Return a small struct in tp_iteritem to support MSVC tailcalling

Author: markshannon

Include/object.h

@@ -385,7 +385,11 @@ typedef Py_hash_t (*hashfunc)(PyObject *);
 typedef PyObject *(*richcmpfunc) (PyObject *, PyObject *, int);
 typedef PyObject *(*getiterfunc) (PyObject *);
 typedef PyObject *(*iternextfunc) (PyObject *);
-typedef PyObject *(*iteritemfunc) (PyObject *, Py_ssize_t *index);
+typedef struct _py_object_index_pair {
+    PyObject *object;
+    Py_ssize_t index;
+} PyObjectIndexPair;
+typedef PyObjectIndexPair (*iteritemfunc) (PyObject *, Py_ssize_t index);
 typedef PyObject *(*descrgetfunc) (PyObject *, PyObject *, PyObject *);
 typedef int (*descrsetfunc) (PyObject *, PyObject *, PyObject *);
 typedef int (*initproc)(PyObject *, PyObject *, PyObject *);

Modules/_testinternalcapi/test_cases.c.h

@@ -3206,16 +3206,15 @@ Construct an immutable array of bytes from:\n\
 static PyObject *bytes_iter(PyObject *seq);
 
 
-static PyObject *
-bytes_iteritem(PyObject *obj, Py_ssize_t *index)
+static PyObjectIndexPair
+bytes_iteritem(PyObject *obj, Py_ssize_t index)
 {
-    Py_ssize_t i = *index;
     PyBytesObject *a = _PyBytes_CAST(obj);
-    if (i >= Py_SIZE(a)) {
-        return NULL;
+    if (index >= Py_SIZE(a)) {
+        return (PyObjectIndexPair) { .object = NULL, .index = index };
     }
-    *index = i+1;
-    return _PyLong_FromUnsignedChar((unsigned char)a->ob_sval[i]);
+    PyObject *l = _PyLong_FromUnsignedChar((unsigned char)a->ob_sval[index]);
+    return (PyObjectIndexPair) { .object = l, .index = index + 1 };
 }
 
 PyTypeObject PyBytes_Type = {

Objects/bytesobject.c

@@ -3913,13 +3913,11 @@ list_ass_subscript(PyObject *self, PyObject *item, PyObject *value)
     return res;
 }
 
-static PyObject *
-list_iteritem(PyObject *obj, Py_ssize_t *index)
+static PyObjectIndexPair
+list_iteritem(PyObject *obj, Py_ssize_t index)
 {
-    Py_ssize_t i = *index;
-    PyObject *result = list_get_item_ref((PyListObject *)obj, i);
-    *index = i+1;
-    return result;
+    PyObject *result = list_get_item_ref((PyListObject *)obj, index);
+    return (PyObjectIndexPair) { .object = result, .index = index + 1 };
 }
 
 static PyMappingMethods list_as_mapping = {

Objects/listobject.c

@@ -873,17 +873,16 @@ static PySequenceMethods tuple_as_sequence = {
     tuple_contains,                             /* sq_contains */
 };
 
-static PyObject *
-tuple_iteritem(PyObject *obj, Py_ssize_t *index)
+static PyObjectIndexPair
+tuple_iteritem(PyObject *obj, Py_ssize_t index)
 {
-    Py_ssize_t i = *index;
-    if (i >= PyTuple_GET_SIZE(obj)) {
-        return NULL;
+    ;
+    if (index >= PyTuple_GET_SIZE(obj)) {
+        return (PyObjectIndexPair) { .object = NULL, index = index };
     }
-    PyObject *result = PyTuple_GET_ITEM(obj, i);
+    PyObject *result = PyTuple_GET_ITEM(obj, index);
     Py_INCREF(result);
-    *index = i+1;
-    return result;
+    return (PyObjectIndexPair) { .object = result, .index = index + 1 };
 }
 
 static PyObject*

Objects/tupleobject.c

@@ -13982,24 +13982,18 @@ unicode_subtype_new(PyTypeObject *type, PyObject *unicode)
     return NULL;
 }
 
-static PyObject *
-unicode_iteritem(PyObject *obj, Py_ssize_t *index)
+static PyObjectIndexPair
+unicode_iteritem(PyObject *obj, Py_ssize_t index)
 {
-    Py_ssize_t i = *index;
-    if (i >= PyUnicode_GET_LENGTH(obj)) {
-        return NULL;
+    if (index >= PyUnicode_GET_LENGTH(obj)) {
+        return (PyObjectIndexPair) { .object = NULL, .index = index };
     }
     const void *data = PyUnicode_DATA(obj);
     int kind = PyUnicode_KIND(obj);
-    Py_UCS4 ch = PyUnicode_READ(kind, data, i);
+    Py_UCS4 ch = PyUnicode_READ(kind, data, index);
     PyObject *result = unicode_char(ch);
-    if (result == NULL) {
-        *index = -1;
-    }
-    else {
-        *index = i+1;
-    }
-    return result;
+    index = (result == NULL) ? -1 : index + 1;
+    return (PyObjectIndexPair) { .object = result, .index = index };
 }
 
 void

Objects/unicodeobject.c

@@ -3643,12 +3643,9 @@ dummy_func(
         replaced op(_FOR_ITER_VIRTUAL, (iter, null_or_index -- iter, null_or_index, next)) {
             PyObject *iter_o = PyStackRef_AsPyObjectBorrow(iter);
             Py_ssize_t index = PyStackRef_UntagInt(null_or_index);
-            PyObject *next_o;
-            { // Make it clear that index doesn't escape; for MSVC tailcall
-                Py_ssize_t tmp = index;
-                next_o = Py_TYPE(iter_o)->tp_iteritem(iter_o, &tmp);
-                index = tmp;
-            }
+            PyObjectIndexPair next_index = Py_TYPE(iter_o)->tp_iteritem(iter_o, index);
+            PyObject *next_o = next_index.object;
+            index = next_index.index;
             if (next_o == NULL) {
                 if (index < 0) {
                     ERROR_NO_POP();
@@ -3669,7 +3666,9 @@ dummy_func(
         op(_FOR_ITER_VIRTUAL_TIER_TWO, (iter, null_or_index -- iter, null_or_index, next)) {
             PyObject *iter_o = PyStackRef_AsPyObjectBorrow(iter);
             Py_ssize_t index = PyStackRef_UntagInt(null_or_index);
-            PyObject *next_o = Py_TYPE(iter_o)->tp_iteritem(iter_o, &index);
+            PyObjectIndexPair next_index = Py_TYPE(iter_o)->tp_iteritem(iter_o, index);
+            PyObject *next_o = next_index.object;
+            index = next_index.index;
             if (next_o == NULL) {
                 if (index < 0) {
                     ERROR_NO_POP();

Python/bytecodes.c

@@ -3759,15 +3759,17 @@ _PyStackRef _PyForIter_VirtualIteratorNext(PyThreadState* tstate, _PyInterpreter
     if (PyStackRef_IsTaggedInt(index)) {
         intptr_t i = PyStackRef_UntagInt(index);
         assert(i >= 0);
-        PyObject *next = Py_TYPE(iter_o)->tp_iteritem(iter_o, &i);
+        PyObjectIndexPair next_index = Py_TYPE(iter_o)->tp_iteritem(iter_o, i);
+        i = next_index.index;
+        PyObject *next = next_index.object;
         if (next == NULL) {
             return i < 0 ? PyStackRef_ERROR : PyStackRef_NULL;
         }
         *index_ptr = PyStackRef_TagInt(i);
         return PyStackRef_FromPyObjectSteal(next);
     }
-    PyObject *next_o = (*Py_TYPE(iter_o)->tp_iternext)(iter_o);
-    if (next_o == NULL) {
+    PyObject *next = (*Py_TYPE(iter_o)->tp_iternext)(iter_o);
+    if (next == NULL) {
         if (_PyErr_Occurred(tstate)) {
             if (_PyErr_ExceptionMatches(tstate, PyExc_StopIteration)) {
                 _PyEval_MonitorRaise(tstate, frame, frame->instr_ptr);
@@ -3779,7 +3781,7 @@ _PyStackRef _PyForIter_VirtualIteratorNext(PyThreadState* tstate, _PyInterpreter
         }
         return PyStackRef_NULL;
     }
-    return PyStackRef_FromPyObjectSteal(next_o);
+    return PyStackRef_FromPyObjectSteal(next);
 }
 
 /* Check if a 'cls' provides the given special method. */