On Jul 1, 2013, at 6:17 PM, Howard Hinnant <[email protected]> wrote:
> Inspired by your approach, I started from scratch and came up with the
> enclosed. The compiler will run out of memory before it runs into the
> recursive template instantiation limit, and this works with far higher
> numbers. It passes all the current intseq tests.
I did some timing tests for the compiler using:
#include <utility>
#include <iostream>
int
main()
{
typedef std::make_index_sequence<10000> T;
std::cout << typeid(T).name() << "\n";
}
and:
#include <utility>
#include <iostream>
int
main()
{
typedef std::make_index_sequence<100000> T;
std::cout << typeid(T).name() << "\n";
}
These are extremely long sequences! I discovered that if I add to the patch
enclosed in my previous email:
template <class _Tp, _Tp _Sp, _Tp _Ep>
struct __make_integer_sequence_unchecked<_Tp, _Sp, _Ep, 4>
{
typedef integer_sequence<_Tp, _Sp, _Sp+1, _Sp+2, _Sp+3> type;
};
template <class _Tp, _Tp _Sp, _Tp _Ep>
struct __make_integer_sequence_unchecked<_Tp, _Sp, _Ep, 3>
{
typedef integer_sequence<_Tp, _Sp, _Sp+1, _Sp+2> type;
};
template <class _Tp, _Tp _Sp, _Tp _Ep>
struct __make_integer_sequence_unchecked<_Tp, _Sp, _Ep, 2>
{
typedef integer_sequence<_Tp, _Sp, _Sp+1> type;
};
that there is between a 50% to 100% improvement in compile time, and lower
compiler memory usage (unmeasured), with no other difference in behavior.
There seems to be a sweet spot somewhere in this neighborhood (3 <= _Diff <= 8)
between complexity, added benefit, and even performance with lower numbers. I
rather arbitrarily selected 4 as the optimum.
However I'm unsure how this compares to Richard's algorithm as I never was able
to get that working. Perhaps though if someone does, we could make an informed
comparison and choose the best. Regardless this is much better than the
algorithm Marshall first posted, which I was responsible for. Its genesis can
be found in <__tuple>'s __make_tuple_indices. When we get
make_integer_sequence settled, I plan on updating <__tuple> accordingly.
Howard
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