P4
`HeapRegion::block_is_obj(const HeapWord* p)` returns where the block pointed by `p` is a proper obj (having a valid klass pointer).
Because `p` points block-start, it must be in the allocated part of the region `[bottom, top)`, and never points *into* an object (i.e. not `is_continues_humongous()`).
Because `p` points block-start, it must be in the allocated part of the region `[bottom, top)`, and never points *into* an object (i.e. not `is_continues_humongous()`).
