Transition shape when object's capacity changes

This commit adds a `capacity` field to shapes, and adds shape
transitions whenever an object's capacity changes. Objects which are
allocated out of a bigger size pool will also make a transition from the
root shape to the shape with the correct capacity for their size pool
when they are allocated.

This commit will allow us to remove numiv from objects completely, and
will also mean we can guarantee that if two objects share shapes, their
IVs are in the same positions (an embedded and extended object cannot
share shapes). This will enable us to implement ivar sets in YJIT using
object shapes.

Co-Authored-By: Aaron Patterson <tenderlove@ruby-lang.org>

1 files changed, 38 insertions, 23 deletions

diff --git a/variable.c b/variable.c
index c9c4be1c43..bdde4d9607 100644
--- a/variable.c
+++ b/variable.c
@@ -1092,7 +1092,7 @@ rb_generic_shape_id(VALUE obj)
             shape_id = ivtbl->shape_id;
         }
         else if (OBJ_FROZEN(obj)) {
-            shape_id = FROZEN_ROOT_SHAPE_ID;
+            shape_id = SPECIAL_CONST_SHAPE_ID;
         }
     }
     RB_VM_LOCK_LEAVE();
@@ -1364,26 +1364,21 @@ rb_obj_transient_heap_evacuate(VALUE obj, int promote)
 #endif
 
 void
-rb_ensure_iv_list_size(VALUE obj, uint32_t len, uint32_t newsize)
+rb_ensure_iv_list_size(VALUE obj, uint32_t current_capacity, uint32_t new_capacity)
 {
     VALUE *ptr = ROBJECT_IVPTR(obj);
     VALUE *newptr;
 
     if (RBASIC(obj)->flags & ROBJECT_EMBED) {
-        newptr = obj_ivar_heap_alloc(obj, newsize);
-        MEMCPY(newptr, ptr, VALUE, len);
+        newptr = obj_ivar_heap_alloc(obj, new_capacity);
+        MEMCPY(newptr, ptr, VALUE, current_capacity);
         RB_FL_UNSET_RAW(obj, ROBJECT_EMBED);
         ROBJECT(obj)->as.heap.ivptr = newptr;
     }
     else {
-        newptr = obj_ivar_heap_realloc(obj, len, newsize);
+        newptr = obj_ivar_heap_realloc(obj, current_capacity, new_capacity);
     }
-
-#if USE_RVARGC
-    ROBJECT(obj)->numiv = newsize;
-#else
-    ROBJECT(obj)->as.heap.numiv = newsize;
-#endif
+    ROBJECT_SET_NUMIV(obj, new_capacity);
 }
 
 struct gen_ivtbl *
@@ -1407,12 +1402,25 @@ rb_ensure_generic_iv_list_size(VALUE obj, uint32_t newsize)
 }
 
 // @note May raise when there are too many instance variables.
-void
-rb_init_iv_list(VALUE obj)
+rb_shape_t *
+rb_grow_iv_list(VALUE obj)
 {
-    uint32_t newsize = (uint32_t)(rb_shape_get_shape(obj)->next_iv_index * 2.0);
     uint32_t len = ROBJECT_NUMIV(obj);
-    rb_ensure_iv_list_size(obj, len, newsize < len ? len : newsize);
+    RUBY_ASSERT(len > 0);
+    uint32_t newsize = (uint32_t)(len * 2);
+    rb_ensure_iv_list_size(obj, len, newsize);
+    rb_shape_t * res;
+
+#if USE_RVARGC
+    ROBJECT_SET_NUMIV(obj, newsize);
+#else
+    ROBJECT(obj)->as.heap.numiv = newsize;
+#endif
+
+    res = rb_shape_transition_shape_capa(rb_shape_get_shape(obj), newsize);
+    rb_shape_set_shape(obj, res);
+    RUBY_ASSERT(!RB_TYPE_P(obj, T_OBJECT) || ROBJECT_IV_CAPACITY(obj) == ROBJECT_NUMIV(obj));
+    return res;
 }
 
 static VALUE
@@ -1423,9 +1431,10 @@ obj_ivar_set(VALUE obj, ID id, VALUE val)
     // Get the current shape
     rb_shape_t * shape = rb_shape_get_shape_by_id(ROBJECT_SHAPE_ID(obj));
 
+    bool found = true;
     if (!rb_shape_get_iv_index(shape, id, &index)) {
-        shape = rb_shape_get_next(shape, obj, id);
-        index = shape->next_iv_index - 1;
+        index = shape->next_iv_index;
+        found = false;
     }
 
     uint32_t len = ROBJECT_NUMIV(obj);
@@ -1434,12 +1443,16 @@ obj_ivar_set(VALUE obj, ID id, VALUE val)
     // on this object until the buffer has been allocated, otherwise
     // GC could read off the end of the buffer.
     if (len <= index) {
-        uint32_t newsize = (uint32_t)((len + 1) * 1.25);
-        rb_ensure_iv_list_size(obj, len, newsize);
+        shape = rb_grow_iv_list(obj);
+    }
+
+    if (!found) {
+        shape = rb_shape_get_next(shape, obj, id);
+        RUBY_ASSERT(index == (shape->next_iv_index - 1));
+        rb_shape_set_shape(obj, shape);
     }
 
     RB_OBJ_WRITE(obj, &ROBJECT_IVPTR(obj)[index], val);
-    rb_shape_set_shape(obj, shape);
 
     return val;
 }
@@ -1475,7 +1488,7 @@ rb_shape_set_shape_id(VALUE obj, shape_id_t shape_id)
         RCLASS_EXT(obj)->shape_id = shape_id;
         break;
       default:
-        if (shape_id != FROZEN_ROOT_SHAPE_ID) {
+        if (shape_id != SPECIAL_CONST_SHAPE_ID) {
             struct gen_ivtbl *ivtbl = 0;
             RB_VM_LOCK_ENTER();
             {
@@ -1599,8 +1612,10 @@ iterate_over_shapes_with_callback(rb_shape_t *shape, rb_ivar_foreach_callback_fu
             callback(shape->edge_name, val, itr_data->arg);
         }
         return;
-      case SHAPE_IVAR_UNDEF:
+      case SHAPE_INITIAL_CAPACITY:
+      case SHAPE_CAPACITY_CHANGE:
       case SHAPE_FROZEN:
+      case SHAPE_IVAR_UNDEF:
         iterate_over_shapes_with_callback(rb_shape_get_shape_by_id(shape->parent_id), callback, itr_data);
         return;
     }
@@ -3922,7 +3937,7 @@ rb_iv_tbl_copy(VALUE dst, VALUE src)
     RUBY_ASSERT(rb_type(dst) == rb_type(src));
     RUBY_ASSERT(RB_TYPE_P(dst, T_CLASS) || RB_TYPE_P(dst, T_MODULE));
 
-    RUBY_ASSERT(RCLASS_SHAPE_ID(dst) == ROOT_SHAPE_ID);
+    RUBY_ASSERT(RCLASS_SHAPE_ID(dst) == ROOT_SHAPE_ID || rb_shape_get_shape_by_id(RCLASS_SHAPE_ID(dst))->type == SHAPE_INITIAL_CAPACITY);
     RUBY_ASSERT(!RCLASS_IVPTR(dst));
 
     rb_ivar_foreach(src, tbl_copy_i, dst);