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+++ b/src/hotspot/share/gc/shenandoah/shenandoahScanRemembered.inline.hpp
@@ -0,0 +1,405 @@
+/*
+ * Copyright Amazon.com Inc. or its affiliates. All Rights Reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ *
+ */
+
+#ifndef SHARE_GC_SHENANDOAH_SHENANDOAHSCANREMEMBEREDINLINE_HPP
+#define SHARE_GC_SHENANDOAH_SHENANDOAHSCANREMEMBEREDINLINE_HPP
+
+#include "memory/iterator.hpp"
+#include "oops/oop.hpp"
+#include "oops/objArrayOop.hpp"
+#include "gc/shared/collectorCounters.hpp"
+#include "gc/shenandoah/shenandoahCardStats.hpp"
+#include "gc/shenandoah/shenandoahCardTable.hpp"
+#include "gc/shenandoah/shenandoahHeap.hpp"
+#include "gc/shenandoah/shenandoahHeapRegion.hpp"
+#include "gc/shenandoah/shenandoahOldGeneration.hpp"
+#include "gc/shenandoah/shenandoahScanRemembered.hpp"
+#include "gc/shenandoah/mode/shenandoahMode.hpp"
+#include "logging/log.hpp"
+
+// Process all objects starting within count clusters beginning with first_cluster and for which the start address is
+// less than end_of_range.  For any non-array object whose header lies on a dirty card, scan the entire object,
+// even if its end reaches beyond end_of_range. Object arrays, on the other hand, are precisely dirtied and
+// only the portions of the array on dirty cards need to be scanned.
+//
+// Do not CANCEL within process_clusters.  It is assumed that if a worker thread accepts responsibility for processing
+// a chunk of work, it will finish the work it starts.  Otherwise, the chunk of work will be lost in the transition to
+// degenerated execution, leading to dangling references.
+template <typename ClosureType>
+void ShenandoahScanRemembered::process_clusters(size_t first_cluster, size_t count, HeapWord* end_of_range,
+                                                               ClosureType* cl, bool use_write_table, uint worker_id) {
+
+  assert(ShenandoahHeap::heap()->old_generation()->is_parsable(), "Old generation regions must be parsable for remembered set scan");
+  // If old-gen evacuation is active, then MarkingContext for old-gen heap regions is valid.  We use the MarkingContext
+  // bits to determine which objects within a DIRTY card need to be scanned.  This is necessary because old-gen heap
+  // regions that are in the candidate collection set have not been coalesced and filled.  Thus, these heap regions
+  // may contain zombie objects.  Zombie objects are known to be dead, but have not yet been "collected".  Scanning
+  // zombie objects is unsafe because the Klass pointer is not reliable, objects referenced from a zombie may have been
+  // collected (if dead), or relocated (if live), or if dead but not yet collected, we don't want to "revive" them
+  // by marking them (when marking) or evacuating them (when updating references).
+
+  // start and end addresses of range of objects to be scanned, clipped to end_of_range
+  const size_t start_card_index = first_cluster * ShenandoahCardCluster::CardsPerCluster;
+  const HeapWord* start_addr = _rs->addr_for_card_index(start_card_index);
+  // clip at end_of_range (exclusive)
+  HeapWord* end_addr = MIN2(end_of_range, (HeapWord*)start_addr + (count * ShenandoahCardCluster::CardsPerCluster
+                                                                   * CardTable::card_size_in_words()));
+  assert(start_addr < end_addr, "Empty region?");
+
+  const size_t whole_cards = (end_addr - start_addr + CardTable::card_size_in_words() - 1)/CardTable::card_size_in_words();
+  const size_t end_card_index = start_card_index + whole_cards - 1;
+  log_debug(gc, remset)("Worker %u: cluster = " SIZE_FORMAT " count = " SIZE_FORMAT " eor = " INTPTR_FORMAT
+                        " start_addr = " INTPTR_FORMAT " end_addr = " INTPTR_FORMAT " cards = " SIZE_FORMAT,
+                        worker_id, first_cluster, count, p2i(end_of_range), p2i(start_addr), p2i(end_addr), whole_cards);
+
+  // use_write_table states whether we are using the card table that is being
+  // marked by the mutators. If false, we are using a snapshot of the card table
+  // that is not subject to modifications. Even when this arg is true, and
+  // the card table is being actively marked, SATB marking ensures that we need not
+  // worry about cards marked after the processing here has passed them.
+  const CardValue* const ctbm = _rs->get_card_table_byte_map(use_write_table);
+
+  // If old gen evacuation is active, ctx will hold the completed marking of
+  // old generation objects. We'll only scan objects that are marked live by
+  // the old generation marking. These include objects allocated since the
+  // start of old generation marking (being those above TAMS).
+  const ShenandoahHeap* heap = ShenandoahHeap::heap();
+  const ShenandoahMarkingContext* ctx = heap->old_generation()->is_mark_complete() ?
+                                        heap->marking_context() : nullptr;
+
+  // The region we will scan is the half-open interval [start_addr, end_addr),
+  // and lies entirely within a single region.
+  const ShenandoahHeapRegion* region = ShenandoahHeap::heap()->heap_region_containing(start_addr);
+  assert(region->contains(end_addr - 1), "Slice shouldn't cross regions");
+
+  // This code may have implicit assumptions of examining only old gen regions.
+  assert(region->is_old(), "We only expect to be processing old regions");
+  assert(!region->is_humongous(), "Humongous regions can be processed more efficiently;"
+                                  "see process_humongous_clusters()");
+  // tams and ctx below are for old generation marking. As such, young gen roots must
+  // consider everything above tams, since it doesn't represent a TAMS for young gen's
+  // SATB marking.
+  const HeapWord* tams = (ctx == nullptr ? region->bottom() : ctx->top_at_mark_start(region));
+
+  NOT_PRODUCT(ShenandoahCardStats stats(whole_cards, card_stats(worker_id));)
+
+  // In the case of imprecise marking, we remember the lowest address
+  // scanned in a range of dirty cards, as we work our way left from the
+  // highest end_addr. This serves as another upper bound on the address we will
+  // scan as we move left over each contiguous range of dirty cards.
+  HeapWord* upper_bound = nullptr;
+
+  // Starting at the right end of the address range, walk backwards accumulating
+  // a maximal dirty range of cards, then process those cards.
+  ssize_t cur_index = (ssize_t) end_card_index;
+  assert(cur_index >= 0, "Overflow");
+  assert(((ssize_t)start_card_index) >= 0, "Overflow");
+  while (cur_index >= (ssize_t)start_card_index) {
+
+    // We'll continue the search starting with the card for the upper bound
+    // address identified by the last dirty range that we processed, if any,
+    // skipping any cards at higher addresses.
+    if (upper_bound != nullptr) {
+      ssize_t right_index = _rs->card_index_for_addr(upper_bound);
+      assert(right_index >= 0, "Overflow");
+      cur_index = MIN2(cur_index, right_index);
+      assert(upper_bound < end_addr, "Program logic");
+      end_addr  = upper_bound;   // lower end_addr
+      upper_bound = nullptr;     // and clear upper_bound
+      if (end_addr <= start_addr) {
+        assert(right_index <= (ssize_t)start_card_index, "Program logic");
+        // We are done with our cluster
+        return;
+      }
+    }
+
+    if (ctbm[cur_index] == CardTable::dirty_card_val()) {
+      // ==== BEGIN DIRTY card range processing ====
+
+      const size_t dirty_r = cur_index;  // record right end of dirty range (inclusive)
+      while (--cur_index >= (ssize_t)start_card_index && ctbm[cur_index] == CardTable::dirty_card_val()) {
+        // walk back over contiguous dirty cards to find left end of dirty range (inclusive)
+      }
+      // [dirty_l, dirty_r] is a "maximal" closed interval range of dirty card indices:
+      // it may not be maximal if we are using the write_table, because of concurrent
+      // mutations dirtying the card-table. It may also not be maximal if an upper bound
+      // was established by the scan of the previous chunk.
+      const size_t dirty_l = cur_index + 1;   // record left end of dirty range (inclusive)
+      // Check that we identified a boundary on our left
+      assert(ctbm[dirty_l] == CardTable::dirty_card_val(), "First card in range should be dirty");
+      assert(dirty_l == start_card_index || use_write_table
+             || ctbm[dirty_l - 1] == CardTable::clean_card_val(),
+             "Interval isn't maximal on the left");
+      assert(dirty_r >= dirty_l, "Error");
+      assert(ctbm[dirty_r] == CardTable::dirty_card_val(), "Last card in range should be dirty");
+      // Record alternations, dirty run length, and dirty card count
+      NOT_PRODUCT(stats.record_dirty_run(dirty_r - dirty_l + 1);)
+
+      // Find first object that starts this range:
+      // [left, right) is a maximal right-open interval of dirty cards
+      HeapWord* left = _rs->addr_for_card_index(dirty_l);        // inclusive
+      HeapWord* right = _rs->addr_for_card_index(dirty_r + 1);   // exclusive
+      // Clip right to end_addr established above (still exclusive)
+      right = MIN2(right, end_addr);
+      assert(right <= region->top() && end_addr <= region->top(), "Busted bounds");
+      const MemRegion mr(left, right);
+
+      // NOTE: We'll not call block_start() repeatedly
+      // on a very large object if its head card is dirty. If not,
+      // (i.e. the head card is clean) we'll call it each time we
+      // process a new dirty range on the object. This is always
+      // the case for large object arrays, which are typically more
+      // common.
+      HeapWord* p = _scc->block_start(dirty_l);
+      oop obj = cast_to_oop(p);
+
+      // PREFIX: The object that straddles into this range of dirty cards
+      // from the left may be subject to special treatment unless
+      // it is an object array.
+      if (p < left && !obj->is_objArray()) {
+        // The mutator (both compiler and interpreter, but not JNI?)
+        // typically dirty imprecisely (i.e. only the head of an object),
+        // but GC closures typically dirty the object precisely. (It would
+        // be nice to have everything be precise for maximum efficiency.)
+        //
+        // To handle this, we check the head card of the object here and,
+        // if dirty, (arrange to) scan the object in its entirety. If we
+        // find the head card clean, we'll scan only the portion of the
+        // object lying in the dirty card range below, assuming this was
+        // the result of precise marking by GC closures.
+
+        // index of the "head card" for p
+        const size_t hc_index = _rs->card_index_for_addr(p);
+        if (ctbm[hc_index] == CardTable::dirty_card_val()) {
+          // Scan or skip the object, depending on location of its
+          // head card, and remember that we'll have processed all
+          // the objects back up to p, which is thus an upper bound
+          // for the next iteration of a dirty card loop.
+          upper_bound = p;   // remember upper bound for next chunk
+          if (p < start_addr) {
+            // if object starts in a previous slice, it'll be handled
+            // in its entirety by the thread processing that slice; we can
+            // skip over it and avoid an unnecessary extra scan.
+            assert(obj == cast_to_oop(p), "Inconsistency detected");
+            p += obj->size();
+          } else {
+            // the object starts in our slice, we scan it in its entirety
+            assert(obj == cast_to_oop(p), "Inconsistency detected");
+            if (ctx == nullptr || ctx->is_marked(obj)) {
+              // Scan the object in its entirety
+              p += obj->oop_iterate_size(cl);
+            } else {
+              assert(p < tams, "Error 1 in ctx/marking/tams logic");
+              // Skip over any intermediate dead objects
+              p = ctx->get_next_marked_addr(p, tams);
+              assert(p <= tams, "Error 2 in ctx/marking/tams logic");
+            }
+          }
+          assert(p > left, "Should have processed into interior of dirty range");
+        }
+      }
+
+      size_t i = 0;
+      HeapWord* last_p = nullptr;
+
+      // BODY: Deal with (other) objects in this dirty card range
+      while (p < right) {
+        obj = cast_to_oop(p);
+        // walk right scanning eligible objects
+        if (ctx == nullptr || ctx->is_marked(obj)) {
+          // we need to remember the last object ptr we scanned, in case we need to
+          // complete a partial suffix scan after mr, see below
+          last_p = p;
+          // apply the closure to the oops in the portion of
+          // the object within mr.
+          p += obj->oop_iterate_size(cl, mr);
+          NOT_PRODUCT(i++);
+        } else {
+          // forget the last object pointer we remembered
+          last_p = nullptr;
+          assert(p < tams, "Tams and above are implicitly marked in ctx");
+          // object under tams isn't marked: skip to next live object
+          p = ctx->get_next_marked_addr(p, tams);
+          assert(p <= tams, "Error 3 in ctx/marking/tams logic");
+        }
+      }
+
+      // SUFFIX: Fix up a possible incomplete scan at right end of window
+      // by scanning the portion of a non-objArray that wasn't done.
+      if (p > right && last_p != nullptr) {
+        assert(last_p < right, "Error");
+        // check if last_p suffix needs scanning
+        const oop last_obj = cast_to_oop(last_p);
+        if (!last_obj->is_objArray()) {
+          // scan the remaining suffix of the object
+          const MemRegion last_mr(right, p);
+          assert(p == last_p + last_obj->size(), "Would miss portion of last_obj");
+          last_obj->oop_iterate(cl, last_mr);
+          log_develop_debug(gc, remset)("Fixed up non-objArray suffix scan in [" INTPTR_FORMAT ", " INTPTR_FORMAT ")",
+                                        p2i(last_mr.start()), p2i(last_mr.end()));
+        } else {
+          log_develop_debug(gc, remset)("Skipped suffix scan of objArray in [" INTPTR_FORMAT ", " INTPTR_FORMAT ")",
+                                        p2i(right), p2i(p));
+        }
+      }
+      NOT_PRODUCT(stats.record_scan_obj_cnt(i);)
+
+      // ==== END   DIRTY card range processing ====
+    } else {
+      // ==== BEGIN CLEAN card range processing ====
+
+      // If we are using the write table (during update refs, e.g.), a mutator may dirty
+      // a card at any time. This is fine for the algorithm below because it is only
+      // counting contiguous runs of clean cards (and only for non-product builds).
+      assert(use_write_table || ctbm[cur_index] == CardTable::clean_card_val(), "Error");
+
+      // walk back over contiguous clean cards
+      size_t i = 0;
+      while (--cur_index >= (ssize_t)start_card_index && ctbm[cur_index] == CardTable::clean_card_val()) {
+        NOT_PRODUCT(i++);
+      }
+      // Record alternations, clean run length, and clean card count
+      NOT_PRODUCT(stats.record_clean_run(i);)
+
+      // ==== END CLEAN card range processing ====
+    }
+  }
+}
+
+// Given that this range of clusters is known to span a humongous object spanned by region r, scan the
+// portion of the humongous object that corresponds to the specified range.
+template <typename ClosureType>
+inline void
+ShenandoahScanRemembered::process_humongous_clusters(ShenandoahHeapRegion* r, size_t first_cluster, size_t count,
+                                                                    HeapWord *end_of_range, ClosureType *cl, bool use_write_table) {
+  ShenandoahHeapRegion* start_region = r->humongous_start_region();
+  HeapWord* p = start_region->bottom();
+  oop obj = cast_to_oop(p);
+  assert(r->is_humongous(), "Only process humongous regions here");
+  assert(start_region->is_humongous_start(), "Should be start of humongous region");
+  assert(p + obj->size() >= end_of_range, "Humongous object ends before range ends");
+
+  size_t first_card_index = first_cluster * ShenandoahCardCluster::CardsPerCluster;
+  HeapWord* first_cluster_addr = _rs->addr_for_card_index(first_card_index);
+  size_t spanned_words = count * ShenandoahCardCluster::CardsPerCluster * CardTable::card_size_in_words();
+  start_region->oop_iterate_humongous_slice_dirty(cl, first_cluster_addr, spanned_words, use_write_table);
+}
+
+
+// This method takes a region & determines the end of the region that the worker can scan.
+template <typename ClosureType>
+inline void
+ShenandoahScanRemembered::process_region_slice(ShenandoahHeapRegion *region, size_t start_offset, size_t clusters,
+                                                              HeapWord *end_of_range, ClosureType *cl, bool use_write_table,
+                                                              uint worker_id) {
+
+  // This is called only for young gen collection, when we scan old gen regions
+  assert(region->is_old(), "Expecting an old region");
+  HeapWord *start_of_range = region->bottom() + start_offset;
+  size_t start_cluster_no = cluster_for_addr(start_of_range);
+  assert(addr_for_cluster(start_cluster_no) == start_of_range, "process_region_slice range must align on cluster boundary");
+
+  // region->end() represents the end of memory spanned by this region, but not all of this
+  //   memory is eligible to be scanned because some of this memory has not yet been allocated.
+  //
+  // region->top() represents the end of allocated memory within this region.  Any addresses
+  //   beyond region->top() should not be scanned as that memory does not hold valid objects.
+
+  if (use_write_table) {
+    // This is update-refs servicing.
+    if (end_of_range > region->get_update_watermark()) {
+      end_of_range = region->get_update_watermark();
+    }
+  } else {
+    // This is concurrent mark servicing.  Note that TAMS for this region is TAMS at start of old-gen
+    // collection.  Here, we need to scan up to TAMS for most recently initiated young-gen collection.
+    // Since all LABs are retired at init mark, and since replacement LABs are allocated lazily, and since no
+    // promotions occur until evacuation phase, TAMS for most recent young-gen is same as top().
+    if (end_of_range > region->top()) {
+      end_of_range = region->top();
+    }
+  }
+
+  log_debug(gc)("Remembered set scan processing Region " SIZE_FORMAT ", from " PTR_FORMAT " to " PTR_FORMAT ", using %s table",
+                region->index(), p2i(start_of_range), p2i(end_of_range),
+                use_write_table? "read/write (updating)": "read (marking)");
+
+  // Note that end_of_range may point to the middle of a cluster because we limit scanning to
+  // region->top() or region->get_update_watermark(). We avoid processing past end_of_range.
+  // Objects that start between start_of_range and end_of_range, including humongous objects, will
+  // be fully processed by process_clusters. In no case should we need to scan past end_of_range.
+  if (start_of_range < end_of_range) {
+    if (region->is_humongous()) {
+      ShenandoahHeapRegion* start_region = region->humongous_start_region();
+      process_humongous_clusters(start_region, start_cluster_no, clusters, end_of_range, cl, use_write_table);
+    } else {
+      process_clusters(start_cluster_no, clusters, end_of_range, cl, use_write_table, worker_id);
+    }
+  }
+}
+
+inline bool ShenandoahRegionChunkIterator::has_next() const {
+  return _index < _total_chunks;
+}
+
+inline bool ShenandoahRegionChunkIterator::next(struct ShenandoahRegionChunk *assignment) {
+  if (_index >= _total_chunks) {
+    return false;
+  }
+  size_t new_index = Atomic::add(&_index, (size_t) 1, memory_order_relaxed);
+  if (new_index > _total_chunks) {
+    // First worker that hits new_index == _total_chunks continues, other
+    // contending workers return false.
+    return false;
+  }
+  // convert to zero-based indexing
+  new_index--;
+  assert(new_index < _total_chunks, "Error");
+
+  // Find the group number for the assigned chunk index
+  size_t group_no;
+  for (group_no = 0; new_index >= _group_entries[group_no]; group_no++)
+    ;
+  assert(group_no < _num_groups, "Cannot have group no greater or equal to _num_groups");
+
+  // All size computations measured in HeapWord
+  size_t region_size_words = ShenandoahHeapRegion::region_size_words();
+  size_t group_region_index = _region_index[group_no];
+  size_t group_region_offset = _group_offset[group_no];
+
+  size_t index_within_group = (group_no == 0)? new_index: new_index - _group_entries[group_no - 1];
+  size_t group_chunk_size = _group_chunk_size[group_no];
+  size_t offset_of_this_chunk = group_region_offset + index_within_group * group_chunk_size;
+  size_t regions_spanned_by_chunk_offset = offset_of_this_chunk / region_size_words;
+  size_t offset_within_region = offset_of_this_chunk % region_size_words;
+
+  size_t region_index = group_region_index + regions_spanned_by_chunk_offset;
+
+  assignment->_r = _heap->get_region(region_index);
+  assignment->_chunk_offset = offset_within_region;
+  assignment->_chunk_size = group_chunk_size;
+  return true;
+}
+
+#endif   // SHARE_GC_SHENANDOAH_SHENANDOAHSCANREMEMBEREDINLINE_HPP
