[Feature #18045] Implement size classes for GC

This commits implements size classes in the GC for the Variable Width
Allocation feature. Unless `USE_RVARGC` compile flag is set, only a
single size class is created, maintaining current behaviour. See the
redmine ticket for more details.

Co-authored-by: Aaron Patterson <tenderlove@ruby-lang.org>

class.c

@@ -185,8 +185,7 @@ class_alloc(VALUE flags, VALUE klass)
     RVARGC_NEWOBJ_OF(obj, struct RClass, klass, (flags & T_MASK) | FL_PROMOTED1 /* start from age == 2 */ | (RGENGC_WB_PROTECTED_CLASS ? FL_WB_PROTECTED : 0), payload_size);
 
 #if USE_RVARGC
-    obj->ptr = (rb_classext_t *)rb_rvargc_payload_data_ptr((VALUE)obj + rb_slot_size());
+    obj->ptr = (rb_classext_t *)rb_gc_rvargc_object_data((VALUE)obj);
-    RB_OBJ_WRITTEN(obj, Qundef, (VALUE)obj + rb_slot_size());
 #else
     obj->ptr = ZALLOC(rb_classext_t);
 #endif

internal/gc.h

@@ -101,6 +101,7 @@ static inline void *ruby_sized_xrealloc2_inlined(void *ptr, size_t new_count, si
 static inline void ruby_sized_xfree_inlined(void *ptr, size_t size);
 VALUE rb_class_allocate_instance(VALUE klass);
 void rb_gc_ractor_newobj_cache_clear(rb_ractor_newobj_cache_t *newobj_cache);
+void *rb_gc_rvargc_object_data(VALUE obj);
 
 RUBY_SYMBOL_EXPORT_BEGIN
 /* gc.c (export) */
@@ -116,7 +117,6 @@ void rb_gc_mark_vm_stack_values(long n, const VALUE *values);
 void *ruby_sized_xrealloc(void *ptr, size_t new_size, size_t old_size) RUBY_ATTR_RETURNS_NONNULL RUBY_ATTR_ALLOC_SIZE((2));
 void *ruby_sized_xrealloc2(void *ptr, size_t new_count, size_t element_size, size_t old_count) RUBY_ATTR_RETURNS_NONNULL RUBY_ATTR_ALLOC_SIZE((2, 3));
 void ruby_sized_xfree(void *x, size_t size);
-int rb_slot_size(void);
 RUBY_SYMBOL_EXPORT_END
 
 MJIT_SYMBOL_EXPORT_BEGIN

misc/lldb_cruby.py

@@ -533,14 +533,15 @@ class HeapPageIter:
         self.target = target
         self.start = page.GetChildMemberWithName('start').GetValueAsUnsigned();
         self.num_slots = page.GetChildMemberWithName('total_slots').unsigned
+        self.slot_size = page.GetChildMemberWithName('size_pool').GetChildMemberWithName('slot_size').unsigned
         self.counter = 0
         self.tRBasic = target.FindFirstType("struct RBasic")
         self.tRValue = target.FindFirstType("struct RVALUE")
 
     def is_valid(self):
         heap_page_header_size = self.target.FindFirstType("struct heap_page_header").GetByteSize()
-        rvalue_size = self.tRValue.GetByteSize()
+        rvalue_size = self.slot_size
-        heap_page_obj_limit = int((HEAP_PAGE_SIZE - heap_page_header_size) / rvalue_size)
+        heap_page_obj_limit = int((HEAP_PAGE_SIZE - heap_page_header_size) / self.slot_size)
 
         return (heap_page_obj_limit - 1) <= self.num_slots <= heap_page_obj_limit
 
@@ -549,7 +550,7 @@ class HeapPageIter:
 
     def __next__(self):
         if self.counter < self.num_slots:
-            obj_addr_i = self.start + (self.counter * self.tRValue.GetByteSize())
+            obj_addr_i = self.start + (self.counter * self.slot_size)
             obj_addr = lldb.SBAddress(obj_addr_i, self.target)
             slot_info = (self.counter, obj_addr_i, self.target.CreateValueFromAddress("object", obj_addr, self.tRBasic))
             self.counter += 1