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 <tenderlove@ruby-lang.org>

compile.c

@@ -2461,7 +2461,17 @@ iseq_set_sequence(rb_iseq_t *iseq, LINK_ANCHOR *const anchor)
                       case TS_IVC: /* inline ivar cache */
                         {
                             unsigned int ic_index = FIX2UINT(operands[j]);
-                            vm_ic_attr_index_initialize(((IVC)&body->is_entries[ic_index]), INVALID_SHAPE_ID);
+
+                            IVC cache = ((IVC)&body->is_entries[ic_index]);
+
+                            if (insn == BIN(setinstancevariable)) {
+                                cache->iv_set_name = SYM2ID(operands[j - 1]);
+                            }
+                            else {
+                                cache->iv_set_name = 0;
+                            }
+
+                            vm_ic_attr_index_initialize(cache, INVALID_SHAPE_ID);
                         }
                       case TS_ISE: /* inline storage entry: `once` insn */
                       case TS_ICVARC: /* inline cvar cache */
@@ -11529,7 +11539,17 @@ ibf_load_code(const struct ibf_load *load, rb_iseq_t *iseq, ibf_offset_t bytecod
                     code[code_index] = (VALUE)ic;
 
                     if (operand_type == TS_IVC) {
-                        vm_ic_attr_index_initialize(((IVC)code[code_index]), INVALID_SHAPE_ID);
+                        IVC cache = (IVC)ic;
+
+                        if (insn == BIN(setinstancevariable)) {
+                            ID iv_name = (ID)code[code_index - 1];
+                            cache->iv_set_name = iv_name;
+                        }
+                        else {
+                            cache->iv_set_name = 0;
+                        }
+
+                        vm_ic_attr_index_initialize(cache, INVALID_SHAPE_ID);
                     }
 
                 }

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)->ivc_size; i++) {
+        IVC cache = (IVC)&ISEQ_BODY(initialize_iseq)->is_entries[i];
 
-    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);
-
-        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);
 

mjit_c.rb

@@ -310,6 +310,7 @@ module RubyVM::MJIT
     @iseq_inline_iv_cache_entry ||= CType::Struct.new(
       "iseq_inline_iv_cache_entry", Primitive.cexpr!("SIZEOF(struct iseq_inline_iv_cache_entry)"),
       value: [CType::Immediate.parse("uintptr_t"), Primitive.cexpr!("OFFSETOF((*((struct iseq_inline_iv_cache_entry *)NULL)), value)")],
+      iv_set_name: [self.ID, Primitive.cexpr!("OFFSETOF((*((struct iseq_inline_iv_cache_entry *)NULL)), iv_set_name)")],
     )
   end
 

vm_core.h

@@ -275,6 +275,7 @@ struct iseq_inline_constant_cache {
 
 struct iseq_inline_iv_cache_entry {
     uintptr_t value; // attr_index in lower bits, dest_shape_id in upper bits
+    ID iv_set_name;
 };
 
 struct iseq_inline_cvar_cache_entry {

yjit/src/cruby_bindings.inc.rs

@@ -853,6 +853,7 @@ pub struct iseq_inline_constant_cache {
 #[derive(Debug, Copy, Clone)]
 pub struct iseq_inline_iv_cache_entry {
     pub value: usize,
+    pub iv_set_name: ID,
 }
 #[repr(C)]
 #[derive(Debug, Copy, Clone)]