https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66059
--- Comment #8 from Daniel Frey <d.frey at gmx dot de> --- (In reply to Jonathan Wakely from comment #7) > (In reply to Daniel Frey from comment #3) > > A better O(log N) library-only solution than the linked one is available at > > https://github.com/taocpp/sequences/blob/master/include/tao/seq/ > > make_integer_sequence.hpp > > The version I came up with is very close to Xeo's at stackoverflow. I tried > something more like yours and it used a LOT more memory. > > Here's what I tested: > > [...snip...] > > As you can see, defining DUP makes it 10 times slower and using 5 times as > much memory. Looking at your implementation I see only one relevant > difference, you pass N/2 as a template argument instead of calculating it > with sizeof... i.e. Yeah, avoiding sizeof... is crucial. I don't really know why, I just know it is. And only for GCC, Clang doesn't care if you use sizeof... or pass in the size explicitly. This behavior from GCC might be worth a look from the compiler people, as there is no reason why it should use so much more memory. > and indeed this is even faster than what I just committed. I didn't realise > sizeof... would have such an impact. > > My one can be optimised to be almost as fast by avoiding sizeof... in > _Itup_cat. I guess my version (without the sizeof...) is still better the larger the numbers get, but it won't make a real difference in practice as it's kind of unrealistic to generate index sequences with 100000 or more elements.
