Extract yjit_force_iv_index and make it work when object is frozen

In an effort to simplify the logic YJIT generates for accessing instance
variable, YJIT ensures that a given name-to-index mapping exists at
compile time. In the case that the mapping doesn't exist, it was created
by using rb_ivar_set() with Qundef on the sample object we see at
compile time. This hack isn't fine if the sample object happens to be
frozen, in which case YJIT would raise a FrozenError unexpectedly.

To deal with this, make a new function that only reserves the mapping
but doesn't touch the object. This is rb_obj_ensure_iv_index_mapping().
This new function superceeds the functionality of rb_iv_index_tbl_lookup()
so it was removed.

Reported by and includes a test case from John Hawthorn <john@hawthorn.email>

Fixes: GH-282

1 files changed, 15 insertions, 0 deletions

diff --git a/bootstraptest/test_yjit.rb b/bootstraptest/test_yjit.rb
index a190d8ffb1..765580a030 100644
--- a/bootstraptest/test_yjit.rb
+++ b/bootstraptest/test_yjit.rb
@@ -2160,3 +2160,18 @@ assert_equal '[2]', %q{
 
   5.times.map { default_expression(value: 2) }.uniq
 }
+
+# attr_reader on frozen object
+assert_equal 'false', %q{
+  class Foo
+    attr_reader :exception
+
+    def failed?
+      !exception.nil?
+    end
+  end
+
+  foo = Foo.new.freeze
+  foo.failed?
+  foo.failed?
+}