In-place extension of arrays only for certain alignment?

[Steven Schveighoffer]

On 8/16/22 2:11 PM, Ali Çehreli wrote:
> Related to my DConf 2022 lightning talk, I am noticing that D runtime's 
> in-place array extension optimization is available only for array data 
> that are at certain memory alignments.
> 

No, it's based on 2 factors:

1. Is it a page-size-or-greater block?
2. Is there a free page after it?

The smaller blocks are stored in page-sized pools and *cannot* be 
combined together. e.g. you can't have 2 16-byte blocks joined to form a 
32 byte block.

The reason why your `bad` version fails is because when it must 
reallocate, it still is only allocating 1 element.

Now a page is 4096 bytes typically. So why does the 2048 amount work? 
Because in order to store a 2048 byte array, you need 2048 bytes AND the 
metadata (e.g. typeinfo for destruction and append capacity). This 
requires a full page.

Note that once you reach page size, the optimization can happen, *even 
if you are only appending to an end slice*. Here's something to try: 
when the capacity is less than the "magic" number of elements, `reserve` 
that number of elements. Then the "drop one element each loop" should 
use the optimization.

-Steve