Allow getting the generic_iv_tbl_

This will allow the Rust part of the binding to inspect the number of
entries in the generic_iv_tbl_, useful when displaying statistics in the
eBPF timeline.

internal/mmtk.h

@@ -91,6 +91,7 @@ typedef struct MMTk_RubyUpcalls {
     void (*update_global_symbols_table)(void);
     void (*update_overloaded_cme_table)(void);
     void (*update_ci_table)(void);
+    struct st_table *(*get_generic_iv_tbl)(void);
     struct st_table *(*get_frozen_strings_table)(void);
     struct st_table *(*get_finalizer_table)(void);
     struct st_table *(*get_obj_to_id_table)(void);

mmtk_support.c

@@ -1675,6 +1675,7 @@ void rb_mmtk_update_global_symbols_table(void); // Defined in gc.c
 void rb_mmtk_update_overloaded_cme_table(void); // Defined in default.c
 void rb_mmtk_update_ci_table(void); // Defined in default.c
 
+st_table* rb_mmtk_get_generic_iv_tbl(void); // Defined in variable.c
 st_table* rb_mmtk_get_frozen_strings_table(void); // Defined in default.c
 st_table* rb_mmtk_get_finalizer_table(void); // Defined in default.c
 st_table* rb_mmtk_get_obj_to_id_table(void); // Defined in default.c
@@ -1714,6 +1715,7 @@ MMTk_RubyUpcalls ruby_upcalls = {
     rb_mmtk_update_global_symbols_table,
     rb_mmtk_update_overloaded_cme_table,
     rb_mmtk_update_ci_table,
+    rb_mmtk_get_generic_iv_tbl,
     rb_mmtk_get_frozen_strings_table,
     rb_mmtk_get_finalizer_table,
     rb_mmtk_get_obj_to_id_table,

variable.c

@@ -1172,6 +1172,11 @@ rb_ref_update_generic_ivar(VALUE obj)
 }
 
 #if USE_MMTK
+st_table*
+rb_mmtk_get_generic_iv_tbl(void)
+{
+    return generic_iv_tbl_;
+}
 
 static int
 rb_mmtk_cleanup_generic_iv_tbl_check(st_data_t key, st_data_t value, st_data_t argp, int error)