Re: [boost] [fusion] improving compile times

Subject: Re: [boost] [fusion] improving compile times
From: Eric Niebler (eric_at_[hidden])
Date: 2009-06-02 13:09:37

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. I read 
form the Wikipedia entry that Clang's C++ support is 2-3 years from 
being usable, though.

> 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.

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