Copy the serial number from the super class to the singleton class

This helps hit inline method caches more frequently.  Before this
commit:

```
[aaron@TC ruby (trunk)]$ time ./ruby -v benchmark/bm_vm2_poly_singleton.rb
ruby 2.4.0dev (2016-09-12 trunk 56141) [x86_64-darwin15]

real  0m3.679s
user  0m3.632s
sys 0m0.022s
```

After this commit:

```
[aaron@TC ruby (trunk)]$ time ./ruby -v benchmark/bm_vm2_poly_singleton.rb
ruby 2.4.0dev (2016-09-12 trunk 56141) [x86_64-darwin15]
last_commit=Copy the serial number from the super class to the singleton class

real  0m2.246s
user  0m2.203s
sys 0m0.020s
```

[Feature #12364]
[ruby-core:75425]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@56144 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

3 files changed, 22 insertions, 0 deletions

diff --git a/ChangeLog b/ChangeLog
index 83800b1b8a..7eabe4460e 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,9 @@
+Tue Sep 13 00:39:47 2016  Aaron Patterson <tenderlove@ruby-lang.org>
+
+	* class.c (singleton_class_of): Copy superclass serial number to
+	  singleton class.  This improves singleton class IMC hit rates.
+	  [Feature #12364] [ruby-core:75425]
+
 Mon Sep 12 13:46:23 2016  Anton Davydov  <mail@davydovanton.com>
 
 	* lib/uri/mailto.rb: Removed needless `return` and use `.`` instead of `::`
diff --git a/benchmark/bm_vm2_poly_singleton.rb b/benchmark/bm_vm2_poly_singleton.rb
new file mode 100644
index 0000000000..0dba4320c4
--- /dev/null
+++ b/benchmark/bm_vm2_poly_singleton.rb
@@ -0,0 +1,14 @@
+class C1
+  def m; 1; end
+end
+
+o1 = C1.new
+o2 = C1.new
+o2.singleton_class
+
+i = 0
+while i<6_000_000 # benchmark loop 2
+  o = (i % 2 == 0) ? o1 : o2
+  o.m; o.m; o.m; o.m; o.m; o.m; o.m; o.m
+  i += 1
+end
diff --git a/class.c b/class.c
index 6c7dfa5c17..ce2ede5c2b 100644
--- a/class.c
+++ b/class.c
@@ -1591,7 +1591,9 @@ singleton_class_of(VALUE obj)
     klass = RBASIC(obj)->klass;
     if (!(FL_TEST(klass, FL_SINGLETON) &&
 	  rb_ivar_get(klass, id_attached) == obj)) {
+	rb_serial_t serial = RCLASS_SERIAL(klass);
 	klass = rb_make_metaclass(obj, klass);
+	RCLASS_SERIAL(klass) = serial;
     }
 
     if (OBJ_TAINTED(obj)) {