Need for speed

[Ali Çehreli]

On 4/1/21 12:55 PM, H. S. Teoh wrote:

 > - Constructing large arrays by appending 1 element at a time with `~`.
 >    Obviously, this requires many array reallocations and the associated
 >    copying

And that may not be a contributing factor. :) The following program sees 
just 15 allocations and 1722 element copies for 1 million appending 
operations:

import std.stdio;

void main() {
   int[] arr;
   auto place = arr.ptr;
   size_t relocated = 0;
   size_t copied = 0;
   foreach (i; 0 .. 1_000_000) {
     arr ~= i;
     if (arr.ptr != place) {
       ++relocated;
       copied += arr.length - 1;
       place = arr.ptr;
     }
   }

   writeln("relocated: ", relocated);
   writeln("copied   : ", copied);
}

This is because the GC does not allocate if there are unused pages right 
after the array. (However, increasing the element count to 10 million 
increases allocations slightly to 18 but element copies jump to 8 million.)

Ali