I saw format-overflow warnings in g1GCPhaseTimes.cpp when I built jdk/jdk with GCC 9 on Fedora 31 as below:

------------------
In member function 'void G1GCPhaseTimes::log_phase(WorkerDataArray<double>*, uint, outputStream*, bool) const',
    inlined from 'void G1GCPhaseTimes::trace_phase(WorkerDataArray<double>*, bool, uint) const' at /home/ysuenaga/OpenJDK/jdk/src/hotspot/share/gc/g1/g1GCPhaseTimes.cpp:339:14:
/home/ysuenaga/OpenJDK/jdk/src/hotspot/share/gc/g1/g1GCPhaseTimes.cpp:319:17: error: '%s' directive argument is null [-Werror=format-overflow=]
  319 | out->print("%s", Indents[indent + 1]);
      | ~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~
In member function 'void G1GCPhaseTimes::details(T*, const char*) const [with T = WorkerDataArray<long unsigned int>]',
    inlined from 'void G1GCPhaseTimes::log_phase(WorkerDataArray<double>*, uint, outputStream*, bool) const' at /home/ysuenaga/OpenJDK/jdk/src/hotspot/share/gc/g1/g1GCPhaseTimes.cpp:321:14,
    inlined from 'void G1GCPhaseTimes::trace_phase(WorkerDataArray<double>*, bool, uint) const' at /home/ysuenaga/OpenJDK/jdk/src/hotspot/share/gc/g1/g1GCPhaseTimes.cpp:339:14:
/home/ysuenaga/OpenJDK/jdk/src/hotspot/share/gc/g1/g1GCPhaseTimes.cpp:306:13: error: '%s' directive argument is null [-Werror=format-overflow=]
  306 | ls.print("%s", indent);
      |
------------------

I used gcc-9.2.1-1.fc31.x86_64.

-Wformat-overflow has been extended since GCC 9.
https://gcc.gnu.org/gcc-9/changes.html