# HG changeset patch
# User Haoyu Li <leihouyju@gmail.com>
# Date 1574947389 -28800
#      Thu Nov 28 21:23:09 2019 +0800
# Node ID 5fed2afe6aba481046a1541aede6c575253e2291
# Parent  358594a083bddabc040a86cf59929941b769d4b8
improve comments of shadow regions

diff -r 358594a083bd -r 5fed2afe6aba src/hotspot/share/gc/parallel/psCompactionManager.cpp
--- a/src/hotspot/share/gc/parallel/psCompactionManager.cpp	Wed Nov 27 10:11:03 2019 +0800
+++ b/src/hotspot/share/gc/parallel/psCompactionManager.cpp	Thu Nov 28 21:23:09 2019 +0800
@@ -176,8 +176,8 @@
   while (true) {
     MutexLocker ml(_shadow_region_monitor, Mutex::_no_safepoint_check_flag);
     if (_shadow_region_array->is_empty()) {
-      // The corresponding heap region may be available now,
-      // so we don't need to acquire a shadow region anymore,
+      // Check if the corresponding heap region is available now.
+      // If so, we don't need to get a shadow region anymore,
       // and we return 0 to indicate this case
       if (region_ptr->claimed()) {
         return 0;
diff -r 358594a083bd -r 5fed2afe6aba src/hotspot/share/gc/parallel/psParallelCompact.cpp
--- a/src/hotspot/share/gc/parallel/psParallelCompact.cpp	Wed Nov 27 10:11:03 2019 +0800
+++ b/src/hotspot/share/gc/parallel/psParallelCompact.cpp	Thu Nov 28 21:23:09 2019 +0800
@@ -2974,7 +2974,9 @@
         cm->push_region(sd.region(cur));
       } else if (cur->mark_copied()) {
         // Try to copy the content of the shadow region back to its corresponding
-        // heap region if the shadow region is filled
+        // heap region if the shadow region is filled. Otherwise, the GC thread
+        // fills the shadow region will copy the data back (see
+        // MoveAndUpdateShadowClosure::complete_region).
         copy_back(sd.region_to_addr(cur->shadow_region()), sd.region_to_addr(cur));
         cm->push_shadow_region_mt_safe(cur->shadow_region());
         cur->set_completed();
@@ -3219,7 +3221,7 @@
 // the unavailable region becomes available, the data in the shadow region will be copied back.
 // Shadow regions are empty regions in the to-space and regions between top and end of other spaces.
 //
-// More more details, please refer to §4.2 of the VEE'19 paper:
+// For more details, please refer to §4.2 of the VEE'19 paper:
 // Haoyu Li, Mingyu Wu, Binyu Zang, and Haibo Chen. 2019. ScissorGC: scalable and efficient
 // compaction for Java full garbage collection. In Proceedings of the 15th ACM SIGPLAN/SIGOPS
 // International Conference on Virtual Execution Environments (VEE 2019). ACM, New York, NY, USA,
@@ -3392,10 +3394,13 @@
   assert(region_ptr->shadow_state() == ParallelCompactData::RegionData::Shadow, "Region should be shadow");
   // Record the shadow region index
   region_ptr->set_shadow_region(_shadow);
-  // Mark the shadow region filled
+  // Mark the shadow region as filled to indicate the data is ready to be
+  // copied back
   region_ptr->mark_filled();
   // Try to copy the content of the shadow region back to its corresponding
-  // heap region if available
+  // heap region if available; the GC thread that decreases the destination
+  // count to zero will do the copying otherwise (see
+  // PSParallelCompact::decrement_destination_counts).
   if (((region_ptr->available() && region_ptr->claim()) || region_ptr->claimed()) && region_ptr->mark_copied()) {
     region_ptr->set_completed();
     PSParallelCompact::copy_back(PSParallelCompact::summary_data().region_to_addr(_shadow), dest_addr);