Speedup TypedData_Get_Struct

While profiling `Monitor#synchronize` and `Mutex#synchronize`
I noticed a fairly significant amount of time spent in
`rb_check_typeddata`.

By implementing a fast path that assumes the object is valid
and that can be inlined, it does make a significant difference:

Before:

```
  Mutex     13.548M (± 3.6%) i/s   (73.81 ns/i) -     68.566M in   5.067444
Monitor     10.497M (± 6.5%) i/s   (95.27 ns/i) -     52.529M in   5.032698s
```

After:

```
  Mutex     20.887M (± 0.3%) i/s   (47.88 ns/i) -    106.021M in   5.075989s
Monitor     16.245M (±13.3%) i/s   (61.56 ns/i) -     80.705M in   5.099680s
```

```ruby
require 'bundler/inline'

gemfile do
  gem "benchmark-ips"
end

mutex = Mutex.new
require "monitor"
monitor = Monitor.new

Benchmark.ips do |x|
  x.report("Mutex") { mutex.synchronize { } }
  x.report("Monitor") { monitor.synchronize { } }
end
```

1 files changed, 32 insertions, 13 deletions

diff --git a/include/ruby/internal/core/rtypeddata.h b/include/ruby/internal/core/rtypeddata.h
index 24e87e63f9..aaf8f7997c 100644
--- a/include/ruby/internal/core/rtypeddata.h
+++ b/include/ruby/internal/core/rtypeddata.h
@@ -507,19 +507,6 @@ RBIMPL_SYMBOL_EXPORT_END()
         sizeof(type))
 #endif
 
-/**
- * Obtains a C struct from inside of a wrapper Ruby object.
- *
- * @param      obj            An instance of ::RTypedData.
- * @param      type           Type name of the C struct.
- * @param      data_type      The data type describing `type`.
- * @param      sval           Variable name of obtained C struct.
- * @exception  rb_eTypeError  `obj` is not a kind of `data_type`.
- * @return     Unwrapped C struct that `obj` holds.
- */
-#define TypedData_Get_Struct(obj,type,data_type,sval) \
-    ((sval) = RBIMPL_CAST((type *)rb_check_typeddata((obj), (data_type))))
-
 static inline bool
 RTYPEDDATA_EMBEDDED_P(VALUE obj)
 {
@@ -614,6 +601,38 @@ RTYPEDDATA_TYPE(VALUE obj)
     return (const struct rb_data_type_struct *)(RTYPEDDATA(obj)->type & TYPED_DATA_PTR_MASK);
 }
 
+RBIMPL_ATTR_PURE_UNLESS_DEBUG()
+RBIMPL_ATTR_ARTIFICIAL()
+/**
+ * @private
+ *
+ * This  is an  implementation detail  of  TypedData_Get_Struct().  Don't use  it
+ * directly.
+ */
+static inline void *
+rbimpl_check_typeddata(VALUE obj, const rb_data_type_t *type)
+{
+    if (RB_LIKELY(RB_TYPE_P(obj, T_DATA) && RTYPEDDATA_P(obj) && RTYPEDDATA_TYPE(obj) == type)) {
+        return RTYPEDDATA_GET_DATA(obj);
+    }
+
+    return rb_check_typeddata(obj, type);
+}
+
+
+/**
+ * Obtains a C struct from inside of a wrapper Ruby object.
+ *
+ * @param      obj            An instance of ::RTypedData.
+ * @param      type           Type name of the C struct.
+ * @param      data_type      The data type describing `type`.
+ * @param      sval           Variable name of obtained C struct.
+ * @exception  rb_eTypeError  `obj` is not a kind of `data_type`.
+ * @return     Unwrapped C struct that `obj` holds.
+ */
+#define TypedData_Get_Struct(obj,type,data_type,sval) \
+    ((sval) = RBIMPL_CAST((type *)rbimpl_check_typeddata((obj), (data_type))))
+
 /**
  * While  we don't  stop  you from  using  this  function, it  seems  to be  an
  * implementation  detail of  #TypedData_Make_Struct, which  is preferred  over