RFR: Use allocation counter instead of timestamp to track region's age

[Aleksey Shipilev]

On 08/02/2017 07:49 PM, Zhengyu Gu wrote:
> Use allocation counter to track heap regions' age for generational and LRU GC.
> 
> Webrev: http://cr.openjdk.java.net/~zgu/shenandoah/alloc_counter/webrev.00/index.html

Pull the recent updates, partial heuristics now has much less fields:

*) Just unary minus here:

   69     return -1 * compare_by_alloc_seq_ascending(a, b);

*) This SIZE_FORMAT_W(8) here, because it should match the header?

  83           SIZE_FORMAT_W(20) ", " SIZE_FORMAT_W(20) ", %8u, {",

*) Typos, "Global allocation":

  37   // Gobal alloaction counter, increased for each allocation

*) Excess whitespace. Also, comment should mention they are also set on allocation paths.

  60   // Seq numbers are used for generational and Least Recently Used heuristics.
  61   // They are set when a region is discarded
  62   uint64_t  _first_alloc_seq_num;
  63   uint64_t  _last_alloc_seq_num;

*) Do we actually care about "- 1" here?

 104   static uint64_t alloc_seq_num() {
 105     // Last used seq number
 106     return AllocSeqNum - 1;
 107   }

*) Just "...seq_num = ++AllocSeqNum"?
*) I guess it is safer to compare top() and bottom() directly here, instead of calling superclass?

  36     _last_alloc_seq_num = AllocSeqNum;
  37     ++ AllocSeqNum;
  38     if (is_empty()) {

Thanks,
-Aleksey