json::serialize takes an extraordinary amount of time on clang / libc++

[madmongo1]

PLEASE DON'T FORGET TO "STAR" THIS REPOSITORY :)

When reporting a bug please include the following:

Version of Boost

1.79

Steps necessary to reproduce the problem

Compile the linked program https://github.com/madmongo1/json-bug-1

with:
clang 13
-stdlib=libc++

All relevant compiler information

clang 13
-stdlib=libc++

Run the executable in the same directory as the bundled log file demo.log.

The program reads one line of text from the file, parses it to json and then serialises the json value back to a string.

With GCC, the serialisation is instantaneous.

With clang (in both debug and release configurations), the parsing is instantaneous, but the serialisation takes many minutes.

It does make progress, but incredibly slowly. I have waited 5-10 minutes. In that time, the program was only able serialise approx 140k of text.

[sehe]

The problem is clearly linked to libc++, because I cannot reproduce it unless I add -stdlib=libc++ (which isn't default on my system).

[vinniefalco]

What is the std::string growth policy of libc++ compared to clang?

See:

json/include/boost/json/impl/serialize.ipp

Line 52 in 83c364a

s.reserve(

[madmongo1]

I use -stdlib=libc++ with clang as it's seemingly the only way to get coroutine support.

[sehe]

Whatever it is, it's taking 96% of execution (even if I reserve the output string to 4'472'628 in serialize.ipp, so it's intermediate strings?).

[vinniefalco]

libc++ reserve( capacity() + 1 ) grows capacity 16 bytes at a time

[sehe]

parsing
...serialising
...serialised: 4472628 chars

real    0m0,079s
user    0m0,059s
sys     0m0,020s

[sehe]

And no leaks reported (valgrind)

==9036== All heap blocks were freed -- no leaks are possible

[grisumbras]

Fixed in 5ca4fb3