.. -c vector_make.cpp
>>
>> real    0m1.208s
>> user    0m0.684s
>> sys     0m0.309s
>
> 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 :-)

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

-- 
Dave Abrahams
BoostPro Computing
http://www.boostpro.com