Zhengyu Gu

> This looks good but one question for ZGC, does ZGC need an OopMapCache::cleanup_old_entries() ? We still call `OopMapCache::cleanup_old_entries() ` after STW pause, but now concurrent phase also can accumulate...

I would suggest to move `OopMapCache::trigger_cleanup();` from `VM_ShenandoahReferenceOperation::doit_epilogue()` to `VM_ShenandoahOperation::doit_prologue()` and add the call to ` VM_ZOperation` and ` VM_XOperation`'s `doit_epilogue()`

/integrate Approved