Set max_iv_count (used for object shapes) based on inline caches

With this change, we're storing the iv name on an inline cache on
setinstancevariable instructions. This allows us to check the inline
cache to count instance variables set in initialize and give us an
estimate of iv capacity for an object.

For the purpose of estimating the number of instance variables required
for an object, we're assuming that all initialize methods will call
`super`.

This change allows us to estimate the number of instance variables
required without disassembling instruction sequences.

Co-Authored-By: Aaron Patterson <[email protected]>

1 files changed, 6 insertions, 19 deletions

diff --git a/iseq.c b/iseq.c
index 381bf6ed20..869ed4da6a 100644
--- a/iseq.c
+++ b/iseq.c
@@ -2509,33 +2509,20 @@ rb_iseq_disasm(const rb_iseq_t *iseq)
 attr_index_t
 rb_estimate_iv_count(VALUE klass, const rb_iseq_t * initialize_iseq)
 {
-    bool calls_super = false;
-
     struct rb_id_table * iv_names = rb_id_table_create(0);
 
-    VALUE * code = ISEQ_BODY(initialize_iseq)->iseq_encoded;
-
-    for (unsigned int i = 0; i < ISEQ_BODY(initialize_iseq)->iseq_size; ) {
-        VALUE insn = code[i];
-        int original_insn = rb_vm_insn_addr2insn((const void *)insn);
+    for (unsigned int i = 0; i < ISEQ_BODY(initialize_iseq)->ivc_size; i++) {
+        IVC cache = (IVC)&ISEQ_BODY(initialize_iseq)->is_entries[i];
 
-        if (BIN(setinstancevariable) == original_insn) {
-            ID name = (ID)code[i + 1];
-            rb_id_table_insert(iv_names, name, Qtrue);
+        if (cache->iv_set_name) {
+            rb_id_table_insert(iv_names, cache->iv_set_name, Qtrue);
         }
-        else if (BIN(invokesuper) == original_insn) {
-            calls_super = true;
-        }
-
-        i += insn_len(original_insn);
     }
 
     attr_index_t count = (attr_index_t)rb_id_table_size(iv_names);
 
-    if (calls_super) {
-        VALUE superclass = rb_class_superclass(klass);
-        count += RCLASS_EXT(superclass)->max_iv_count;
-    }
+    VALUE superclass = rb_class_superclass(klass);
+    count += RCLASS_EXT(superclass)->max_iv_count;
 
     rb_id_table_free(iv_names);