bug#29889: 27.0.50; Slow visual selection

[Sujith <m.sujith@gmail.com>]

Eli Zaretskii <eliz@gnu.org> writes:
> Since mark_object appears high in the profile, could you please
> rerun the experiment after setting gc-cons-threshold and
> gc-cons-percentage so as to avoid GC for the time of the expdriment?

After doing (setq gc-cons-threshold 1000000000), the issue doesn't
seem to happen. The cursor moved around freely except for one
interruption - maybe the GC kicked in then.

Profile report using 'perf record -p `pidof emacs`' just before
starting to move the cursor:

# Samples: 39K of event 'cycles:ppp'
# Event count (approx.): 21976730020
#
# Overhead  Command  Shared Object               Symbol                                   
# ........  .......  ..........................  .........................................
#
    35.09%  emacs    emacs-27.0.50               [.] balance_an_interval
     9.36%  emacs    emacs-27.0.50               [.] Flength
     7.38%  emacs    emacs-27.0.50               [.] lisp_align_free
     5.12%  emacs    emacs-27.0.50               [.] next_interval
     5.12%  emacs    emacs-27.0.50               [.] concat
     3.74%  emacs    emacs-27.0.50               [.] copy_intervals
     2.54%  emacs    emacs-27.0.50               [.] Fcons
     2.13%  emacs    libc-2.26.so                [.] __memmove_sse2_unaligned_erms
     1.94%  emacs    emacs-27.0.50               [.] assq_no_quit
     1.44%  emacs    emacs-27.0.50               [.] copy_properties
     1.39%  emacs    emacs-27.0.50               [.] mem_insert
     1.35%  emacs    emacs-27.0.50               [.] sweep_intervals
     1.34%  emacs    libc-2.26.so                [.] _int_malloc
     1.17%  emacs    emacs-27.0.50               [.] make_interval
     1.14%  emacs    emacs-27.0.50               [.] Fmake_list
     1.02%  emacs    emacs-27.0.50               [.] sweep_conses
     0.76%  emacs    emacs-27.0.50               [.] x_produce_glyphs
     0.76%  emacs    emacs-27.0.50               [.] split_interval_right
     0.76%  emacs    libc-2.26.so                [.] _int_free
     0.65%  emacs    emacs-27.0.50               [.] lookup_char_property
     0.61%  emacs    libc-2.26.so                [.] malloc
     0.57%  emacs    emacs-27.0.50               [.] mem_find.part.9
     0.55%  emacs    emacs-27.0.50               [.] Fcopy_sequence