Lazily create singletons on instance_{exec,eval} (#5146)

* Lazily create singletons on instance_{exec,eval}

Previously when instance_exec or instance_eval was called on an object,
that object would be given a singleton class so that method
definitions inside the block would be added to the object rather than
its class.

This commit aims to improve performance by delaying the creation of the
singleton class unless/until one is needed for method definition. Most
of the time instance_eval is used without any method definition.

This was implemented by adding a flag to the cref indicating that it
represents a singleton of the object rather than a class itself. In this
case CREF_CLASS returns the object's existing class, but in cases that
we are defining a method (either via definemethod or
VM_SPECIAL_OBJECT_CBASE which is used for undef and alias).

This also happens to fix what I believe is a bug. Previously
instance_eval behaved differently with regards to constant access for
true/false/nil than for all other objects. I don't think this was
intentional.

    String::Foo = "foo"
    "".instance_eval("Foo")   # => "foo"
    Integer::Foo = "foo"
    123.instance_eval("Foo")  # => "foo"
    TrueClass::Foo = "foo"
    true.instance_eval("Foo") # NameError: uninitialized constant Foo

This also slightly changes the error message when trying to define a method
through instance_eval on an object which can't have a singleton class.

Before:

    $ ruby -e '123.instance_eval { def foo; end }'
    -e:1:in `block in <main>': no class/module to add method (TypeError)

After:

    $ ./ruby -e '123.instance_eval { def foo; end }'
    -e:1:in `block in <main>': can't define singleton (TypeError)

IMO this error is a small improvement on the original and better matches
the (both old and new) message when definging a method using `def self.`

    $ ruby -e '123.instance_eval{ def self.foo; end }'
    -e:1:in `block in <main>': can't define singleton (TypeError)

Co-authored-by: Matthew Draper <matthew@trebex.net>

* Remove "under" argument from yield_under

* Move CREF_SINGLETON_SET into vm_cref_new

* Simplify vm_get_const_base

* Fix leaf VM_SPECIAL_OBJECT_CONST_BASE

Co-authored-by: Matthew Draper <matthew@trebex.net>

10 files changed, 197 insertions, 78 deletions

diff --git a/bootstraptest/test_eval.rb b/bootstraptest/test_eval.rb
index 5d2593c306..efc22efff4 100644
--- a/bootstraptest/test_eval.rb
+++ b/bootstraptest/test_eval.rb
@@ -116,6 +116,33 @@ assert_equal %q{1}, %q{
     Const
   }
 }
+assert_equal %q{1}, %q{
+  class TrueClass
+    Const = 1
+  end
+  true.instance_eval %{
+    Const
+  }
+}
+assert_equal %q{[:Const]}, %q{
+  mod = Module.new
+  mod.instance_eval %{
+    Const = 1
+  }
+  raise if defined?(Module::Const)
+  mod.singleton_class.constants
+}
+assert_equal %q{can't define singleton}, %q{
+  begin
+    123.instance_eval %{
+      Const = 1
+    }
+    "bad"
+  rescue TypeError => e
+    raise "bad" if defined?(Integer::Const)
+    e.message
+  end
+}
 assert_equal %q{top}, %q{
   Const = :top
   class C
diff --git a/eval_intern.h b/eval_intern.h
index 58400b5f25..e858d79c3e 100644
--- a/eval_intern.h
+++ b/eval_intern.h
@@ -173,11 +173,28 @@ rb_ec_tag_jump(const rb_execution_context_t *ec, enum ruby_tag_type st)
 
 #define CREF_FL_PUSHED_BY_EVAL IMEMO_FL_USER1
 #define CREF_FL_OMOD_SHARED    IMEMO_FL_USER2
+#define CREF_FL_SINGLETON      IMEMO_FL_USER3
+
+static inline int CREF_SINGLETON(const rb_cref_t *cref);
 
 static inline VALUE
 CREF_CLASS(const rb_cref_t *cref)
 {
-    return cref->klass;
+    if (CREF_SINGLETON(cref)) {
+        return CLASS_OF(cref->klass_or_self);
+    } else {
+        return cref->klass_or_self;
+    }
+}
+
+static inline VALUE
+CREF_CLASS_FOR_DEFINITION(const rb_cref_t *cref)
+{
+    if (CREF_SINGLETON(cref)) {
+        return rb_singleton_class(cref->klass_or_self);
+    } else {
+        return cref->klass_or_self;
+    }
 }
 
 static inline rb_cref_t *
@@ -217,6 +234,18 @@ CREF_PUSHED_BY_EVAL_SET(rb_cref_t *cref)
 }
 
 static inline int
+CREF_SINGLETON(const rb_cref_t *cref)
+{
+    return cref->flags & CREF_FL_SINGLETON;
+}
+
+static inline void
+CREF_SINGLETON_SET(rb_cref_t *cref)
+{
+    cref->flags |= CREF_FL_SINGLETON;
+}
+
+static inline int
 CREF_OMOD_SHARED(const rb_cref_t *cref)
 {
     return cref->flags & CREF_FL_OMOD_SHARED;
diff --git a/gc.c b/gc.c
index 9bdeaac234..f889cbb81b 100644
--- a/gc.c
+++ b/gc.c
@@ -6808,7 +6808,7 @@ gc_mark_imemo(rb_objspace_t *objspace, VALUE obj)
 	}
 	return;
       case imemo_cref:
-	gc_mark(objspace, RANY(obj)->as.imemo.cref.klass);
+	gc_mark(objspace, RANY(obj)->as.imemo.cref.klass_or_self);
 	gc_mark(objspace, (VALUE)RANY(obj)->as.imemo.cref.next);
 	gc_mark(objspace, RANY(obj)->as.imemo.cref.refinements);
 	return;
@@ -9677,7 +9677,7 @@ gc_ref_update_imemo(rb_objspace_t *objspace, VALUE obj)
         }
         break;
       case imemo_cref:
-        UPDATE_IF_MOVED(objspace, RANY(obj)->as.imemo.cref.klass);
+        UPDATE_IF_MOVED(objspace, RANY(obj)->as.imemo.cref.klass_or_self);
         TYPED_UPDATE_IF_MOVED(objspace, struct rb_cref_struct *, RANY(obj)->as.imemo.cref.next);
         UPDATE_IF_MOVED(objspace, RANY(obj)->as.imemo.cref.refinements);
         break;
diff --git a/insns.def b/insns.def
index f759c1a5c5..87624a0500 100644
--- a/insns.def
+++ b/insns.def
@@ -350,6 +350,7 @@ putspecialobject
 (rb_num_t value_type)
 ()
 (VALUE val)
+// attr bool leaf = (value_type == VM_SPECIAL_OBJECT_VMCORE); /* others may raise when allocating singleton */
 {
     enum vm_special_object_type type;
 
@@ -715,7 +716,7 @@ defineclass
     /* enter scope */
     vm_push_frame(ec, class_iseq, VM_FRAME_MAGIC_CLASS | VM_ENV_FLAG_LOCAL, klass,
 		  GET_BLOCK_HANDLER(),
-		  (VALUE)vm_cref_push(ec, klass, NULL, FALSE),
+		  (VALUE)vm_cref_push(ec, klass, NULL, FALSE, FALSE),
 		  class_iseq->body->iseq_encoded, GET_SP(),
 		  class_iseq->body->local_table_size,
 		  class_iseq->body->stack_max);
diff --git a/method.h b/method.h
index 031d2ce89f..8bff5b3b8d 100644
--- a/method.h
+++ b/method.h
@@ -44,7 +44,7 @@ typedef struct rb_scope_visi_struct {
 typedef struct rb_cref_struct {
     VALUE flags;
     VALUE refinements;
-    VALUE klass;
+    VALUE klass_or_self;
     struct rb_cref_struct * next;
     const rb_scope_visibility_t scope_visi;
 } rb_cref_t;
diff --git a/test/ruby/test_eval.rb b/test/ruby/test_eval.rb
index bf551c6845..d55977c986 100644
--- a/test/ruby/test_eval.rb
+++ b/test/ruby/test_eval.rb
@@ -219,6 +219,12 @@ class TestEval < Test::Unit::TestCase
     end
   end
 
+  def test_instance_exec_cvar
+    [Object.new, [], 7, :sym, true, false, nil].each do |obj|
+      assert_equal(13, obj.instance_exec{@@cvar})
+    end
+  end
+
   def test_instance_eval_method
     bug2788 = '[ruby-core:28324]'
     [Object.new, [], nil, true, false].each do |o|
@@ -253,6 +259,70 @@ class TestEval < Test::Unit::TestCase
     assert_equal(2, bar)
   end
 
+  def test_instance_exec_block_basic
+    forall_TYPE do |o|
+      assert_equal nil,   o.instance_exec { nil }
+      assert_equal true,  o.instance_exec { true }
+      assert_equal false, o.instance_exec { false }
+      assert_equal o,     o.instance_exec { self }
+      assert_equal 1,     o.instance_exec { 1 }
+      assert_equal :sym,  o.instance_exec { :sym }
+
+      assert_equal 11,    o.instance_exec { 11 }
+      assert_equal 12,    o.instance_exec { @ivar } unless o.frozen?
+      assert_equal 13,    o.instance_exec { @@cvar }
+      assert_equal 14,    o.instance_exec { $gvar__eval }
+      assert_equal 15,    o.instance_exec { Const }
+      assert_equal 16,    o.instance_exec { 7 + 9 }
+      assert_equal 17,    o.instance_exec { 17.to_i }
+      assert_equal "18",  o.instance_exec { "18" }
+      assert_equal "19",  o.instance_exec { "1#{9}" }
+
+      1.times {
+        assert_equal 12,  o.instance_exec { @ivar } unless o.frozen?
+        assert_equal 13,  o.instance_exec { @@cvar }
+        assert_equal 14,  o.instance_exec { $gvar__eval }
+        assert_equal 15,  o.instance_exec { Const }
+      }
+    end
+  end
+
+  def test_instance_exec_method_definition
+    klass = Class.new
+    o = klass.new
+
+    o.instance_exec do
+      def foo
+        :foo_result
+      end
+    end
+
+    assert_respond_to o, :foo
+    refute_respond_to klass, :foo
+    refute_respond_to klass.new, :foo
+
+    assert_equal :foo_result, o.foo
+  end
+
+  def test_instance_exec_eval_method_definition
+    klass = Class.new
+    o = klass.new
+
+    o.instance_exec do
+      eval %{
+        def foo
+          :foo_result
+        end
+      }
+    end
+
+    assert_respond_to o, :foo
+    refute_respond_to klass, :foo
+    refute_respond_to klass.new, :foo
+
+    assert_equal :foo_result, o.foo
+  end
+
   #
   # From ruby/test/ruby/test_eval.rb
   #
diff --git a/test/ruby/test_undef.rb b/test/ruby/test_undef.rb
index e0add7c3ab..074b92be55 100644
--- a/test/ruby/test_undef.rb
+++ b/test/ruby/test_undef.rb
@@ -35,4 +35,20 @@ class TestUndef < Test::Unit::TestCase
       end
     end
   end
+
+  def test_singleton_undef
+    klass = Class.new do
+      def foo
+        :ok
+      end
+    end
+
+    klass.new.foo
+
+    klass.new.instance_eval do
+      undef foo
+    end
+
+    klass.new.foo
+  end
 end
diff --git a/vm.c b/vm.c
index 19f5ce95f0..ad687d579a 100644
--- a/vm.c
+++ b/vm.c
@@ -223,7 +223,7 @@ vm_passed_block_handler(rb_execution_context_t *ec)
 }
 
 static rb_cref_t *
-vm_cref_new0(VALUE klass, rb_method_visibility_t visi, int module_func, rb_cref_t *prev_cref, int pushed_by_eval, int use_prev_prev)
+vm_cref_new0(VALUE klass, rb_method_visibility_t visi, int module_func, rb_cref_t *prev_cref, int pushed_by_eval, int use_prev_prev, int singleton)
 {
     VALUE refinements = Qnil;
     int omod_shared = FALSE;
@@ -248,24 +248,27 @@ vm_cref_new0(VALUE klass, rb_method_visibility_t visi, int module_func, rb_cref_
 	}
     }
 
+    VM_ASSERT(singleton || klass);
+
     cref = (rb_cref_t *)rb_imemo_new(imemo_cref, klass, (VALUE)(use_prev_prev ? CREF_NEXT(prev_cref) : prev_cref), scope_visi.value, refinements);
 
     if (pushed_by_eval) CREF_PUSHED_BY_EVAL_SET(cref);
     if (omod_shared) CREF_OMOD_SHARED_SET(cref);
+    if (singleton) CREF_SINGLETON_SET(cref);
 
     return cref;
 }
 
 static rb_cref_t *
-vm_cref_new(VALUE klass, rb_method_visibility_t visi, int module_func, rb_cref_t *prev_cref, int pushed_by_eval)
+vm_cref_new(VALUE klass, rb_method_visibility_t visi, int module_func, rb_cref_t *prev_cref, int pushed_by_eval, int singleton)
 {
-    return vm_cref_new0(klass, visi, module_func, prev_cref, pushed_by_eval, FALSE);
+    return vm_cref_new0(klass, visi, module_func, prev_cref, pushed_by_eval, FALSE, singleton);
 }
 
 static rb_cref_t *
 vm_cref_new_use_prev(VALUE klass, rb_method_visibility_t visi, int module_func, rb_cref_t *prev_cref, int pushed_by_eval)
 {
-    return vm_cref_new0(klass, visi, module_func, prev_cref, pushed_by_eval, TRUE);
+    return vm_cref_new0(klass, visi, module_func, prev_cref, pushed_by_eval, TRUE, FALSE);
 }
 
 static int
@@ -277,18 +280,18 @@ ref_delete_symkey(VALUE key, VALUE value, VALUE unused)
 static rb_cref_t *
 vm_cref_dup(const rb_cref_t *cref)
 {
-    VALUE klass = CREF_CLASS(cref);
     const rb_scope_visibility_t *visi = CREF_SCOPE_VISI(cref);
     rb_cref_t *next_cref = CREF_NEXT(cref), *new_cref;
     int pushed_by_eval = CREF_PUSHED_BY_EVAL(cref);
+    int singleton = CREF_SINGLETON(cref);
 
-    new_cref = vm_cref_new(klass, visi->method_visi, visi->module_func, next_cref, pushed_by_eval);
+    new_cref = vm_cref_new(cref->klass_or_self, visi->method_visi, visi->module_func, next_cref, pushed_by_eval, singleton);
 
     if (!NIL_P(CREF_REFINEMENTS(cref))) {
         VALUE ref = rb_hash_dup(CREF_REFINEMENTS(cref));
         rb_hash_foreach(ref, ref_delete_symkey, Qnil);
         CREF_REFINEMENTS_SET(new_cref, ref);
-	CREF_OMOD_SHARED_UNSET(new_cref);
+        CREF_OMOD_SHARED_UNSET(new_cref);
     }
 
     return new_cref;
@@ -298,12 +301,12 @@ vm_cref_dup(const rb_cref_t *cref)
 rb_cref_t *
 rb_vm_cref_dup_without_refinements(const rb_cref_t *cref)
 {
-    VALUE klass = CREF_CLASS(cref);
     const rb_scope_visibility_t *visi = CREF_SCOPE_VISI(cref);
     rb_cref_t *next_cref = CREF_NEXT(cref), *new_cref;
     int pushed_by_eval = CREF_PUSHED_BY_EVAL(cref);
+    int singleton = CREF_SINGLETON(cref);
 
-    new_cref = vm_cref_new(klass, visi->method_visi, visi->module_func, next_cref, pushed_by_eval);
+    new_cref = vm_cref_new(cref->klass_or_self, visi->method_visi, visi->module_func, next_cref, pushed_by_eval, singleton);
 
     if (!NIL_P(CREF_REFINEMENTS(cref))) {
         CREF_REFINEMENTS_SET(new_cref, Qnil);
@@ -316,11 +319,11 @@ rb_vm_cref_dup_without_refinements(const rb_cref_t *cref)
 static rb_cref_t *
 vm_cref_new_toplevel(rb_execution_context_t *ec)
 {
-    rb_cref_t *cref = vm_cref_new(rb_cObject, METHOD_VISI_PRIVATE /* toplevel visibility is private */, FALSE, NULL, FALSE);
+    rb_cref_t *cref = vm_cref_new(rb_cObject, METHOD_VISI_PRIVATE /* toplevel visibility is private */, FALSE, NULL, FALSE, FALSE);
     VALUE top_wrapper = rb_ec_thread_ptr(ec)->top_wrapper;
 
     if (top_wrapper) {
-	cref = vm_cref_new(top_wrapper, METHOD_VISI_PRIVATE, FALSE, cref, FALSE);
+	cref = vm_cref_new(top_wrapper, METHOD_VISI_PRIVATE, FALSE, cref, FALSE, FALSE);
     }
 
     return cref;
@@ -3742,7 +3745,7 @@ Init_VM(void)
 	th->ec->cfp->self = th->top_self;
 
 	VM_ENV_FLAGS_UNSET(th->ec->cfp->ep, VM_FRAME_FLAG_CFRAME);
-	VM_STACK_ENV_WRITE(th->ec->cfp->ep, VM_ENV_DATA_INDEX_ME_CREF, (VALUE)vm_cref_new(rb_cObject, METHOD_VISI_PRIVATE, FALSE, NULL, FALSE));
+	VM_STACK_ENV_WRITE(th->ec->cfp->ep, VM_ENV_DATA_INDEX_ME_CREF, (VALUE)vm_cref_new(rb_cObject, METHOD_VISI_PRIVATE, FALSE, NULL, FALSE, FALSE));
 
 	/*
 	 * The Binding of the top level scope
diff --git a/vm_eval.c b/vm_eval.c
index 73739fc3a5..0abb4644f9 100644
--- a/vm_eval.c
+++ b/vm_eval.c
@@ -1860,7 +1860,7 @@ eval_string_wrap_protect(VALUE data)
 {
     const struct eval_string_wrap_arg *const arg = (struct eval_string_wrap_arg*)data;
     rb_cref_t *cref = rb_vm_cref_new_toplevel();
-    cref->klass = arg->klass;
+    cref->klass_or_self = arg->klass;
     return eval_string_with_cref(arg->top_self, rb_str_new_cstr(arg->str), cref, rb_str_new_cstr("eval"), 1);
 }
 
@@ -1922,7 +1922,7 @@ rb_eval_cmd_kw(VALUE cmd, VALUE arg, int kw_splat)
 /* block eval under the class/module context */
 
 static VALUE
-yield_under(VALUE under, VALUE self, int argc, const VALUE *argv, int kw_splat)
+yield_under(VALUE self, int singleton, int argc, const VALUE *argv, int kw_splat)
 {
     rb_execution_context_t *ec = GET_EC();
     rb_control_frame_t *cfp = ec->cfp;
@@ -1963,7 +1963,9 @@ yield_under(VALUE under, VALUE self, int argc, const VALUE *argv, int kw_splat)
 	VM_FORCE_WRITE_SPECIAL_CONST(&VM_CF_LEP(ec->cfp)[VM_ENV_DATA_INDEX_SPECVAL], new_block_handler);
     }
 
-    cref = vm_cref_push(ec, under, ep, TRUE);
+    VM_ASSERT(singleton || RB_TYPE_P(self, T_MODULE) || RB_TYPE_P(self, T_CLASS));
+    cref = vm_cref_push(ec, self, ep, TRUE, singleton);
+
     return vm_yield_with_cref(ec, argc, argv, kw_splat, cref, is_lambda);
 }
 
@@ -1981,7 +1983,7 @@ rb_yield_refine_block(VALUE refinement, VALUE refinements)
 	struct rb_captured_block new_captured = *captured;
 	VALUE new_block_handler = VM_BH_FROM_ISEQ_BLOCK(&new_captured);
 	const VALUE *ep = captured->ep;
-	rb_cref_t *cref = vm_cref_push(ec, refinement, ep, TRUE);
+	rb_cref_t *cref = vm_cref_push(ec, refinement, ep, TRUE, FALSE);
 	CREF_REFINEMENTS_SET(cref, refinements);
 	VM_FORCE_WRITE_SPECIAL_CONST(&VM_CF_LEP(ec->cfp)[VM_ENV_DATA_INDEX_SPECVAL], new_block_handler);
 	new_captured.self = refinement;
@@ -1991,19 +1993,20 @@ rb_yield_refine_block(VALUE refinement, VALUE refinements)
 
 /* string eval under the class/module context */
 static VALUE
-eval_under(VALUE under, VALUE self, VALUE src, VALUE file, int line)
+eval_under(VALUE self, int singleton, VALUE src, VALUE file, int line)
 {
-    rb_cref_t *cref = vm_cref_push(GET_EC(), under, NULL, SPECIAL_CONST_P(self) && !NIL_P(under));
+    rb_cref_t *cref = vm_cref_push(GET_EC(), self, NULL, FALSE, singleton);
     SafeStringValue(src);
+
     return eval_string_with_cref(self, src, cref, file, line);
 }
 
 static VALUE
-specific_eval(int argc, const VALUE *argv, VALUE klass, VALUE self, int kw_splat)
+specific_eval(int argc, const VALUE *argv, VALUE self, int singleton, int kw_splat)
 {
     if (rb_block_given_p()) {
 	rb_check_arity(argc, 0, 0);
-        return yield_under(klass, self, 1, &self, kw_splat);
+        return yield_under(self, singleton, 1, &self, kw_splat);
     }
     else {
 	VALUE file = Qundef;
@@ -2019,23 +2022,7 @@ specific_eval(int argc, const VALUE *argv, VALUE klass, VALUE self, int kw_splat
 	    file = argv[1];
 	    if (!NIL_P(file)) StringValue(file);
 	}
-	return eval_under(klass, self, code, file, line);
-    }
-}
-
-static VALUE
-singleton_class_for_eval(VALUE self)
-{
-    if (SPECIAL_CONST_P(self)) {
-	return rb_special_singleton_class(self);
-    }
-    switch (BUILTIN_TYPE(self)) {
-      case T_FLOAT: case T_BIGNUM: case T_SYMBOL:
-	return Qnil;
-      case T_STRING:
-	if (FL_TEST_RAW(self, RSTRING_FSTR)) return Qnil;
-      default:
-	return rb_singleton_class(self);
+	return eval_under(self, singleton, code, file, line);
     }
 }
 
@@ -2075,15 +2062,13 @@ singleton_class_for_eval(VALUE self)
 static VALUE
 rb_obj_instance_eval_internal(int argc, const VALUE *argv, VALUE self)
 {
-    VALUE klass = singleton_class_for_eval(self);
-    return specific_eval(argc, argv, klass, self, RB_PASS_CALLED_KEYWORDS);
+    return specific_eval(argc, argv, self, TRUE, RB_PASS_CALLED_KEYWORDS);
 }
 
 VALUE
 rb_obj_instance_eval(int argc, const VALUE *argv, VALUE self)
 {
-    VALUE klass = singleton_class_for_eval(self);
-    return specific_eval(argc, argv, klass, self, RB_NO_KEYWORDS);
+    return specific_eval(argc, argv, self, TRUE, RB_NO_KEYWORDS);
 }
 
 /*
@@ -2107,15 +2092,13 @@ rb_obj_instance_eval(int argc, const VALUE *argv, VALUE self)
 static VALUE
 rb_obj_instance_exec_internal(int argc, const VALUE *argv, VALUE self)
 {
-    VALUE klass = singleton_class_for_eval(self);
-    return yield_under(klass, self, argc, argv, RB_PASS_CALLED_KEYWORDS);
+    return yield_under(self, TRUE, argc, argv, RB_PASS_CALLED_KEYWORDS);
 }
 
 VALUE
 rb_obj_instance_exec(int argc, const VALUE *argv, VALUE self)
 {
-    VALUE klass = singleton_class_for_eval(self);
-    return yield_under(klass, self, argc, argv, RB_NO_KEYWORDS);
+    return yield_under(self, TRUE, argc, argv, RB_NO_KEYWORDS);
 }
 
 /*
@@ -2148,13 +2131,13 @@ rb_obj_instance_exec(int argc, const VALUE *argv, VALUE self)
 static VALUE
 rb_mod_module_eval_internal(int argc, const VALUE *argv, VALUE mod)
 {
-    return specific_eval(argc, argv, mod, mod, RB_PASS_CALLED_KEYWORDS);
+    return specific_eval(argc, argv, mod, FALSE, RB_PASS_CALLED_KEYWORDS);
 }
 
 VALUE
 rb_mod_module_eval(int argc, const VALUE *argv, VALUE mod)
 {
-    return specific_eval(argc, argv, mod, mod, RB_NO_KEYWORDS);
+    return specific_eval(argc, argv, mod, FALSE, RB_NO_KEYWORDS);
 }
 
 /*
@@ -2182,13 +2165,13 @@ rb_mod_module_eval(int argc, const VALUE *argv, VALUE mod)
 static VALUE
 rb_mod_module_exec_internal(int argc, const VALUE *argv, VALUE mod)
 {
-    return yield_under(mod, mod, argc, argv, RB_PASS_CALLED_KEYWORDS);
+    return yield_under(mod, FALSE, argc, argv, RB_PASS_CALLED_KEYWORDS);
 }
 
 VALUE
 rb_mod_module_exec(int argc, const VALUE *argv, VALUE mod)
 {
-    return yield_under(mod, mod, argc, argv, RB_NO_KEYWORDS);
+    return yield_under(mod, FALSE, argc, argv, RB_NO_KEYWORDS);
 }
 
 /*
diff --git a/vm_insnhelper.c b/vm_insnhelper.c
index 741deb6b42..c6b079e7d5 100644
--- a/vm_insnhelper.c
+++ b/vm_insnhelper.c
@@ -892,7 +892,7 @@ rb_vm_rewrite_cref(rb_cref_t *cref, VALUE old_klass, VALUE new_klass, rb_cref_t
 }
 
 static rb_cref_t *
-vm_cref_push(const rb_execution_context_t *ec, VALUE klass, const VALUE *ep, int pushed_by_eval)
+vm_cref_push(const rb_execution_context_t *ec, VALUE klass, const VALUE *ep, int pushed_by_eval, int singleton)
 {
     rb_cref_t *prev_cref = NULL;
 
@@ -907,40 +907,30 @@ vm_cref_push(const rb_execution_context_t *ec, VALUE klass, const VALUE *ep, int
 	}
     }
 
-    return vm_cref_new(klass, METHOD_VISI_PUBLIC, FALSE, prev_cref, pushed_by_eval);
+    return vm_cref_new(klass, METHOD_VISI_PUBLIC, FALSE, prev_cref, pushed_by_eval, singleton);
 }
 
 static inline VALUE
 vm_get_cbase(const VALUE *ep)
 {
     const rb_cref_t *cref = vm_get_cref(ep);
-    VALUE klass = Qundef;
 
-    while (cref) {
-	if ((klass = CREF_CLASS(cref)) != 0) {
-	    break;
-	}
-	cref = CREF_NEXT(cref);
-    }
-
-    return klass;
+    return CREF_CLASS_FOR_DEFINITION(cref);
 }
 
 static inline VALUE
 vm_get_const_base(const VALUE *ep)
 {
     const rb_cref_t *cref = vm_get_cref(ep);
-    VALUE klass = Qundef;
 
     while (cref) {
-	if (!CREF_PUSHED_BY_EVAL(cref) &&
-	    (klass = CREF_CLASS(cref)) != 0) {
-	    break;
-	}
-	cref = CREF_NEXT(cref);
+        if (!CREF_PUSHED_BY_EVAL(cref)) {
+            return CREF_CLASS_FOR_DEFINITION(cref);
+        }
+        cref = CREF_NEXT(cref);
     }
 
-    return klass;
+    return Qundef;
 }
 
 static inline void
@@ -1060,7 +1050,7 @@ vm_get_cvar_base(const rb_cref_t *cref, const rb_control_frame_t *cfp, int top_l
 
     while (CREF_NEXT(cref) &&
 	   (NIL_P(CREF_CLASS(cref)) || FL_TEST(CREF_CLASS(cref), FL_SINGLETON) ||
-	    CREF_PUSHED_BY_EVAL(cref))) {
+	    CREF_PUSHED_BY_EVAL(cref) || CREF_SINGLETON(cref))) {
 	cref = CREF_NEXT(cref);
     }
     if (top_level_raise && !CREF_NEXT(cref)) {
@@ -4657,14 +4647,14 @@ vm_define_method(const rb_execution_context_t *ec, VALUE obj, ID id, VALUE iseqv
     rb_method_visibility_t visi;
     rb_cref_t *cref = vm_ec_cref(ec);
 
-    if (!is_singleton) {
-        klass = CREF_CLASS(cref);
-        visi = vm_scope_visibility_get(ec);
-    }
-    else { /* singleton */
+    if (is_singleton) {
         klass = rb_singleton_class(obj); /* class and frozen checked in this API */
         visi = METHOD_VISI_PUBLIC;
     }
+    else {
+        klass = CREF_CLASS_FOR_DEFINITION(cref);
+        visi = vm_scope_visibility_get(ec);
+    }
 
     if (NIL_P(klass)) {
         rb_raise(rb_eTypeError, "no class/module to add method");