This commit implements the Object Shapes technique in CRuby.

Object Shapes is used for accessing instance variables and representing the
"frozenness" of objects.  Object instances have a "shape" and the shape
represents some attributes of the object (currently which instance variables are
set and the "frozenness").  Shapes form a tree data structure, and when a new
instance variable is set on an object, that object "transitions" to a new shape
in the shape tree.  Each shape has an ID that is used for caching. The shape
structure is independent of class, so objects of different types can have the
same shape.

For example:

```ruby
class Foo
  def initialize
    # Starts with shape id 0
    @a = 1 # transitions to shape id 1
    @b = 1 # transitions to shape id 2
  end
end

class Bar
  def initialize
    # Starts with shape id 0
    @a = 1 # transitions to shape id 1
    @b = 1 # transitions to shape id 2
  end
end

foo = Foo.new # `foo` has shape id 2
bar = Bar.new # `bar` has shape id 2
```

Both `foo` and `bar` instances have the same shape because they both set
instance variables of the same name in the same order.

This technique can help to improve inline cache hits as well as generate more
efficient machine code in JIT compilers.

This commit also adds some methods for debugging shapes on objects.  See
`RubyVM::Shape` for more details.

For more context on Object Shapes, see [Feature: #18776]

Co-Authored-By: Aaron Patterson <tenderlove@ruby-lang.org>
Co-Authored-By: Eileen M. Uchitelle <eileencodes@gmail.com>
Co-Authored-By: John Hawthorn <john@hawthorn.email>

vm_callinfo.h

@@ -10,6 +10,7 @@
 
 #include "debug_counter.h"
 #include "internal/class.h"
+#include "shape.h"
 
 enum vm_call_flag_bits {
     VM_CALL_ARGS_SPLAT_bit,     /* m(*args) */
@@ -284,14 +285,32 @@ struct rb_callcache {
     const vm_call_handler call_;
 
     union {
-        const unsigned int attr_index;
+        struct {
+          const attr_index_t index;
+          shape_id_t dest_shape_id;
+        } attr;
         const enum method_missing_reason method_missing_reason; /* used by method_missing */
         VALUE v;
     } aux_;
 };
 
-#define VM_CALLCACHE_UNMARKABLE IMEMO_FL_USER0
-#define VM_CALLCACHE_ON_STACK   IMEMO_FL_USER1
+#define VM_CALLCACHE_UNMARKABLE FL_FREEZE
+#define VM_CALLCACHE_ON_STACK   FL_EXIVAR
+
+extern const struct rb_callcache *rb_vm_empty_cc(void);
+extern const struct rb_callcache *rb_vm_empty_cc_for_super(void);
+
+#define vm_cc_empty() rb_vm_empty_cc()
+
+static inline void
+vm_cc_attr_index_initialize(const struct rb_callcache *cc, shape_id_t shape_id)
+{
+    VM_ASSERT(IMEMO_TYPE_P(cc, imemo_callcache));
+    VM_ASSERT(cc != vm_cc_empty());
+    IMEMO_SET_CACHED_SHAPE_ID((VALUE)cc, shape_id);
+    *(attr_index_t *)&cc->aux_.attr.index = 0;
+    *(shape_id_t *)&cc->aux_.attr.dest_shape_id = shape_id;
+}
 
 static inline const struct rb_callcache *
 vm_cc_new(VALUE klass,
@@ -299,6 +318,7 @@ vm_cc_new(VALUE klass,
           vm_call_handler call)
 {
     const struct rb_callcache *cc = (const struct rb_callcache *)rb_imemo_new(imemo_callcache, (VALUE)cme, (VALUE)call, 0, klass);
+    vm_cc_attr_index_initialize(cc, INVALID_SHAPE_ID);
     RB_DEBUG_COUNTER_INC(cc_new);
     return cc;
 }
@@ -350,30 +370,71 @@ vm_cc_call(const struct rb_callcache *cc)
     return cc->call_;
 }
 
-static inline unsigned int
+static inline attr_index_t
 vm_cc_attr_index(const struct rb_callcache *cc)
 {
     VM_ASSERT(IMEMO_TYPE_P(cc, imemo_callcache));
-    return cc->aux_.attr_index - 1;
+    return cc->aux_.attr.index - 1;
 }
 
 static inline bool
 vm_cc_attr_index_p(const struct rb_callcache *cc)
 {
     VM_ASSERT(IMEMO_TYPE_P(cc, imemo_callcache));
-    return cc->aux_.attr_index > 0;
+    return cc->aux_.attr.index != 0;
 }
 
-static inline uint32_t
-vm_ic_entry_index(const struct iseq_inline_iv_cache_entry *ic)
+static inline shape_id_t
+vm_cc_attr_index_source_shape_id(const struct rb_callcache *cc)
 {
-    return ic->entry->index;
+    VM_ASSERT(IMEMO_TYPE_P(cc, imemo_callcache));
+
+    return IMEMO_CACHED_SHAPE_ID((VALUE)cc);
+}
+
+static inline shape_id_t
+vm_cc_attr_shape_id(const struct rb_callcache *cc)
+{
+    VM_ASSERT(IMEMO_TYPE_P(cc, imemo_callcache));
+    return vm_cc_attr_index_source_shape_id(cc);
+}
+
+static inline shape_id_t
+vm_cc_attr_index_dest_shape_id(const struct rb_callcache *cc)
+{
+    VM_ASSERT(IMEMO_TYPE_P(cc, imemo_callcache));
+
+    return cc->aux_.attr.dest_shape_id;
+}
+
+static inline attr_index_t
+vm_ic_attr_index(const struct iseq_inline_iv_cache_entry *ic)
+{
+    return ic->attr_index - 1;
 }
 
 static inline bool
-vm_ic_entry_p(const struct iseq_inline_iv_cache_entry *ic)
+vm_ic_attr_index_p(const struct iseq_inline_iv_cache_entry *ic)
 {
-    return ic->entry;
+    return ic->attr_index > 0;
+}
+
+static inline shape_id_t
+vm_ic_attr_shape_id(const struct iseq_inline_iv_cache_entry *ic)
+{
+    return ic->source_shape_id;
+}
+
+static inline shape_id_t
+vm_ic_attr_index_source_shape_id(const struct iseq_inline_iv_cache_entry *ic)
+{
+    return ic->source_shape_id;
+}
+
+static inline shape_id_t
+vm_ic_attr_index_dest_shape_id(const struct iseq_inline_iv_cache_entry *ic)
+{
+    return ic->dest_shape_id;
 }
 
 static inline unsigned int
@@ -407,10 +468,6 @@ vm_cc_valid_p(const struct rb_callcache *cc, const rb_callable_method_entry_t *c
     }
 }
 
-extern const struct rb_callcache *rb_vm_empty_cc(void);
-extern const struct rb_callcache *rb_vm_empty_cc_for_super(void);
-#define vm_cc_empty() rb_vm_empty_cc()
-
 /* callcache: mutate */
 
 static inline void
@@ -422,26 +479,29 @@ vm_cc_call_set(const struct rb_callcache *cc, vm_call_handler call)
 }
 
 static inline void
-vm_cc_attr_index_set(const struct rb_callcache *cc, int index)
+vm_cc_attr_index_set(const struct rb_callcache *cc, attr_index_t index, shape_id_t source_shape_id, shape_id_t dest_shape_id)
 {
     VM_ASSERT(IMEMO_TYPE_P(cc, imemo_callcache));
     VM_ASSERT(cc != vm_cc_empty());
-    *(int *)&cc->aux_.attr_index = index + 1;
+    IMEMO_SET_CACHED_SHAPE_ID((VALUE)cc, source_shape_id);
+    *(attr_index_t *)&cc->aux_.attr.index = (index + 1);
+    *(shape_id_t *)&cc->aux_.attr.dest_shape_id = dest_shape_id;
 }
 
 static inline void
-vm_ic_entry_set(struct iseq_inline_iv_cache_entry *ic, struct rb_iv_index_tbl_entry *entry, const rb_iseq_t *iseq)
+vm_ic_attr_index_set(const rb_iseq_t *iseq, const struct iseq_inline_iv_cache_entry *ic, attr_index_t index, shape_id_t source_shape_id, shape_id_t dest_shape_id)
 {
-    ic->entry = entry;
-    RB_OBJ_WRITTEN(iseq, Qundef, entry->class_value);
+    *(shape_id_t *)&ic->source_shape_id = source_shape_id;
+    *(shape_id_t *)&ic->dest_shape_id = dest_shape_id;
+    *(attr_index_t *)&ic->attr_index = index + 1;
 }
 
 static inline void
-vm_cc_attr_index_initialize(const struct rb_callcache *cc)
+vm_ic_attr_index_initialize(const struct iseq_inline_iv_cache_entry *ic, shape_id_t shape_id)
 {
-    VM_ASSERT(IMEMO_TYPE_P(cc, imemo_callcache));
-    VM_ASSERT(cc != vm_cc_empty());
-    *(int *)&cc->aux_.attr_index = 0;
+    *(shape_id_t *)&ic->source_shape_id = shape_id;
+    *(shape_id_t *)&ic->dest_shape_id = shape_id;
+    *(attr_index_t *)&ic->attr_index = 0;
 }
 
 static inline void