Created on behalf of yude.lyd@alibaba-inc.com
----
When we call block_start(addr), we ask the BOT to give us the starting address of the block that covers 'addr'.
I think BOT is not efficiently handling the queries if we call block_start(addr) sequentially, that is, call it multiple times
with ascending addresses and close to each other.


For example, for two addresses addr1 and addr2, we might have the following heap layout:


| q | addr1 | addr2
v v v
——————————————————————————————————————————
| | |
——————————————————————————————————————————
^
| gc alloc block ------------------- |


It's possible that BOT only records the large gc-allocated blocks but not individual objects in them.
So when we call block_start() with either addr1 or addr2, it will return q.
BOT has the ability to fix itself. When we call block_start(addr1), it will fix from q to addr1, looking
for all of the real objects in the range and update the entries. After that BOT will not return the
gc-allocated block address q but the real objects' addresses.
So calling block_start(addr1) and block_start(addr2) in different orders result in different work:

Case 1: {
call block_start(addr1)

block_start returns q

enter slow path {
  fix bot from q to addr1
  return head_of(addr1)
}

call block_start(addr2)

block_start returns q

enter slow path {
  fix bot from q to addr2
  return head_of(addr2)
}
}

Case 2: {
call block_start(addr2)

block_start returns q

enter slow path {
  fix bot from q to addr2
  return head_of(addr2)
}

call block_start(addr1)

will not enter slow path because range q to addr1 is already fixed
return head_of(addr1)
}


The difference between Case 1 and 2 is the repeated traversal between q and addr1.
This affects G1ScanHRForRegionClosure::scan_heap_roots and potentially any future code that walks card table for dirty oops in
ascending address order. It won't matter a lot if the address are sparse enough. But, say if we do -XX:G1ConcRefinementGreenZone=1000000
and log the function forward_to_block_containing_addr_slow(), we will find there are some entries with the same q.