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.
Somewhat off topic:
What I'd really like to see is someone explain how metaprogramming improves compile speed. Steven said earlier that the slowdown depends on the template:
Interesting.
I would have guessed that the more template instances that, are created, then the slower the compile time. However, Steven's remark made me wonder. What I'm guessing is that if the template metaprogram produces a lot of intermediate results, then it might be better to use preprocessor metaprogramming to just produce the final result. Is that about right or is it more complicated?
My experience matches yours: more instantiations --> longer compiles. I wonder what Steven's experience is. Steven? -- Eric Niebler BoostPro Computing http://www.boostpro.com