Support GC.auto_compact = :empty on debug builds

This commit adds `GC.auto_compact = :empty` which will run
auto-compaction sorting pages by empty slots so the most amount of
objects will be moved. This will make it easier to write tests for
auto-compaction.

gc.c

@@ -1342,6 +1342,9 @@ int ruby_gc_debug_indent = 0;
 VALUE rb_mGC;
 int ruby_disable_gc = 0;
 int ruby_enable_autocompact = 0;
+#if RGENGC_CHECK_MODE
+gc_compact_compare_func ruby_autocompact_compare_func;
+#endif
 
 void rb_iseq_mark_and_move(rb_iseq_t *iseq, bool referece_updating);
 void rb_iseq_free(const rb_iseq_t *iseq);
@@ -9549,6 +9552,9 @@ gc_start(rb_objspace_t *objspace, unsigned int reason)
     /* Explicitly enable compaction (GC.compact) */
     if (do_full_mark && ruby_enable_autocompact) {
         objspace->flags.during_compacting = TRUE;
+#if RGENGC_CHECK_MODE
+        objspace->rcompactor.compare_func = ruby_autocompact_compare_func;
+#endif
     }
     else {
         objspace->flags.during_compacting = !!(reason & GPR_FLAG_COMPACT);
@@ -11883,6 +11889,18 @@ gc_set_auto_compact(VALUE _, VALUE v)
     GC_ASSERT(GC_COMPACTION_SUPPORTED);
 
     ruby_enable_autocompact = RTEST(v);
+
+#if RGENGC_CHECK_MODE
+    ruby_autocompact_compare_func = NULL;
+
+    if (SYMBOL_P(v)) {
+        ID id = RB_SYM2ID(v);
+        if (id == rb_intern("empty")) {
+            ruby_autocompact_compare_func = compare_free_slots;
+        }
+    }
+#endif
+
     return v;
 }
 #else

tool/lib/envutil.rb

@@ -245,9 +245,9 @@ def under_gc_stress(stress = true)
   end
   module_function :under_gc_stress
 
-  def under_gc_compact_stress(&block)
+  def under_gc_compact_stress(val = :empty, &block)
     auto_compact = GC.auto_compact
-    GC.auto_compact = true
+    GC.auto_compact = val
     under_gc_stress(&block)
   ensure
     GC.auto_compact = auto_compact