Implement heap_free_slots in GC.stat_heap

[Feature #20408]

Author: peterzhu2118

gc/default/default.c

@@ -7593,6 +7593,7 @@ rb_gc_impl_stat(void *objspace_ptr, VALUE hash_or_sym)
 enum gc_stat_heap_sym {
     gc_stat_heap_sym_slot_size,
     gc_stat_heap_sym_heap_live_slots,
+    gc_stat_heap_sym_heap_free_slots,
     gc_stat_heap_sym_heap_eden_pages,
     gc_stat_heap_sym_heap_eden_slots,
     gc_stat_heap_sym_total_allocated_pages,
@@ -7612,6 +7613,7 @@ setup_gc_stat_heap_symbols(void)
 #define S(s) gc_stat_heap_symbols[gc_stat_heap_sym_##s] = ID2SYM(rb_intern_const(#s))
         S(slot_size);
         S(heap_live_slots);
+        S(heap_free_slots);
         S(heap_eden_pages);
         S(heap_eden_slots);
         S(total_allocated_pages);
@@ -7634,6 +7636,7 @@ stat_one_heap(rb_heap_t *heap, VALUE hash, VALUE key)
 
     SET(slot_size, heap->slot_size);
     SET(heap_live_slots, heap->total_allocated_objects - heap->total_freed_objects - heap->final_slots_count);
+    SET(heap_free_slots, heap->total_slots - (heap->total_allocated_objects - heap->total_freed_objects));
     SET(heap_eden_pages, heap->total_pages);
     SET(heap_eden_slots, heap->total_slots);
     SET(total_allocated_pages, heap->total_allocated_pages);

test/ruby/test_gc.rb

@@ -232,6 +232,7 @@ def test_stat_heap
 
       assert_equal (GC::INTERNAL_CONSTANTS[:BASE_SLOT_SIZE] + GC::INTERNAL_CONSTANTS[:RVALUE_OVERHEAD]) * (2**i), stat_heap[:slot_size]
       assert_operator stat_heap[:heap_live_slots], :<=, stat[:heap_live_slots]
+      assert_operator stat_heap[:heap_free_slots], :<=, stat[:heap_free_slots]
       assert_operator stat_heap[:heap_eden_pages], :<=, stat[:heap_eden_pages]
       assert_operator stat_heap[:heap_eden_slots], :>=, 0
       assert_operator stat_heap[:total_allocated_pages], :>=, 0
@@ -262,7 +263,7 @@ def test_stat_heap_all
       GC.stat_heap(i, stat_heap)
 
       # Remove keys that can vary between invocations
-      %i(total_allocated_objects heap_live_slots).each do |sym|
+      %i(total_allocated_objects heap_live_slots heap_free_slots).each do |sym|
         stat_heap[sym] = stat_heap_all[i][sym] = 0
       end
 
@@ -288,6 +289,7 @@ def test_stat_heap_constraints
     end
 
     assert_equal stat[:heap_live_slots], stat_heap_sum[:heap_live_slots]
+    assert_equal stat[:heap_free_slots], stat_heap_sum[:heap_free_slots]
     assert_equal stat[:heap_eden_pages], stat_heap_sum[:heap_eden_pages]
     assert_equal stat[:heap_available_slots], stat_heap_sum[:heap_eden_slots]
     assert_equal stat[:total_allocated_objects], stat_heap_sum[:total_allocated_objects]