From 45d586ad60ba807dbb8c5f17b0ac156095eb4fbf Mon Sep 17 00:00:00 2001
From: Jean Boussier <jean.boussier@gmail.com>
Date: Sat, 2 May 2026 06:55:30 +0200
Subject: Never increment `max_iv_count` on Object / BasicObject.

Otherwise some code defining ivars on a naked object early during
program boot can cause all objects to be larger than needed.

This seem like it was always the intention, but wasn't quite
properly prevented.

Also stop updating `max_iv_count` during GC marking as it is
redundant.
---
 test/ruby/test_shapes.rb | 42 ++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 42 insertions(+)

(limited to 'test')

diff --git a/test/ruby/test_shapes.rb b/test/ruby/test_shapes.rb
index 67e2c543a3..b3333b1660 100644
--- a/test/ruby/test_shapes.rb
+++ b/test/ruby/test_shapes.rb
@@ -131,6 +131,48 @@ class TestShapes < Test::Unit::TestCase
     assert_operator obj["variation_count"], :<, RubyVM::Shape::SHAPE_MAX_VARIATIONS
   end
 
+  def test_max_iv_count
+    klass = Class.new
+    object = klass.new
+
+    assert_equal 0, RubyVM::Shape.class_max_iv_count(klass)
+    8.times do |i|
+      object.instance_variable_set("@ivar_#{i}", i)
+    end
+    assert_equal 8, RubyVM::Shape.class_max_iv_count(klass)
+
+    subklass = Class.new(klass)
+    assert_equal 8, RubyVM::Shape.class_max_iv_count(subklass)
+  end
+
+  def test_max_iv_count_on_Object
+    object = Object.new
+
+    assert_equal 0, RubyVM::Shape.class_max_iv_count(Object)
+    8.times do |i|
+      object.instance_variable_set("@ivar_#{i}", i)
+    end
+    assert_equal 0, RubyVM::Shape.class_max_iv_count(Object)
+  end
+
+  def test_max_iv_count_on_BasicObject
+    object = BasicObject.new
+
+    assert_equal 0, RubyVM::Shape.class_max_iv_count(BasicObject)
+    8.times do |i|
+      Object.instance_method(:instance_variable_set).bind_call(object, "@ivar_#{i}", i)
+    end
+    assert_equal 0, RubyVM::Shape.class_max_iv_count(BasicObject)
+
+    subklass = Class.new(BasicObject)
+    object = subklass.new
+    assert_equal 0, RubyVM::Shape.class_max_iv_count(subklass)
+    8.times do |i|
+      Object.instance_method(:instance_variable_set).bind_call(object, "@ivar_#{i}", i)
+    end
+    assert_equal 8, RubyVM::Shape.class_max_iv_count(subklass)
+  end
+
   def test_too_many_ivs_on_obj
     assert_separately([], "#{<<~"begin;"}\n#{<<~'end;'}")
     begin;
-- 
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