Dynamic array comparison optimization: check for equal pointers?

[Stanislav Blinov]

On Tuesday, 2 June 2020 at 19:38:04 UTC, Johan wrote:

> I was expecting something like:
> ```
>   if ( sizes are not equal )
>      return false;
>   if ( pointers are equal  )
>      return true;
>   return memcmp;
> ```

That's, worst case, two branches and a scan, and best case one 
branch which may or may not be slower than the actual scan.

> I am surprised that memcmp itself does not implement the 
> pointers are equal check. Is there a big impact of adding the 
> shortcircuit check to prevent scanning memory?

There can't be a definitive answer for this, it's dependent on 
architecture as well as size of data. As far as array comparison 
goes, if a compiler (talking about ldc/gdc here) can infer 
pointer/length information, it'll be free to throw away the scan. 
In all other cases, I'd rather see the comparison function, well, 
do the comparison, and leave branches up to the user. It's much 
better than to inflict the branches on everyone unconditionally 
(pun intended): if your data shows that you can benefit from the 
short-circuit, you can always do just that. Short of rethinking 
the design, of course ;) I mean, I can't imagine a scenario where 
a program would have to routinely compare identical memory 
ranges. Doesn't mean there aren't such scenarios though.