bug#55636: 27.2; etags performance fix when working with very big TAGS files

[WAROQUIERS Philippe]

> > So, what we can observe there is that the C function Fexpand_file_name 
> > makes 236_905 calls
> >
> > to Ffind_File_name_handler
> >
> > This Ffind_file_name_handler function makes 1.2 million calls to 
> > fast_string_match.
> >
> >
> >
> > On this screenshot, we cannot see the CPU directly consumed by the 
> > functions,
> >
> > but looking at the direct cpu shows that expand-file-name  cost is mostly 
> > due to the fast_string_match closure
>
> The Lisp profile tells quite a different story: according to that,
> expand-file-name is not a hotspot.
>
> So I still don't see  that expand-file-name is the place to optimize
> your use case.
>
> Did you succeed in reproducing the 10-sec delay with the original
> code, and then ten-fold speedup if expand-file-name is called only on
> non-absolute file names?

I have tried to reproduce the 10 seconds delay to no success.
With or without the expand-file-name called only on non absolute file names,
 I now see emacs always using about 4 seconds of cpu to load the TAGS files.

So, at this point:
   * I do not understand why we observed 3 different speed:
      sub-second, 4 seconds and 10 seconds
   * we now see that loading the TAGS files takes around 4 seconds, so is still 
a heavy
      operation that maybe could be optimised but not clear anymore what is the
      costly part.

Thanks
Philippe

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