The constructor of MemAllocator::Allocation calls JavaThread::current()

https://github.com/openjdk/jdk/blob/a9d21c61fb12a11e18c6bb8aa903e5a8e42473f1/src/hotspot/share/gc/shared/memAllocator.cpp#L73

  Allocation(const MemAllocator& allocator, oop* obj_ptr)
    : _allocator(allocator),
      _thread(JavaThread::current()),

This can cause slow down when we allocate a lot of objects inside a tight loop in native code (E.g., JDK-8310823)

Since the code assumes that the current thread by a Java thread, we can replace the JavaThread::current() call with:

    assert(Thread::current()->is_Java_thread(), "must be used by JavaThreads only");
    assert(Thread::current() == allocator._thread, "must be");
    _thread = JavaThread::cast(allocator._thread);