为什么大小..(T）这么慢?在没有大小的情况下实现C++14 make_index_sequence.(T）

我发现C++14的实现make_index_sequence"算法"：

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!!!
};

但是，当我更换尺寸时...（i） 从"concate"到具体数字，然后make_index_sequence<1024 *64> - 编译速度非常快。

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 ) ... > {};
// 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 > >
                           >{};

问：为什么尺寸...（一）这么慢？

我用 gcc 4.8.1 进行测试更新：

对于第一种情况：（仅限 1024 和 1024*16）。

g++  -Wall  -c "ctx_fptr.cpp"   -g  -O2 -std=c++11 -ftime-report
Execution times (seconds)
 garbage collection    :   0.06 ( 1%) usr   0.00 ( 0%) sys   0.06 ( 0%) wall       0 kB ( 0%) ggc
 preprocessing         :   0.03 ( 0%) usr   0.04 ( 2%) sys   0.09 ( 1%) wall     293 kB ( 0%) ggc
 parser                :  10.41 (97%) usr   1.61 (95%) sys  12.01 (96%) wall 2829842 kB (99%) ggc
 name lookup           :   0.12 ( 1%) usr   0.04 ( 2%) sys   0.23 ( 2%) wall    7236 kB ( 0%) ggc
 dead store elim1      :   0.01 ( 0%) usr   0.00 ( 0%) sys   0.00 ( 0%) wall       0 kB ( 0%) ggc
 symout                :   0.15 ( 1%) usr   0.00 ( 0%) sys   0.15 ( 1%) wall   12891 kB ( 0%) ggc
 unaccounted todo      :   0.00 ( 0%) usr   0.01 ( 1%) sys   0.00 ( 0%) wall       0 kB ( 0%) ggc
 TOTAL                 :  10.78             1.70            12.55            2850835 kB

对于第二种情况：（所有 1024、1024*16 和 1024*64 ）

g++  -Wall  -c "ctx_fptr.cpp"   -g  -O2 -std=c++11 -ftime-report 
Execution times (seconds)
 preprocessing         :   0.02 ( 2%) usr   0.01 ( 5%) sys   0.05 ( 4%) wall     293 kB ( 0%) ggc
 parser                :   0.54 (45%) usr   0.10 (53%) sys   0.71 (50%) wall   95339 kB (58%) ggc
 name lookup           :   0.47 (39%) usr   0.04 (21%) sys   0.47 (33%) wall   20197 kB (12%) ggc
 tree PRE              :   0.01 ( 1%) usr   0.00 ( 0%) sys   0.00 ( 0%) wall       1 kB ( 0%) ggc
 varconst              :   0.00 ( 0%) usr   0.01 ( 5%) sys   0.00 ( 0%) wall      17 kB ( 0%) ggc
 symout                :   0.17 (14%) usr   0.03 (16%) sys   0.18 (13%) wall   47092 kB (29%) ggc
 TOTAL                 :   1.21             0.19             1.41             163493 kB