Adding GC.compact and compacting GC support.

This commit adds the new method `GC.compact` and compacting GC support.
Please see this issue for caveats:

  https://bugs.ruby-lang.org/issues/15626

[Feature #15626]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@67479 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

NEWS

@@ -98,6 +98,15 @@ JIT::
 
   * Default value of +--jit-min-calls+ is changed from 5 to 10,000
 
+GC::
+
+  * New `GC.compact` method for compacting the heap.
+    This function compacts live objects in the heap so that fewer pages may
+    be used, and the heap may be more CoW friendly. [Feature #15626]
+
+    Details on the algorithm and caveats can be found here:
+    https://bugs.ruby-lang.org/issues/15626
+
 === Miscellaneous changes
 
 * Require compilers to support C99 [Misc #15347]

class.c

@@ -539,6 +539,7 @@ boot_defclass(const char *name, VALUE super)
 
     rb_name_class(obj, id);
     rb_const_set((rb_cObject ? rb_cObject : obj), id, obj);
+    rb_vm_add_root_module(id, obj);
     return obj;
 }
 
@@ -730,6 +731,9 @@ rb_define_class_id_under(VALUE outer, ID id, VALUE super)
 		     " (%"PRIsVALUE" is given but was %"PRIsVALUE")",
 		     outer, rb_id2str(id), RCLASS_SUPER(klass), super);
 	}
+        /* Class may have been defined in Ruby and not pin-rooted */
+        rb_vm_add_root_module(id, klass);
+
 	return klass;
     }
     if (!super) {
@@ -740,6 +744,7 @@ rb_define_class_id_under(VALUE outer, ID id, VALUE super)
     rb_set_class_path_string(klass, outer, rb_id2str(id));
     rb_const_set(outer, id, klass);
     rb_class_inherited(super, klass);
+    rb_vm_add_root_module(id, klass);
     rb_gc_register_mark_object(klass);
 
     return klass;
@@ -777,10 +782,13 @@ rb_define_module(const char *name)
 	    rb_raise(rb_eTypeError, "%s is not a module (%"PRIsVALUE")",
 		     name, rb_obj_class(module));
 	}
+        /* Module may have been defined in Ruby and not pin-rooted */
+        rb_vm_add_root_module(id, module);
 	return module;
     }
     module = rb_define_module_id(id);
     rb_vm_add_root_module(id, module);
+    rb_gc_register_mark_object(module);
     rb_const_set(rb_cObject, id, module);
 
     return module;

constant.h

@@ -31,8 +31,8 @@ typedef enum {
 typedef struct rb_const_entry_struct {
     rb_const_flag_t flag;
     int line;
-    const VALUE value;            /* should be mark */
-    const VALUE file;             /* should be mark */
+    VALUE value;            /* should be mark */
+    VALUE file;             /* should be mark */
 } rb_const_entry_t;
 
 VALUE rb_mod_private_constant(int argc, const VALUE *argv, VALUE obj);