8314329: AgeTable: add is_clear() & allocation spec, and relax assert to allow use of 0-index slot

AgeTable changes:
1. add CHeap allocation spec
2. add a non-product const is_clear() method to check if the age table has only zero entries
3. relax the assertion that the 0th index of the age table is never used, to allow use of that slot

Reviewed-by: wkemper, kdnilsen, ayang, shade

Author: Y. Srinivas Ramakrishna

src/hotspot/share/gc/shared/ageTable.cpp

@@ -71,6 +71,15 @@ void AgeTable::clear() {
   }
 }
 
+#ifndef PRODUCT
+bool AgeTable::is_clear() const {
+  for (const size_t* p = sizes; p < sizes + table_size; ++p) {
+    if (*p != 0) return false;
+  }
+  return true;
+}
+#endif // !PRODUCT
+
 void AgeTable::merge(const AgeTable* subTable) {
   for (int i = 0; i < table_size; i++) {
     sizes[i]+= subTable->sizes[i];

src/hotspot/share/gc/shared/ageTable.hpp

@@ -25,6 +25,7 @@
 #ifndef SHARE_GC_SHARED_AGETABLE_HPP
 #define SHARE_GC_SHARED_AGETABLE_HPP
 
+#include "memory/allocation.hpp"
 #include "oops/markWord.hpp"
 #include "oops/oop.hpp"
 #include "runtime/perfDataTypes.hpp"
@@ -36,7 +37,7 @@
 //
 // Note: all sizes are in oops
 
-class AgeTable {
+class AgeTable: public CHeapObj<mtGC> {
   friend class VMStructs;
 
  public:
@@ -53,16 +54,20 @@ class AgeTable {
   // clear table
   void clear();
 
+#ifndef PRODUCT
+  // check whether it's clear
+  bool is_clear() const;
+#endif // !PRODUCT
+
   // add entry
   inline void add(oop p, size_t oop_size);
 
   void add(uint age, size_t oop_size) {
-    assert(age > 0 && age < table_size, "invalid age of object");
+    assert(age < table_size, "invalid age of object");
     sizes[age] += oop_size;
   }
 
-  // Merge another age table with the current one.  Used
-  // for parallel young generation gc.
+  // Merge another age table with the current one.
   void merge(const AgeTable* subTable);
 
   // Calculate new tenuring threshold based on age information.