vm_getivar: assume the cached shape_id like have a common ancestor

When an inline cache misses, it is very likely that the stale shape_id
and the current instance shape_id have a close common ancestor.

For example if the instance variable is sometimes frozen sometimes
not, one of the two shape will be the direct parent of the other.

Another pattern that commonly cause IC misses is "memoization",
in such case the object will have a "base common shape" and then
a number of close descendants.

In addition, when we find a common ancestor, we store it in the
inline cache instead of the current shape. This help prevent the
cache from flip-flopping, ensuring the next lookup will be marginally
faster and more generally avoid writing in memory too much.

However, now that shapes have an ancestors index, we only check
for a few ancestors before falling back to use the index.

So overall this change speeds up what is assumed to be the more common
case, but makes what is assumed to be the less common case a bit slower.

```
compare-ruby: ruby 3.3.0dev (2023-10-26T05:30:17Z master 701ca070b4) [arm64-darwin22]
built-ruby: ruby 3.3.0dev (2023-10-26T09:25:09Z shapes_double_sear.. a723a85235) [arm64-darwin22]
warming up......

|                                     |compare-ruby|built-ruby|
|:------------------------------------|-----------:|---------:|
|vm_ivar_stable_shape                 |     11.672M|   11.679M|
|                                     |           -|     1.00x|
|vm_ivar_memoize_unstable_shape       |      7.551M|   10.506M|
|                                     |           -|     1.39x|
|vm_ivar_memoize_unstable_shape_miss  |     11.591M|   11.624M|
|                                     |           -|     1.00x|
|vm_ivar_unstable_undef               |      9.037M|    7.981M|
|                                     |       1.13x|         -|
|vm_ivar_divergent_shape              |      8.034M|    6.657M|
|                                     |       1.21x|         -|
|vm_ivar_divergent_shape_imbalanced   |     10.471M|    9.231M|
|                                     |       1.13x|         -|
```

Co-Authored-By: John Hawthorn <john@hawthorn.email>

1 files changed, 13 insertions, 7 deletions

diff --git a/vm_insnhelper.c b/vm_insnhelper.c
index e28323e588..785dbfedec 100644
--- a/vm_insnhelper.c
+++ b/vm_insnhelper.c
@@ -1286,8 +1286,6 @@ vm_getivar(VALUE obj, ID id, const rb_iseq_t *iseq, IVC ic, const struct rb_call
         }
 #endif
 
-        rb_shape_t *shape = rb_shape_get_shape_by_id(shape_id);
-
         if (shape_id == OBJ_TOO_COMPLEX_SHAPE_ID) {
             st_table *table = NULL;
             switch (BUILTIN_TYPE(obj)) {
@@ -1314,14 +1312,22 @@ vm_getivar(VALUE obj, ID id, const rb_iseq_t *iseq, IVC ic, const struct rb_call
             }
         }
         else {
-            if (rb_shape_get_iv_index(shape, id, &index)) {
+            shape_id_t previous_cached_id = cached_id;
+            if (rb_shape_get_iv_index_with_hint(shape_id, id, &index, &cached_id)) {
                 // This fills in the cache with the shared cache object.
                 // "ent" is the shared cache object
-                fill_ivar_cache(iseq, ic, cc, is_attr, index, shape_id);
+                if (cached_id != previous_cached_id) {
+                    fill_ivar_cache(iseq, ic, cc, is_attr, index, cached_id);
+                }
 
-                // We fetched the ivar list above
-                val = ivar_list[index];
-                RUBY_ASSERT(!UNDEF_P(val));
+                if (index == ATTR_INDEX_NOT_SET) {
+                    val = default_value;
+                }
+                else {
+                    // We fetched the ivar list above
+                    val = ivar_list[index];
+                    RUBY_ASSERT(!UNDEF_P(val));
+                }
             }
             else {
                 if (is_attr) {