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, 22 insertions, 30 deletions

diff --git a/vm_insnhelper.c b/vm_insnhelper.c
index cff4b9138a..7b24392932 100644
--- a/vm_insnhelper.c
+++ b/vm_insnhelper.c
@@ -50,11 +50,6 @@ MJIT_STATIC VALUE
 ruby_vm_special_exception_copy(VALUE exc)
 {
     VALUE e = rb_obj_alloc(rb_class_real(RBASIC_CLASS(exc)));
-    rb_shape_t * shape = rb_shape_get_shape(exc);
-    if (rb_shape_frozen_shape_p(shape)) {
-        shape = rb_shape_get_shape_by_id(shape->parent_id);
-    }
-    rb_shape_set_shape(e, shape);
     rb_obj_copy_ivar(e, exc);
     return e;
 }
@@ -1310,37 +1305,33 @@ vm_setivar_slowpath(VALUE obj, ID id, VALUE val, const rb_iseq_t *iseq, IVC ic,
             rb_shape_t* shape = rb_shape_get_shape(obj);
             shape_id_t next_shape_id = ROBJECT_SHAPE_ID(obj);
 
-            rb_shape_t* next_shape = rb_shape_get_next(shape, obj, id);
+            if (!rb_shape_get_iv_index(shape, id, &index)) {
+                if (UNLIKELY(shape->next_iv_index >= num_iv)) {
+                    RUBY_ASSERT(shape->next_iv_index == num_iv);
 
-            if (shape != next_shape) {
-                RUBY_ASSERT(next_shape->parent_id == rb_shape_id(shape));
-                next_shape_id = rb_shape_id(next_shape);
-            }
+                    shape = rb_grow_iv_list(obj);
+                    RUBY_ASSERT(shape->type == SHAPE_CAPACITY_CHANGE);
+                }
+
+                index = shape->next_iv_index;
 
-            if (rb_shape_get_iv_index(next_shape, id, &index)) { // based off the hash stored in the transition tree
                 if (index >= MAX_IVARS) {
                     rb_raise(rb_eArgError, "too many instance variables");
                 }
 
-                populate_cache(index, next_shape_id, id, iseq, ic, cc, is_attr);
-            }
-            else {
-                rb_bug("Didn't find instance variable %s\n", rb_id2name(id));
-            }
-
-            // Ensure the IV buffer is wide enough to store the IV
-            if (UNLIKELY(index >= num_iv)) {
-                RUBY_ASSERT(index == num_iv);
-                rb_init_iv_list(obj);
-            }
+                rb_shape_t * next_shape = rb_shape_get_next(shape, obj, id);
+                RUBY_ASSERT(next_shape->type == SHAPE_IVAR);
+                RUBY_ASSERT(index == (next_shape->next_iv_index - 1));
+                next_shape_id = rb_shape_id(next_shape);
 
-            if (shape != next_shape) {
                 rb_shape_set_shape(obj, next_shape);
             }
+
+            populate_cache(index, next_shape_id, id, iseq, ic, cc, is_attr);
+
             VALUE *ptr = ROBJECT_IVPTR(obj);
             RB_OBJ_WRITE(obj, &ptr[index], val);
             RB_DEBUG_COUNTER_INC(ivar_set_ic_miss_iv_hit);
-
             return val;
         }
       case T_CLASS:
@@ -1450,17 +1441,18 @@ vm_setivar(VALUE obj, ID id, VALUE val, shape_id_t dest_shape_id, attr_index_t i
             else if (dest_shape_id != INVALID_SHAPE_ID) {
                 rb_shape_t *dest_shape = rb_shape_get_shape_by_id(dest_shape_id);
                 shape_id_t source_shape_id = dest_shape->parent_id;
-                if (shape_id == source_shape_id && dest_shape->edge_name == id && dest_shape->type == SHAPE_IVAR) {
+
+                RUBY_ASSERT(dest_shape->type == SHAPE_IVAR || dest_shape->type == SHAPE_IVAR_UNDEF);
+
+                if (shape_id == source_shape_id && dest_shape->edge_name == id) {
                     RUBY_ASSERT(dest_shape_id != INVALID_SHAPE_ID && shape_id != INVALID_SHAPE_ID);
-                    if (UNLIKELY(index >= ROBJECT_NUMIV(obj))) {
-                        rb_init_iv_list(obj);
-                    }
+                    RUBY_ASSERT(ROBJECT_IV_CAPACITY(obj) == ROBJECT_NUMIV(obj));
 
                     ROBJECT_SET_SHAPE_ID(obj, dest_shape_id);
 
-                    RUBY_ASSERT(rb_shape_get_next(rb_shape_get_shape_by_id(source_shape_id), obj, id) == dest_shape);
+                    RUBY_ASSERT(rb_shape_get_next_iv_shape(rb_shape_get_shape_by_id(source_shape_id), id) == dest_shape);
+                    RUBY_ASSERT(ROBJECT_IV_CAPACITY(obj) == ROBJECT_NUMIV(obj));
                     RUBY_ASSERT(index < ROBJECT_NUMIV(obj));
-
                 }
                 else {
                     break;