Eric Niebler wrote:
Larry Evans wrote:
On 06/01/09 20:26, Eric Niebler wrote:
I'm attaching a simple patch to vector_n_chooser.hpp that replaces some template metaprogramming with preprocessor metaprogramming in the interest of improving compile times. I found this hotspot through profiling, [snip] Eric,
Could you post your benchmark code that showed the improvement in compile speed? I'd like to eventually try it with a variadic template compiler version of fusion vector.
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.
Be careful. I once spent a couple of days trying to speed up some compilations that were taking ~5mins per file. All my efforts to reduce template instantiations had no effect on the compile time. Eventually I realised that ccache was actually caching the compiles and my timing was ignoring the compile step entirely; those minutes were being spent in the preprocessor. I rewrote some preprocessor metaprogramming, changing an algorithm from O(n^4) to O(n) (at the expense of some runtime memory), and it all went away. I later had to abandon the project because the template metaprogramming was too memory-hungry, but it was a valuable lesson. I don't suppose anyone's written a preprocessing metaprogramming profiler yet? John