Reserve on dynamic arrays

[cym13 via Digitalmars-d-learn]

Does changing the capacity of dynamic arrays actually do anything 
with respect to memory allocation? I have the following data with 
DMD 2.071.1

// With dmd -profile=gc

void main() {
     import std.stdio;

     int[] arr = [1, 1, 1];
     arr.reserve(512);

     foreach (i ; 0..10)
         arr ~= 42;
}

/*
  * bytes allocated, allocations, type, function, file:line
  *              40              10 int[] D main test.d:8
  *              12               1 int[] D main test.d:4
  */

void main() {
     import std.stdio;

     int[] arr = [1, 1, 1];
     arr.reserve(512);

     foreach (i ; 0..20) // Just changing the number of elements
         arr ~= 42;
}

/*
  * bytes allocated, allocations, type, function, file:line
  *              80              20 int[] D main test.d:8
  *              12               1 int[] D main test.d:4
  */

void main() {
     import std.stdio;

     int[] arr = [1, 1, 1];
     // Same as above, without reserve

     foreach (i ; 0..20)
         arr ~= 42;
}

/*
  * bytes allocated, allocations, type, function, file:line
  *              80              20 int[] D main test.d:8
  *              12               1 int[] D main test.d:4
  */

As one can see there seem to be absolutely no difference in 
allocations wether we reserve or not. I've ran more experiences, 
reserving way less or more than I appened to no avail: 
allocations seem to just ignore it.

Isn't the point of .reserve to perform preallocation?