8240668: G1 list of all PerRegionTable does not have to be a double linkedlist any more

Reviewed-by: kbarrett, tschatzl

src/hotspot/share/gc/g1/heapRegionRemSet.cpp

@@ -102,60 +102,20 @@ void OtherRegionsTable::link_to_all(PerRegionTable* prt) {
   // We always append to the beginning of the list for convenience;
   // the order of entries in this list does not matter.
   if (_first_all_fine_prts != NULL) {
-    assert(_first_all_fine_prts->prev() == NULL, "invariant");
-    _first_all_fine_prts->set_prev(prt);
     prt->set_next(_first_all_fine_prts);
   } else {
     // this is the first element we insert. Adjust the "last" pointer
     _last_all_fine_prts = prt;
     assert(prt->next() == NULL, "just checking");
   }
-  // the new element is always the first element without a predecessor
-  prt->set_prev(NULL);
   _first_all_fine_prts = prt;
 
-  assert(prt->prev() == NULL, "just checking");
   assert(_first_all_fine_prts == prt, "just checking");
   assert((_first_all_fine_prts == NULL && _last_all_fine_prts == NULL) ||
          (_first_all_fine_prts != NULL && _last_all_fine_prts != NULL),
          "just checking");
   assert(_last_all_fine_prts == NULL || _last_all_fine_prts->next() == NULL,
          "just checking");
-  assert(_first_all_fine_prts == NULL || _first_all_fine_prts->prev() == NULL,
-         "just checking");
-}
-
-void OtherRegionsTable::unlink_from_all(PerRegionTable* prt) {
-  if (prt->prev() != NULL) {
-    assert(_first_all_fine_prts != prt, "just checking");
-    prt->prev()->set_next(prt->next());
-    // removing the last element in the list?
-    if (_last_all_fine_prts == prt) {
-      _last_all_fine_prts = prt->prev();
-    }
-  } else {
-    assert(_first_all_fine_prts == prt, "just checking");
-    _first_all_fine_prts = prt->next();
-    // list is empty now?
-    if (_first_all_fine_prts == NULL) {
-      _last_all_fine_prts = NULL;
-    }
-  }
-
-  if (prt->next() != NULL) {
-    prt->next()->set_prev(prt->prev());
-  }
-
-  prt->set_next(NULL);
-  prt->set_prev(NULL);
-
-  assert((_first_all_fine_prts == NULL && _last_all_fine_prts == NULL) ||
-         (_first_all_fine_prts != NULL && _last_all_fine_prts != NULL),
-         "just checking");
-  assert(_last_all_fine_prts == NULL || _last_all_fine_prts->next() == NULL,
-         "just checking");
-  assert(_first_all_fine_prts == NULL || _first_all_fine_prts->prev() == NULL,
-         "just checking");
 }
 
 CardIdx_t OtherRegionsTable::card_within_region(OopOrNarrowOopStar within_region, HeapRegion* hr) {

src/hotspot/share/gc/g1/heapRegionRemSet.hpp

@@ -81,8 +81,8 @@ class OtherRegionsTable {
   PerRegionTable** _fine_grain_regions;
   size_t           _n_fine_entries;
 
-  // The fine grain remembered sets are doubly linked together using
-  // their 'next' and 'prev' fields.
+  // The fine grain remembered sets are linked together using
+  // their 'next' fields.
   // This allows fast bulk freeing of all the fine grain remembered
   // set entries, and fast finding of all of them without iterating
   // over the _fine_grain_regions table.
@@ -113,8 +113,6 @@ class OtherRegionsTable {
 
   // link/add the given fine grain remembered set into the "all" list
   void link_to_all(PerRegionTable * prt);
-  // unlink/remove the given fine grain remembered set into the "all" list
-  void unlink_from_all(PerRegionTable * prt);
 
   bool contains_reference_locked(OopOrNarrowOopStar from) const;
 
@@ -167,9 +165,6 @@ class PerRegionTable: public CHeapObj<mtGC> {
   // next pointer for free/allocated 'all' list
   PerRegionTable* _next;
 
-  // prev pointer for the allocated 'all' list
-  PerRegionTable* _prev;
-
   // next pointer in collision list
   PerRegionTable * _collision_list_next;
 
@@ -181,7 +176,7 @@ protected:
     _hr(hr),
     _bm(HeapRegion::CardsPerRegion, mtGC),
     _occupied(0),
-    _next(NULL), _prev(NULL),
+    _next(NULL),
     _collision_list_next(NULL)
   {}
 
@@ -243,16 +238,11 @@ public:
 
   PerRegionTable* next() const { return _next; }
   void set_next(PerRegionTable* next) { _next = next; }
-  PerRegionTable* prev() const { return _prev; }
-  void set_prev(PerRegionTable* prev) { _prev = prev; }
 
   // Accessor and Modification routines for the pointer for the
   // singly linked collision list that links the PRTs within the
   // OtherRegionsTable::_fine_grain_regions hash table.
   //
-  // It might be useful to also make the collision list doubly linked
-  // to avoid iteration over the collisions list during scrubbing/deletion.
-  // OTOH there might not be many collisions.
 
   PerRegionTable* collision_list_next() const {
     return _collision_list_next;

src/hotspot/share/gc/g1/heapRegionRemSet.inline.hpp

@@ -62,7 +62,6 @@ inline bool PerRegionTable::add_reference(OopOrNarrowOopStar from) {
 inline void PerRegionTable::init(HeapRegion* hr, bool clear_links_to_all_list) {
   if (clear_links_to_all_list) {
     set_next(NULL);
-    set_prev(NULL);
   }
   _collision_list_next = NULL;
   _occupied = 0;