"Jaap Suter" <boost@jaapsuter.com> writes:
Hello,
can any compiler expert give me feedback on whether there should be a noticable difference in compilation speed between the following two meta-functions:
template<class N, class M, class Q, class P> struct foo : mpl::plus < N, typename mpl::minus < M, typename mpl::divides < Q, P >::type >::type >::type {};
and
template<int N, int M, int P, int Q> struct foo { enum { value = N + (M - (Q/P)) };
typedef boost::mpl::int<value> type; }
The second version saves me a lot of template instantiations and many header inclusions, at the cost of losing some genericity.
It's likely to be faster, then. Compilation speed tends to vary with number of template instantiations.
On the other hand, if the calculation becomes complicated, I find myself using many enums to hold intermediate values, resulting in code like the following:
template<class T> struct next_number_with_same_num_bits { enum { lowest_bit_set = T::value & (-T::value), lowest_bit_set_index = count_leading_zeroes<T>::type::value, next_t = T::value + lowest_bit_set, next_lowest_set_bit = next_t & (-next_t), mask = next_lowest_set_bit - lowest_bit_set, result_t = next_t | (mask >> (lowest_bit_set_index + 1)) };
typedef boost::mpl::int_<result_t> type; };
which leads MSVC to run out of so-called compiler-keys much quicker than if I reduce the above to the completely unreadable:
template<class T> struct next_number_with_same_num_bits { enum { result_t = (T::value + (T::value & (-T::value))) | ((((T::value + (T::value & (-T::value))) & (-T::value - (T::value & (-T::value)))) - (T::value & (-T::value))) >> (count_leading_zeroes<T>::type::value + 1)) };
typedef boost::mpl::int_<result_t> type; };
Does anybody have any thoughts or comments on this technique, and is anybody else using tricks around MPL to make sure your compiler still handles things?
My advice: use the MPL constructs and optimize later. When you're done you can measure the compilation speed difference when using mpl::plus, minus and divides vs. the ugly version at the bottom. I bet you dollars to donuts that it's not a significant hit. But if it is, you can make some informed choices. -- Dave Abrahams Boost Consulting http://www.boost-consulting.com