GH-143732: SEND specialization (GH-148963)

* SEND specialization. Adds 2 new specialized instructions:

* SEND_VIRTUAL: for sends to virtual iterators e.g lists and tuples
* SEND_ASYNC_GEN: for sends to async generators

Tweak FOR_ITER_VIRTUAL so that SEND_VIRTUAL and FOR_ITER_VIRTUAL use equivalent guards

Author: markshannon

Include/cpython/object.h

@@ -241,7 +241,11 @@ struct _typeobject {
      * Otherwise, limited to MAX_VERSIONS_PER_CLASS (defined elsewhere).
      */
     uint16_t tp_versions_used;
-    _Py_iteritemfunc _tp_iteritem; /* Virtual iterator next function */
+
+     /* Virtual iterator next function.
+      * This function must escape to any code that can result in
+      * the GC being run, such as Py_DECREF.  */
+    _Py_iteritemfunc _tp_iteritem;
 };
 
 #define _Py_ATTR_CACHE_UNUSED (30000)  // (see tp_versions_used)

Include/internal/pycore_abstract.h

@@ -60,6 +60,14 @@ PyAPI_FUNC(int) _Py_convert_optional_to_non_negative_ssize_t(PyObject *, void *)
 // Export for 'math' shared extension.
 PyAPI_FUNC(PyObject*) _PyNumber_Index(PyObject *o);
 
+typedef struct {
+    PyObject *object;
+    PySendResult kind;
+} PySendResultPair;
+
+// Same as PyIter_Send but returns a struct for MSVC tailcall support
+PyAPI_FUNC(PySendResultPair) _PyIter_Send(PyObject *iter, PyObject *arg);
+
 #ifdef __cplusplus
 }
 #endif

Include/internal/pycore_genobject.h

@@ -40,6 +40,8 @@ extern PyTypeObject _PyCoroWrapper_Type;
 extern PyTypeObject _PyAsyncGenWrappedValue_Type;
 extern PyTypeObject _PyAsyncGenAThrow_Type;
 
+PyAPI_FUNC(PySendResult) _PyAsyncGenASend_Send(PyObject *iter, PyObject *arg, PyObject **result);
+
 #ifdef __cplusplus
 }
 #endif

Include/internal/pycore_magic_number.h

@@ -296,6 +296,7 @@ Known values:
     Python 3.15a8 3663 (Merge GET_ITER and GET_YIELD_FROM_ITER. Modify SEND to make it a bit more like FOR_ITER)
     Python 3.15a8 3664 (Fix __qualname__ for __annotate__ functions)
     Python 3.15a8 3665 (Add FOR_ITER_VIRTUAL and GET_ITER specializations)
+    Python 3.15b1 3666 (Add SEND_VIRTUAL and SEND_ASYNC_GEN specializations)
 
 
     Python 3.16 will start with 3700
@@ -309,7 +310,7 @@ PC/launcher.c must also be updated.
 
 */
 
-#define PYC_MAGIC_NUMBER 3665
+#define PYC_MAGIC_NUMBER 3666
 /* This is equivalent to converting PYC_MAGIC_NUMBER to 2 bytes
    (little-endian) and then appending b'\r\n'. */
 #define PYC_MAGIC_NUMBER_TOKEN \