bug#73484: 31.0.50; Abolishing etags-regen-file-extensions

[Eli Zaretskii]

> From: Francesco Potortì <pot@gnu.org>
> Date: Thu, 10 Oct 2024 16:25:28 +0200
> Cc: dmitry@gutov.dev,
>       73484@debbugs.gnu.org,
>       spwhitton@spwhitton.name
> 
>   for (fdp = fdhead; fdp != NULL; fdp = fdp->next)
>     {
>       assert (fdp->infname != NULL);
>       if (streq (uncompressed_name, fdp->infname))
>       goto cleanup;
>     }
> 
> This is a simple O^2 comparison, which is repeated sum(1,N,N-1)=~N^2/2, which 
> for ~375k files means ~70G comparisons.  If you can count the number of times 
> streq is called and 70G is a substantial portion of that number, then we have 
> the culprit.  To check, just remove the above test and see if the running 
> time drops.

Dmitry already made this check, and the run time did drop, see
https://debbugs.gnu.org/cgi/bugreport.cgi?bug=73484#107

> In that case, using a hash rather than a comparison would probably make sense.

Right.

> Alternatively, rather than managing file names in a single loop, do a first 
> loop on all file names to canonicalise them, but without searching for tags 
> (essentially, remove the call to process_file from process_file_name), then 
> uniquify the list of canonicalised file names, then run process_file on them.

I don't think this is possible because command-line options can be
interspersed with file names, and each option affects the processing
of the files whose names follow the option.