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, 3 insertions, 6 deletions

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)) {