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]

1 files changed, 18 insertions, 7 deletions

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 {