why use string for this example of appender?

[ludo]

Hi guys,

I am working on an old software in D1, which defines at some 
point an array.d module. See my github file: 
https://tinyurl.com/5ffbmfvz

If you go line 347, you see an ArrayBuilder struct, which is 
supposed to behave exactly like an array but with faster 
concatenation. The class comment says:
/**
  * Behaves the same as built-in arrays, except about 6x faster 
with concatenation at the expense of the base pointer
  * being 4 system words instead of two (16 instead of 8 bytes on 
a 32-bit system).
  */

I created a unittest, line 586, to test this statement. You can 
git clone + dub test, to see the result. On my machine I get:
*** ArrayBuilder benchmark ***
1) Concatenation with std:          745 μs and 7 hnsecs
2) Concatenation with Arraybuilder: 236 μs and 8 hnsecs
3) Concatenation with Reserve:      583 μs and 5 hnsecs
4) Concatenation with Length:       611 μs and 8 hnsecs
5) Concatenation with Appender:     418 μs

In (1) I use standard array concatenation, in (2) I use the 
ArrayBuilder struct, in (3) I test standard concat with a call to 
reserve() before-hand, in (4) standard + length= beforehand, and 
finally I dug out the Appender class in (5).

None of the D library tools beat the ArrayBuilder class! Note 
though, that Appender is only two times slower, not 6x slower.

Can someone explain to me why ArrayBuilder is so fast (or the 
others so slow)? What is this base pointer being "4 system words 
instead of 2"? I read the code of ArrayBuilder, but I can not 
spot any magic trick... Has this anything to do with standard 
array concat being "safer" (thread-wise, bounds-wise, etc)?

And also, is there a std module which performs as well, some Fast 
Appender class? Could not find any.

Thanks