RFR: 8299703: GenShen: improvements in card scanning [v2]

[Y. Srinivas Ramakrishna]

> **Main changes:**
> 1. `process_clusters()` now finds and processes contiguous ranges of dirty cards, skipping over contiguous ranges of clean cards. For reading the diffs, it might be easiest to look at the new code, rather than view the side-by-side diffs.
> 2. the ShenandoahCardCluster class has been extended by a `block_start()` method which returns the first object in a card (which could be co-initial with the card); this method is used by the refactored `process_clusters()` above.
> 3. ShenandoahCardCluster class's `has_object()` method has been renamed `starts_object()` which more closely reflects the API.
> 4. ShenandoahCardStats class has been modified to better suit the way statistics are gathered in the rewritten `process_clusters()`. The larger-grain API should also result in less overhead for gathering the statistics and might (subject to measurement) allow it to be available in product/release builds (if so, that will be done in a separate follow-up ticket).
> 5. Added some const annotations.
> 
> **Testing & Implementation Notes:**
> 6. Tested with Extremem and SpecJBB, fastdebug, release, and product builds, with and without verification enabled.
> 7. Preliminary performance data with an Extremem workload show roughly 17-18% reduction in wall-clock durations of concurrent remembered set scanning across the distribution (p0, p25, p50, p75), p100 (max) was marginally down at 2%. The trend of the change was as expected since the gains are lost when we have a higher frequency of dirty/clean alternations with short dirty/clean runs.
> 8. More performance data with SPECjbb and a different Extremem workload are being gathered, and will be included. We will also include the impact on the concurrent update refs phase, as well as the overall impact on latency scores.
> 
> **Acknowledgments**:
> 9. Many thanks to @kdnilsen for feedback on an earlier version of the draft PR, which helped catch a crucial misunderstanding on the role of TAMS and marked objects, and helped fix the error that had been dogging me.
> 
> **Epilogue**:
> 10. Further performance improvements are possible, but are deferred for follow-up.

Y. Srinivas Ramakrishna has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 110 commits:

 - Merge branch 'master' into rs_scan
 - More const safety, some asserts, some comments.
 - Change type of loop variable to signed to allow correct termination for the case when start_card_index is 0. Nominal check for overflow when using signed type for card index.
 - Fix the direction of an address comparison, add a couple of assertions,
   and elaborate some comments. Passes heap verification handily now.
 - Fixes related to tams logic and iteration, block_start backwards walk
   loop, etc. from review feedback from @kdnilsen.
   
   More const safety, & elaboration of some comments.
 - TODO: marks the places identified in code walkthrough / review with
   @kdnilsen that need fixing up. These will be addressed in the next
   commit.
 - ...
 - Clean up & refine comments; rename a variable or two; add a couple of
   assertion checks.
 - Correct tracking of oldest address scanned; relax an overly strong
   assertion.
 - const some methods.
 - ... and 100 more: https://git.openjdk.org/shenandoah/compare/1974f58a...79b5f733

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Changes: https://git.openjdk.org/shenandoah/pull/193/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=193&range=01
  Stats: 836 lines in 11 files changed: 325 ins; 274 del; 237 mod
  Patch: https://git.openjdk.org/shenandoah/pull/193.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah pull/193/head:pull/193

PR: https://git.openjdk.org/shenandoah/pull/193