on Thu Apr 05 2012, Thomas Heller <thom.heller-AT-googlemail.com> wrote:
On 04/04/2012 06:08 PM, Dave Abrahams wrote:
on Tue Apr 03 2012, Sumant Tambe<sutambe-AT-gmail.com> wrote:
Some compile-time computations can be better (faster) implemented as recursive constexpr functions. I would like to see some performance comparisons between constexpr and vanilla metaprogramming. Well, yes, that's the point. The question is, what kinds of computations should we try to perform in order to discover where to use the new techniques?
Do we have libraries in Boost with slow-to-compile examples that would make good representative test cases?
Certainly, yes, essentially every library that uses proto/fusion/mpl excessively are compile time hogs.
Well, I'm asking for specifics, please. Like, a path in the Boost SVN repository.
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. http://lists.boost.org/Archives/boost/2010/07/169562.php http://lists.boost.org/Archives/boost/att-169562/Lazy_MPL__strictness_analys...
One example where the meta_function1-style is used almost all over the place is fusion (in the traits for example). This is not something C++11 related though. However, there is still much uncertainty about what metaprogramming techniques are costly in C++03.
Yep. -- Dave Abrahams BoostPro Computing http://www.boostpro.com