Write to second slot on stack

Author: Fidget-Spinner

Include/internal/pycore_object.h

@@ -882,7 +882,7 @@ PyAPI_FUNC(PyObject*) _PyObject_LookupSpecialMethod(PyObject *self, PyObject *at
 extern int _PyObject_IsAbstract(PyObject *);
 
 PyAPI_FUNC(int) _PyObject_GetMethod(PyObject *obj, PyObject *name, PyObject **method);
-PyAPI_FUNC(int) _PyObject_GetMethodStackRef(PyObject *obj, PyObject *name, _PyStackRef *method);
+PyAPI_FUNC(int) _PyObject_GetMethodStackRef(PyObject *obj, PyObject *name, _PyStackRef *method, _PyStackRef *spare);
 extern PyObject* _PyObject_NextNotImplemented(PyObject *);
 
 // Pickle support.

Include/internal/pycore_opcode_metadata.h

@@ -1621,7 +1621,7 @@ _PyObject_GetMethod(PyObject *obj, PyObject *name, PyObject **method)
 }
 
 int
-_PyObject_GetMethodStackRef(PyObject *obj, PyObject *name, _PyStackRef *method)
+_PyObject_GetMethodStackRef(PyObject *obj, PyObject *name, _PyStackRef *method, _PyStackRef *spare)
 {
 #ifdef Py_GIL_DISABLED
     int meth_found = 0;
@@ -1647,6 +1647,7 @@ _PyObject_GetMethodStackRef(PyObject *obj, PyObject *name, _PyStackRef *method)
     }
 
     _PyType_LookupStackRef(tp, name, method);
+    *spare = *method;
     _PyStackRef descr_st = *method;
     descrgetfunc f = NULL;
     if (!PyStackRef_IsNull(descr_st)) {

Objects/object.c

@@ -2054,19 +2054,19 @@ dummy_func(
             #endif  /* ENABLE_SPECIALIZATION */
         }
 
-        op(_LOAD_ATTR, (owner -- attr[1], self_or_null if (oparg & 1))) {
+        op(_LOAD_ATTR, (owner -- attr[1], self_or_null[1] if (oparg & 1))) {
             PyObject *name = GETITEM(FRAME_CO_NAMES, oparg >> 1);
             if (oparg & 1) {
                 /* Designed to work in tandem with CALL, pushes two values. */
                 *attr = PyStackRef_NULL;
-                int is_meth = _PyObject_GetMethodStackRef(PyStackRef_AsPyObjectBorrow(owner), name, attr);
+                int is_meth = _PyObject_GetMethodStackRef(PyStackRef_AsPyObjectBorrow(owner), name, attr, self_or_null);
                 if (is_meth) {
                     /* We can bypass temporary bound method object.
                        meth is unbound method and obj is self.
                        meth | self | arg1 | ... | argN
                      */
                     assert(!PyStackRef_IsNull(*attr));  // No errors on this branch
-                    self_or_null = owner;  // Transfer ownership
+                    *self_or_null = owner;  // Transfer ownership
                     DEAD(owner);
                 }
                 else {
@@ -2078,7 +2078,7 @@ dummy_func(
                     */
                     DECREF_INPUTS();
                     ERROR_IF(PyStackRef_IsNull(*attr), error);
-                    self_or_null = PyStackRef_NULL;
+                    *self_or_null = PyStackRef_NULL;
                 }
             }
             else {
@@ -2090,10 +2090,10 @@ dummy_func(
                 else {
                     *attr = PyStackRef_FromPyObjectSteal(attr_o);
                 }
+                /* We need to define self_or_null on all paths */
+                *self_or_null = PyStackRef_NULL;
                 DECREF_INPUTS();
                 ERROR_IF(PyStackRef_IsNull(*attr), error);
-                /* We need to define self_or_null on all paths */
-                self_or_null = PyStackRef_NULL;
             }
         }
 

Python/bytecodes.c

@@ -1335,6 +1335,11 @@ optimize_and_assemble_code_unit(struct compiler_unit *u, PyObject *const_cache,
         goto error;
     }
 
+    /* Reserve an extra word on the stack to ensure there is space for uops to
+       pass at least one item on the stack to a subsequent uop.
+    */
+    stackdepth++;
+
     /** Assembly **/
     co = _PyAssemble_MakeCodeObject(&u->u_metadata, const_cache, consts,
                                     stackdepth, &optimized_instrs, nlocalsplus,

Python/compile.c

@@ -270,21 +270,15 @@ def cache_effect(self) -> CacheEffect | None:
 
     @contextual
     def stack_effect(self) -> StackEffect | None:
-        # IDENTIFIER [':' IDENTIFIER [TIMES]] ['if' '(' expression ')']
-        # | IDENTIFIER '[' expression ']'
+        # IDENTIFIER [':' IDENTIFIER [[TIMES]] ['if' '(' expression ')']
+        # | IDENTIFIER '[' expression ']' ['if' '(' expression ')']
         if tkn := self.expect(lx.IDENTIFIER):
             type_text = ""
             if self.expect(lx.COLON):
                 type_text = self.require(lx.IDENTIFIER).text.strip()
                 if self.expect(lx.TIMES):
                     type_text += " *"
             cond_text = ""
-            if self.expect(lx.IF):
-                self.require(lx.LPAREN)
-                if not (cond := self.expression()):
-                    raise self.make_syntax_error("Expected condition")
-                self.require(lx.RPAREN)
-                cond_text = cond.text.strip()
             size_text = ""
             if self.expect(lx.LBRACKET):
                 if type_text or cond_text:
@@ -293,6 +287,12 @@ def stack_effect(self) -> StackEffect | None:
                     raise self.make_syntax_error("Expected expression")
                 self.require(lx.RBRACKET)
                 size_text = size.text.strip()
+            if self.expect(lx.IF):
+                self.require(lx.LPAREN)
+                if not (cond := self.expression()):
+                    raise self.make_syntax_error("Expected condition")
+                self.require(lx.RPAREN)
+                cond_text = cond.text.strip()
             return StackEffect(tkn.text, type_text, cond_text, size_text)
         return None