8328602: Parallel: Incorrect assertion in fill_dense_prefix_end

Reviewed-by: gli, iwalulya

Author: albertnetymk

src/hotspot/share/gc/parallel/psParallelCompact.cpp

@@ -1106,16 +1106,28 @@ void PSParallelCompact::summarize_spaces_quick()
 }
 
 void PSParallelCompact::fill_dense_prefix_end(SpaceId id) {
-  // Since both markword and klass takes 1 heap word, the min-obj-size is 2
-  // heap words.
-  // If min-fill-size decreases to 1, this whole method becomes redundant.
-  assert(CollectedHeap::min_fill_size() == 2, "inv");
+  // Comparing two sizes to decide if filling is required:
+  //
+  // The size of the filler (min-obj-size) is 2 heap words with the default
+  // MinObjAlignment, since both markword and klass take 1 heap word.
+  //
+  // The size of the gap (if any) right before dense-prefix-end is
+  // MinObjAlignment.
+  //
+  // Need to fill in the gap only if it's smaller than min-obj-size, and the
+  // filler obj will extend to next region.
+
+  // Note: If min-fill-size decreases to 1, this whole method becomes redundant.
+  assert(CollectedHeap::min_fill_size() >= 2, "inv");
 #ifndef _LP64
-  // In 32-bit system, min-obj-alignment is >= 8 bytes, so the gap (if any)
-  // right before denses-prefix must be greater than min-fill-size; nothing to
-  // do.
+  // In 32-bit system, each heap word is 4 bytes, so MinObjAlignment == 2.
+  // The gap is always equal to min-fill-size, so nothing to do.
   return;
 #endif
+  if (MinObjAlignment > 1) {
+    return;
+  }
+  assert(CollectedHeap::min_fill_size() == 2, "inv");
   HeapWord* const dense_prefix_end = dense_prefix(id);
   RegionData* const region_after_dense_prefix = _summary_data.addr_to_region_ptr(dense_prefix_end);
   idx_t const dense_prefix_bit = _mark_bitmap.addr_to_bit(dense_prefix_end);