bug#40407: [PATCH] slow ENCODE_FILE and DECODE_FILE

[Mattias Engdegård]

ENCODE_FILE and DECODE_FILE turn out to be surprisingly slow, and allocate 
copious amounts of memory, to the point that they often turn up in both memory 
and cpu profiles. (This is on macOS; I haven't checked the situation elsewhere.)

For instance, a single call to file-relative-name, with ASCII-only arguments, 
manages to allocate 140 KiB. There are several conversion steps each involving 
creating temporary buffers as well as the compilation and execution of very 
large "quick-check" regexps. Example:

(progn
  (require 'profiler)
  (profiler-reset)
  (garbage-collect)
  (profiler-start 'mem)
  (file-relative-name "abc")
  (profiler-stop)
  (profiler-report))

This applies to just about every function dealing with files or file names.

The attached patch is somewhat conservatively written but at least a starting 
point. It reduces the memory consumption by file-relative-name in the example 
above to zero. Perhaps we can assume that file names codings are always 
ASCII-compatible; if so, the shortcut can be taken in encode_file_name and 
decode_file_name directly.

There is already a hack in encode_file_name that assumes that no unibyte string 
ever needs encoding; if so, the shortcut could perhaps be extended to 
decode_file_name and simplified.

0001-Avoid-expensive-recoding-for-ASCII-identity-cases.patch
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