Remove write barrier exemption for T_ICLASS

Before this commit, iclasses were "shady", or not protected by write
barriers. Because of that, the GC needs to spend more time marking these
objects than otherwise.

Applications that make heavy use of modules should see reduction in GC
time as they have a significant number of live iclasses on the heap.

 - Put logic for iclass method table ownership into a function
 - Remove calls to WB_UNPROTECT and insert write barriers for iclasses

This commit relies on the following invariant: for any non oirigin
iclass `I`, `RCLASS_M_TBL(I) == RCLASS_M_TBL(RBasic(I)->klass)`. This
invariant did not hold prior to 98286e9 for classes and modules that
have prepended modules.

[Feature #16984]

6 files changed, 35 insertions, 21 deletions

diff --git a/class.c b/class.c
index 0d97603821..9876e996c8 100644
--- a/class.c
+++ b/class.c
@@ -911,8 +911,7 @@ rb_include_class_new(VALUE module, VALUE super)
 {
     VALUE klass = class_alloc(T_ICLASS, rb_cClass);
 
-    RCLASS_M_TBL(OBJ_WB_UNPROTECT(klass)) =
-      RCLASS_M_TBL(OBJ_WB_UNPROTECT(module)); /* TODO: unprotected? */
+    RCLASS_M_TBL(klass) = RCLASS_M_TBL(module);
 
     RCLASS_SET_ORIGIN(klass, klass);
     if (BUILTIN_TYPE(module) == T_ICLASS) {
@@ -1098,13 +1097,12 @@ move_refined_method(ID key, VALUE value, void *data)
 	    const rb_method_entry_t *orig_me = me->def->body.refined.orig_me, *new_me;
 	    RB_OBJ_WRITE(me, &me->def->body.refined.orig_me, NULL);
 	    new_me = rb_method_entry_clone(me);
-	    rb_id_table_insert(tbl, key, (VALUE)new_me);
-	    RB_OBJ_WRITTEN(klass, Qundef, new_me);
+            rb_method_table_insert(klass, tbl, key, new_me);
 	    rb_method_entry_copy(me, orig_me);
 	    return ID_TABLE_CONTINUE;
 	}
 	else {
-	    rb_id_table_insert(tbl, key, (VALUE)me);
+            rb_method_table_insert(klass, tbl, key, me);
 	    return ID_TABLE_DELETE;
 	}
     }
@@ -1119,7 +1117,6 @@ ensure_origin(VALUE klass)
     VALUE origin = RCLASS_ORIGIN(klass);
     if (origin == klass) {
 	origin = class_alloc(T_ICLASS, klass);
-	OBJ_WB_UNPROTECT(origin); /* TODO: conservative shading. Need more survey. */
 	RCLASS_SET_SUPER(origin, RCLASS_SUPER(klass));
 	RCLASS_SET_SUPER(klass, origin);
 	RCLASS_SET_ORIGIN(klass, origin);
diff --git a/eval.c b/eval.c
index 237d9acaa7..e4fec3d6e1 100644
--- a/eval.c
+++ b/eval.c
@@ -1420,8 +1420,7 @@ rb_using_refinement(rb_cref_t *cref, VALUE klass, VALUE module)
     c = iclass = rb_include_class_new(module, superclass);
     RB_OBJ_WRITE(c, &RCLASS_REFINED_CLASS(c), klass);
 
-    RCLASS_M_TBL(OBJ_WB_UNPROTECT(c)) =
-      RCLASS_M_TBL(OBJ_WB_UNPROTECT(module)); /* TODO: check unprotecting */
+    RCLASS_M_TBL(c) = RCLASS_M_TBL(module);
 
     module = RCLASS_SUPER(module);
     while (module && module != klass) {
diff --git a/gc.c b/gc.c
index 788f06f158..11a79e69fe 100644
--- a/gc.c
+++ b/gc.c
@@ -2854,8 +2854,7 @@ obj_free(rb_objspace_t *objspace, VALUE obj)
 	break;
       case T_ICLASS:
 	/* Basically , T_ICLASS shares table with the module */
-        if (FL_TEST(obj, RICLASS_IS_ORIGIN) &&
-                !FL_TEST(obj, RICLASS_ORIGIN_SHARED_MTBL)) {
+        if (RICLASS_OWNS_M_TBL_P(obj)) {
             /* Method table is not shared for origin iclasses of classes */
             rb_id_table_free(RCLASS_M_TBL(obj));
         }
@@ -3974,8 +3973,7 @@ obj_memsize_of(VALUE obj, int use_all_types)
 	}
 	break;
       case T_ICLASS:
-        if (FL_TEST(obj, RICLASS_IS_ORIGIN) &&
-                !FL_TEST(obj, RICLASS_ORIGIN_SHARED_MTBL)) {
+        if (RICLASS_OWNS_M_TBL_P(obj)) {
 	    if (RCLASS_M_TBL(obj)) {
 		size += rb_id_table_memsize(RCLASS_M_TBL(obj));
 	    }
@@ -5504,8 +5502,7 @@ gc_mark_children(rb_objspace_t *objspace, VALUE obj)
 	break;
 
       case T_ICLASS:
-        if (FL_TEST(obj, RICLASS_IS_ORIGIN) &&
-                !FL_TEST(obj, RICLASS_ORIGIN_SHARED_MTBL)) {
+        if (RICLASS_OWNS_M_TBL_P(obj)) {
 	    mark_m_tbl(objspace, RCLASS_M_TBL(obj));
 	}
         if (RCLASS_SUPER(obj)) {
diff --git a/internal/class.h b/internal/class.h
index 40938255af..663a1e8dbf 100644
--- a/internal/class.h
+++ b/internal/class.h
@@ -144,6 +144,12 @@ RICLASS_SET_ORIGIN_SHARED_MTBL(VALUE iclass)
     FL_SET(iclass, RICLASS_ORIGIN_SHARED_MTBL);
 }
 
+static inline bool
+RICLASS_OWNS_M_TBL_P(VALUE iclass)
+{
+    return FL_TEST_RAW(iclass, RICLASS_IS_ORIGIN | RICLASS_ORIGIN_SHARED_MTBL) == RICLASS_IS_ORIGIN;
+}
+
 static inline void
 RCLASS_SET_INCLUDER(VALUE iclass, VALUE klass)
 {
diff --git a/method.h b/method.h
index 6bc494f4b0..dae0a4f43f 100644
--- a/method.h
+++ b/method.h
@@ -188,6 +188,8 @@ struct rb_method_definition_struct {
     uintptr_t method_serial;
 };
 
+struct rb_id_table;
+
 typedef struct rb_method_definition_struct rb_method_definition_t;
 STATIC_ASSERT(sizeof_method_def, offsetof(rb_method_definition_t, body)==8);
 
@@ -230,6 +232,8 @@ const rb_method_entry_t *rb_method_entry_clone(const rb_method_entry_t *me);
 const rb_callable_method_entry_t *rb_method_entry_complement_defined_class(const rb_method_entry_t *src_me, ID called_id, VALUE defined_class);
 void rb_method_entry_copy(rb_method_entry_t *dst, const rb_method_entry_t *src);
 
+void rb_method_table_insert(VALUE klass, struct rb_id_table *table, ID method_id, const rb_method_entry_t *me);
+
 void rb_scope_visibility_set(rb_method_visibility_t);
 
 VALUE rb_unnamed_parameters(int arity);
diff --git a/vm_method.c b/vm_method.c
index 037ac2a655..58516b9ddd 100644
--- a/vm_method.c
+++ b/vm_method.c
@@ -174,11 +174,9 @@ clear_method_cache_by_id_in_class(VALUE klass, ID mid)
                     // invalidate cc by invalidating cc->cme
                     VALUE owner = cme->owner;
                     VM_ASSERT(BUILTIN_TYPE(owner) == T_CLASS);
-                    rb_callable_method_entry_t *new_cme =
-                      (rb_callable_method_entry_t *)rb_method_entry_clone((const rb_method_entry_t *)cme);
-                    struct rb_id_table *mtbl = RCLASS_M_TBL(RCLASS_ORIGIN(owner));
-                    rb_id_table_insert(mtbl, mid, (VALUE)new_cme);
-                    RB_OBJ_WRITTEN(owner, cme, new_cme);
+                    const rb_method_entry_t *new_cme = rb_method_entry_clone((const rb_method_entry_t *)cme);
+                    VALUE origin = RCLASS_ORIGIN(owner);
+                    rb_method_table_insert(origin, RCLASS_M_TBL(origin), mid, new_cme);
                 }
                 vm_me_invalidate_cache((rb_callable_method_entry_t *)cme);
                 RB_DEBUG_COUNTER_INC(cc_invalidate_tree_cme);
@@ -261,6 +259,19 @@ rb_clear_method_cache_all(void)
     rb_objspace_each_objects(invalidate_all_cc, NULL);
 }
 
+void
+rb_method_table_insert(VALUE klass, struct rb_id_table *table, ID method_id, const rb_method_entry_t *me)
+{
+    VALUE table_owner = klass;
+    if (RB_TYPE_P(klass, T_ICLASS) && !RICLASS_OWNS_M_TBL_P(klass)) {
+        table_owner = RBASIC(table_owner)->klass;
+    }
+    VM_ASSERT(RB_TYPE_P(table_owner, T_CLASS) || RB_TYPE_P(table_owner, T_ICLASS) || RB_TYPE_P(table_owner, T_MODULE));
+    VM_ASSERT(table == RCLASS_M_TBL(table_owner));
+    rb_id_table_insert(table, method_id, (VALUE)me);
+    RB_OBJ_WRITTEN(table_owner, Qundef, (VALUE)me);
+}
+
 VALUE
 rb_f_notimplement(int argc, const VALUE *argv, VALUE obj, VALUE marker)
 {
@@ -802,8 +813,7 @@ rb_method_entry_make(VALUE klass, ID mid, VALUE defined_class, rb_method_visibil
 	make_method_entry_refined(klass, me);
     }
 
-    rb_id_table_insert(mtbl, mid, (VALUE)me);
-    RB_OBJ_WRITTEN(klass, Qundef, (VALUE)me);
+    rb_method_table_insert(klass, mtbl, mid, me);
 
     VM_ASSERT(me->def != NULL);
 
@@ -973,6 +983,7 @@ prepare_callable_method_entry(VALUE defined_class, ID id, const rb_method_entry_
                 }
                 cme = rb_method_entry_complement_defined_class(me, me->called_id, defined_class);
                 rb_id_table_insert(mtbl, id, (VALUE)cme);
+                RB_OBJ_WRITTEN(defined_class, Qundef, (VALUE)cme);
                 VM_ASSERT(callable_method_entry_p(cme));
             }
             else {