cpp: slow sphinx-build read when -j $big is used

[marxin]

Describe the bug

Originally thought it's #10967, but the suggested patch hasn't helped me.
The problem is that reading sources... takes ages (~15 minutes and more) if I use -j 160:

ncalls  tottime  percall  cumtime  percall filename:lineno(function)
    497/1    0.001    0.000  311.019  311.019 {built-in method builtins.exec}
        1    0.000    0.000  311.019  311.019 sphinx-build:1(<module>)
        1    0.000    0.000  310.746  310.746 build.py:306(main)
        1    0.004    0.004  310.745  310.745 build.py:268(build_main)
        1    0.000    0.000  310.399  310.399 application.py:339(build)
        1    0.000    0.000  310.399  310.399 __init__.py:301(build_update)
        1    0.000    0.000  310.398  310.398 __init__.py:314(build)
        1    0.000    0.000  310.395  310.395 __init__.py:384(read)
        1    0.009    0.009  310.386  310.386 __init__.py:456(_read_parallel)
      324    0.044    0.000  310.023    0.957 parallel.py:120(_join_one)
      293    0.008    0.000  307.161    1.048 __init__.py:476(merge)
      293    0.050    0.000  273.810    0.935 __init__.py:351(merge_info_from)
      293    0.009    0.000  273.521    0.934 cpp.py:7917(merge_domaindata)
101038/293    1.031    0.000  273.512    0.933 cpp.py:4807(merge_with)
   618010   59.922    0.000  261.630    0.000 cpp.py:4412(_find_named_symbols)
131399834   61.641    0.000  184.644    0.000 cpp.py:4440(matches)
        1    0.005    0.005  167.875  167.875 parallel.py:102(join)
      311    0.239    0.001  142.469    0.458 parallel.py:88(add_task)
131399833   96.795    0.000  123.003    0.000 cfamily.py:84(__eq__)
      586   33.457    0.057   33.643    0.057 {built-in method _pickle.loads}
131500914   16.998    0.000   16.998    0.000 cpp.py:4462(candidates)
131408156/131408152   14.050    0.000   14.050    0.000 {built-in method builtins.getattr}
131412043   12.171    0.000   12.171    0.000 {method 'items' of 'dict' objects}
   201490    0.156    0.000   10.658    0.000 cpp.py:4050(get_newest_id)

How to Reproduce

$ git clone --depth 1 git://gcc.gnu.org/git/gcc.git
$ cd gcc
$ time sphinx-build -b texinfo -d doctrees /home/marxin/Programming/gcc/gcc/doc/gccint output -j 160

Environment Information

5.3

Sphinx extensions

No response

Additional context

No response

[jakobandersen]

I believe this is a duplicate of #10966, as in parallel mode the symbol tables needs to be merged, which requires lookup.

[marxin]

Closing as duplicate.