Add FOR_ITER specialization for virtual iterators. Specialize GET_ITER.

* Add FOR_ITER_VIRTUAL to specialize FOR_ITER for virtual iterators
* Add GET_ITER_SELF to specialize GET_ITER for iterators (including generators)
* Add GET_ITER_VIRTUAL to specialize GET_ITER for iterables as virtual iterators

Author: markshannon

Include/cpython/object.h

@@ -229,6 +229,8 @@ struct _typeobject {
 
     destructor tp_finalize;
     vectorcallfunc tp_vectorcall;
+    iteritemfunc tp_iteritem;
+
 
     /* bitset of which type-watchers care about this type */
     unsigned char tp_watched;

Include/internal/pycore_code.h

@@ -141,6 +141,12 @@ typedef struct {
 
 #define INLINE_CACHE_ENTRIES_FOR_ITER CACHE_ENTRIES(_PyForIterCache)
 
+typedef struct {
+    _Py_BackoffCounter counter;
+} _PyGetIterCache;
+
+#define INLINE_CACHE_ENTRIES_GET_ITER CACHE_ENTRIES(_PyGetIterCache)
+
 typedef struct {
     _Py_BackoffCounter counter;
 } _PySendCache;
@@ -324,6 +330,7 @@ PyAPI_FUNC(void) _Py_Specialize_ContainsOp(_PyStackRef value, _Py_CODEUNIT *inst
 PyAPI_FUNC(void) _Py_GatherStats_GetIter(_PyStackRef iterable);
 PyAPI_FUNC(void) _Py_Specialize_CallFunctionEx(_PyStackRef func_st, _Py_CODEUNIT *instr);
 PyAPI_FUNC(void) _Py_Specialize_Resume(_Py_CODEUNIT *instr, PyThreadState *tstate, _PyInterpreterFrame *frame);
+PyAPI_FUNC(void) _Py_Specialize_GetIter(_PyStackRef iterable, _Py_CODEUNIT *instr);
 
 // Utility functions for reading/writing 32/64-bit values in the inline caches.
 // Great care should be taken to ensure that these functions remain correct and

Include/internal/pycore_magic_number.h

@@ -294,6 +294,7 @@ Known values:
     Python 3.15a4 3661 (Lazy imports IMPORT_NAME opcode changes)
     Python 3.15a8 3662 (Add counter to RESUME)
     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 (Add FOR_ITER_VIRTUAL and GET_ITER specializations)
 
 
     Python 3.16 will start with 3700
@@ -307,7 +308,7 @@ PC/launcher.c must also be updated.
 
 */
 
-#define PYC_MAGIC_NUMBER 3663
+#define PYC_MAGIC_NUMBER 3664
 /* This is equivalent to converting PYC_MAGIC_NUMBER to 2 bytes
    (little-endian) and then appending b'\r\n'. */
 #define PYC_MAGIC_NUMBER_TOKEN \