[vm, gc] As a debugging aid, remember the previous object during heap iteration.

Bug: #36906
Change-Id: Id7214aaa475929e33e5e5ae5bc58c99eb927b74b
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/127743
Reviewed-by: Alexander Markov <alexmarkov@google.com>
Reviewed-by: Siva Annamalai <asiva@google.com>
Commit-Queue: Ryan Macnak <rmacnak@google.com>

Author: rmacnak-google

runtime/vm/heap/pages.cc

@@ -96,10 +96,12 @@ void HeapPage::VisitObjects(ObjectVisitor* visitor) const {
   NoSafepointScope no_safepoint;
   uword obj_addr = object_start();
   uword end_addr = object_end();
+  RawObject* volatile debugging_prev = 0;  // TODO(36906): Remove.
   while (obj_addr < end_addr) {
     RawObject* raw_obj = RawObject::FromAddr(obj_addr);
     visitor->VisitObject(raw_obj);
     obj_addr += raw_obj->HeapSize();
+    debugging_prev = raw_obj;
   }
   ASSERT(obj_addr == end_addr);
 }
@@ -110,9 +112,11 @@ void HeapPage::VisitObjectPointers(ObjectPointerVisitor* visitor) const {
   NoSafepointScope no_safepoint;
   uword obj_addr = object_start();
   uword end_addr = object_end();
+  RawObject* volatile debugging_prev = 0;  // TODO(36906): Remove.
   while (obj_addr < end_addr) {
     RawObject* raw_obj = RawObject::FromAddr(obj_addr);
     obj_addr += raw_obj->VisitPointers(visitor);
+    debugging_prev = raw_obj;
   }
   ASSERT(obj_addr == end_addr);
 }

runtime/vm/heap/scavenger.cc

@@ -956,9 +956,11 @@ void Scavenger::VisitObjectPointers(ObjectPointerVisitor* visitor) const {
          (Thread::Current()->task_kind() == Thread::kCompactorTask));
   MakeNewSpaceIterable();
   uword cur = FirstObjectStart();
+  RawObject* volatile debugging_prev = 0;  // TODO(36906): Remove.
   while (cur < top_) {
     RawObject* raw_obj = RawObject::FromAddr(cur);
     cur += raw_obj->VisitPointers(visitor);
+    debugging_prev = raw_obj;
   }
 }
 
@@ -967,10 +969,12 @@ void Scavenger::VisitObjects(ObjectVisitor* visitor) const {
          (Thread::Current()->task_kind() == Thread::kMarkerTask));
   MakeNewSpaceIterable();
   uword cur = FirstObjectStart();
+  RawObject* volatile debugging_prev = 0;  // TODO(36906): Remove.
   while (cur < top_) {
     RawObject* raw_obj = RawObject::FromAddr(cur);
     visitor->VisitObject(raw_obj);
     cur += raw_obj->HeapSize();
+    debugging_prev = raw_obj;
   }
 }