bug#40407: [PATCH] slow ENCODE_FILE and DECODE_FILE

[Eli Zaretskii]

> From: Mattias Engdegård <mattiase@acm.org>
> Date: Sat, 4 Apr 2020 18:55:26 +0200
> Cc: 40407@debbugs.gnu.org
> 
> 4 apr. 2020 kl. 12.26 skrev Eli Zaretskii <eliz@gnu.org>:
> 
> > After looking at this for some time, I think the problem is rarely if
> > ever seen.  The only function which has the NOCOPY sense inverted is
> > code_convert_string, and it only does that when the CODING_SYSTEM
> > argument is nil, which should almost never happen.
> 
> Oh, that's the easy part. It's the ASCII optimisation that makes it 
> interesting.

How so?