Re: [boost] Metaprogram benchmarks

Subject: Re: [boost] Metaprogram benchmarks
From: Joel de Guzman (joel_at_[hidden])
Date: 2012-04-05 20:56:35

On 4/5/2012 11:55 PM, Dave Abrahams wrote:
> 
> on Thu Apr 05 2012, Thomas Heller <thom.heller-AT-googlemail.com> wrote:
> 
>> I, personally, haven't come around to actually extract meaningful
>> benchmarks from, for example, phoenix.  One thing that immediately
>> comes to my mind is whether to deferr the instantation of a template
>> as long as possible, or to have it instantiated immediately.  Example:
>>
>> template <typename T>
>> struct meta_function1
>> {
>>     typedef T type;
>> };
>>
>> template <>
>> struct meta_function1<int>
>> {
>>     typedef int type;
>> };
>>
>> template <typename T, typename Dummy = void>
>> struct meta_function2
>> {
>>     typedef T type;
>> };
>>
>> template <typename Dummy>
>> struct meta_function2<int, Dummy>
>> {
>>     typedef int type;
>> };
>>
>> The question now is: which is faster meta_function1 or meta_function2?
>> Of course, in this case we won't get meaningful numbers. However, some
>> claim that meta_function2 is generally faster, especially in the
>> context of many full specializations. 
> 
> That's a bit surprising.
> 
> In C++03 compilers, fully lazy evaluation seems to become expensive in
> surprising ways that aren't yet well-understood.

Thomas, Fusion is an easy target. If anyone would like to try this
out --grep or write a script that converts Fusion's meta_function1
style to meta_function2 (lazy) style, please do so and show us the
benchmark results (e.g. by running the Fusion test suite over either
versions). Without such *real* numbers, it will remain a myth and we
will never really know. I'd love to tweak Fusion if such a trick is
found to really improve CT.

Regards,
-- 
Joel de Guzman
http://www.boostpro.com
http://boost-spirit.com