RFR: 8360817: [ubsan] zDirector select_worker_threads - outside the range of representable values issue

[Matthias Baesken]

When running the jtreg test
gc/z/TestMappedCacheHarvest.java
with ubsan-enabled binaries on macOS aarch64, we following ubsan error is reported :


 stderr: [/jdk/src/hotspot/share/gc/z/zDirector.cpp:715:33: runtime error: 6.1962e+10 is outside the range of representable values of type 'unsigned int'
    #0 0x109e368fc in select_worker_threads(ZDirectorStats const&, unsigned int, ZWorkerSelectionType) zDirector.cpp:715
    #1 0x109e3658c in initial_workers(ZDirectorStats const&, ZWorkerSelectionType) zDirector.cpp:804
    #2 0x109e35df0 in ZDirector::run_thread() zDirector.cpp:932
    #3 0x109ecd5f4 in ZThread::run_service() zThread.cpp:28
    #4 0x108cd1e50 in ConcurrentGCThread::run() concurrentGCThread.cpp:47
    #5 0x109ce770c in Thread::call_run() thread.cpp:243
    #6 0x10985be28 in thread_native_entry(Thread*) os_bsd.cpp:599
    #7 0x19fa8ef90 in _pthread_start+0x84 (libsystem_pthread.dylib:arm64e+0x6f90)
    #8 0x19fa89d30 in thread_start+0x4 (libsystem_pthread.dylib:arm64e+0x1d30)


After I added a bit of tracing to `select_worker_threads` and it seems we get WAY too high young_to_old_ratio values; in my locally failing example
young_to_old_ratio:27561965412.478878
this leads to values out of range of uint and that makes ubsan complain about the uint cast 
`uint(young_workers * young_to_old_ratio)` .
We clamp the value anyway to the range 1u to ZOldGCThreads ; so it probably makes sense to avoid such high factors for young_to_old_ratio .

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Commit messages:
 - JDK-8360817

Changes: https://git.openjdk.org/jdk/pull/26186/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26186&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8360817
  Stats: 6 lines in 1 file changed: 5 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/26186.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26186/head:pull/26186

PR: https://git.openjdk.org/jdk/pull/26186