[Bug c++/58999] New: sizeof ...(T) is very slow than clang

[khurshid.normuradov at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58999

            Bug ID: 58999
           Summary: sizeof ...(T) is very slow than clang
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: khurshid.normuradov at gmail dot com

This code doesn't compiled well on gcc, but compiled well with clang, and very
fast.

template< int ... > struct index_sequence{   using type = index_sequence; };

template< typename T> using invoke = typename T :: type ;

template< typename T, typename U > struct concate;
template< int ...i, int ... j>
struct concate< index_sequence<i...>, index_sequence<j...> >
        : index_sequence< i... ,  (j + sizeof ... (i ) )... > {};
  //                                   \          /
  //                                    ----------
 //                                   I think here is slowly.
template< int n>
struct make_index_sequence_help : concate< 
                          invoke< make_index_sequence_help<n/2>>,
                          invoke< make_index_sequence_help<n-n/2>>
                          > {};

template<> struct make_index_sequence_help <0> : index_sequence<>{};
template<> struct make_index_sequence_help <1> : index_sequence<0>{};

template< int n> using make_index_sequence = invoke<
make_index_sequence_help<n> >;


int main()
{
    using iseq = make_index_sequence< 1024 > ; // successfull
    using jseq = make_index_sequence< 1024 * 16 > ; // a lot of compile time!!!
    using kseq = make_index_sequence< 1024 * 64 > ; // can't compile: memory
exhauted!!!
};


When I change sizeof...(i)  to concrete number 's'  - compiled fastest as
clang.


template< int s, typename T, typename U > struct concate;
template< int s, int ...i, int ...j >
struct concate< s, index_sequence<i...>, index_sequence<j...> >
 :  index_sequence< i..., ( j + s ) ... > {};
//                              |_ changed sizeof...(i) to s
// and 
template< int n >
struct make_index_sequence_help : concate<
                                  n / 2 , 
                          invoke< make_index_sequence_help< n / 2 > >,
                          invoke< make_index_sequence_help< n - n/2 > >
                           >{};