GH-145668: Add FOR_ITER specialization for virtual iterators. Specialize GET_ITER. (GH-147967)

* 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
* Add new (internal) _tp_iteritem function slot to PyTypeObject
* Put limited RESUME at start of genexpr for free-threading. Fix up exception handling in genexpr

Author: markshannon

Include/cpython/object.h

@@ -230,6 +230,8 @@ struct _typeobject {
     destructor tp_finalize;
     vectorcallfunc tp_vectorcall;
 
+    /* Below here all fields are internal to the VM */
+
     /* bitset of which type-watchers care about this type */
     unsigned char tp_watched;
 
@@ -239,6 +241,7 @@ struct _typeobject {
      * Otherwise, limited to MAX_VERSIONS_PER_CLASS (defined elsewhere).
      */
     uint16_t tp_versions_used;
+    _Py_iteritemfunc _tp_iteritem; /* Virtual iterator next function */
 };
 
 #define _Py_ATTR_CACHE_UNUSED (30000)  // (see tp_versions_used)

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

@@ -295,6 +295,7 @@ Known values:
     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 (Fix __qualname__ for __annotate__ functions)
+    Python 3.15a8 3665 (Add FOR_ITER_VIRTUAL and GET_ITER specializations)
 
 
     Python 3.16 will start with 3700
@@ -308,7 +309,7 @@ PC/launcher.c must also be updated.
 
 */
 
-#define PYC_MAGIC_NUMBER 3664
+#define PYC_MAGIC_NUMBER 3665
 /* This is equivalent to converting PYC_MAGIC_NUMBER to 2 bytes
    (little-endian) and then appending b'\r\n'. */
 #define PYC_MAGIC_NUMBER_TOKEN \

Include/internal/pycore_opcode_metadata.h

@@ -82,9 +82,10 @@ extern "C" {
 #define RESUME_AFTER_YIELD 1
 #define RESUME_AFTER_YIELD_FROM 2
 #define RESUME_AFTER_AWAIT 3
+#define RESUME_AT_GEN_EXPR_START 4
 
-#define RESUME_OPARG_LOCATION_MASK 0x3
-#define RESUME_OPARG_DEPTH1_MASK 0x4
+#define RESUME_OPARG_LOCATION_MASK 0x7
+#define RESUME_OPARG_DEPTH1_MASK 0x8
 
 #define GET_ITER_YIELD_FROM 1
 #define GET_ITER_YIELD_FROM_NO_CHECK 2