Commits on Mar 26, 2021

1. std::vector -> std::array for constant …

   `std::vector` is not very well suited for static, constant arrays.
   Since creating it from a list requires memory allocation,
   modern C++ compilers miss out on many optimizations.
   In particular, compilers are forced to insert a call to a global
   atomic variable and a bunch of code for initialization at the place of
   its use. Plus access to heap memory cause cache misses
   For example for this function:
   bool f(const int *arr, int x)
   {
       static std::vector<Foo> const values = {A, B, C};
   
       for (auto v : values) {
           if (arr[static_cast<size_t>(v)] == x)
               return true;
       }
       return false;
   }
   
   compiler generates ~60 instructions (gcc 10.2 -O3 -march=native),
   if replace `std::vector` with `std::array` the result code contains only 9 instructions.

   

   Dushistov committed Mar 26, 2021
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