gh-121459: Deferred LOAD_GLOBAL

Include/internal/pycore_ceval.h

@@ -271,7 +271,7 @@ PyAPI_FUNC(PyObject **) _PyObjectArray_FromStackRefArray(_PyStackRef *input, Py_
 PyAPI_FUNC(void) _PyObjectArray_Free(PyObject **array, PyObject **scratch);
 
 PyAPI_FUNC(PyObject *) _PyEval_GetANext(PyObject *aiter);
-PyAPI_FUNC(PyObject *) _PyEval_LoadGlobal(PyObject *globals, PyObject *builtins, PyObject *name);
+PyAPI_FUNC(void) _PyEval_LoadGlobalStackRef(PyObject *globals, PyObject *builtins, PyObject *name, _PyStackRef *writeto);
 PyAPI_FUNC(PyObject *) _PyEval_GetAwaitable(PyObject *iterable, int oparg);
 PyAPI_FUNC(PyObject *) _PyEval_LoadName(PyThreadState *tstate, _PyInterpreterFrame *frame, PyObject *name);
 

Include/internal/pycore_dict.h

@@ -10,6 +10,7 @@ extern "C" {
 
 #include "pycore_object.h"               // PyManagedDictPointer
 #include "pycore_pyatomic_ft_wrappers.h" // FT_ATOMIC_LOAD_SSIZE_ACQUIRE
+#include "pycore_stackref.h"             // _PyStackRef
 
 // Unsafe flavor of PyDict_GetItemWithError(): no error checking
 extern PyObject* _PyDict_GetItemWithError(PyObject *dp, PyObject *key);
@@ -100,10 +101,12 @@ extern void _PyDictKeys_DecRef(PyDictKeysObject *keys);
  */
 extern Py_ssize_t _Py_dict_lookup(PyDictObject *mp, PyObject *key, Py_hash_t hash, PyObject **value_addr);
 extern Py_ssize_t _Py_dict_lookup_threadsafe(PyDictObject *mp, PyObject *key, Py_hash_t hash, PyObject **value_addr);
+extern Py_ssize_t _Py_dict_lookup_threadsafe_stackref(PyDictObject *mp, PyObject *key, Py_hash_t hash, _PyStackRef *value_addr);
 
 extern Py_ssize_t _PyDict_LookupIndex(PyDictObject *, PyObject *);
 extern Py_ssize_t _PyDictKeys_StringLookup(PyDictKeysObject* dictkeys, PyObject *key);
 PyAPI_FUNC(PyObject *)_PyDict_LoadGlobal(PyDictObject *, PyDictObject *, PyObject *);
+PyAPI_FUNC(void) _PyDict_LoadGlobalStackRef(PyDictObject *, PyDictObject *, PyObject *, _PyStackRef *);
 
 /* Consumes references to key and value */
 PyAPI_FUNC(int) _PyDict_SetItem_Take2(PyDictObject *op, PyObject *key, PyObject *value);

Objects/dictobject.c

@@ -1496,6 +1496,70 @@ _Py_dict_lookup_threadsafe(PyDictObject *mp, PyObject *key, Py_hash_t hash, PyOb
     return ix;
 }
 
+Py_ssize_t
+_Py_dict_lookup_threadsafe_stackref(PyDictObject *mp, PyObject *key, Py_hash_t hash, _PyStackRef *value_addr)
+{
+    PyDictKeysObject *dk;
+    DictKeysKind kind;
+    Py_ssize_t ix;
+
+    ensure_shared_on_read(mp);
+
+    dk = _Py_atomic_load_ptr(&mp->ma_keys);
+    kind = dk->dk_kind;
+
+    if (kind != DICT_KEYS_GENERAL) {
+        PyObject *value;
+        ix = _Py_dict_lookup_threadsafe(mp, key, hash, &value);
+        assert (ix >= 0 || value == NULL);
+        *value_addr = PyStackRef_FromPyObjectSteal(value);
+    }
+    else {
+        ix = dictkeys_generic_lookup_threadsafe(mp, dk, key, hash);
+        if (ix == DKIX_KEY_CHANGED) {
+            goto read_failed;
+        }
+        if (ix >= 0) {
+            PyObject **addr_of_value = &(DK_ENTRIES(dk)[ix].me_value);
+            PyObject *value = _Py_atomic_load_ptr(addr_of_value);
+            if (value == NULL) {
+                *value_addr = PyStackRef_NULL;
+            }
+            else if (_Py_IsImmortal(value) ||
+                     _PyObject_HasDeferredRefcount(value)) {
+                *value_addr = PyStackRef_FromPyObjectNew(value);
+            }
+            else {
+                *value_addr = PyStackRef_FromPyObjectSteal(
+                    _Py_TryXGetRef(addr_of_value));
+            }
+            if (PyStackRef_IsNull(*value_addr)) {
+                goto read_failed;
+            }
+            if (dk != _Py_atomic_load_ptr(&mp->ma_keys)) {
+                goto read_failed;
+            }
+        }
+        else {
+            *value_addr = PyStackRef_NULL;
+        }
+    }
+
+    return ix;
+
+    PyObject *value;
+read_failed:
+    // In addition to the normal races of the dict being modified the _Py_TryXGetRef
+    // can all fail if they don't yet have a shared ref count.  That can happen here
+    // or in the *_lookup_* helper.  In that case we need to take the lock to avoid
+    // mutation and do a normal incref which will make them shared.
+    Py_BEGIN_CRITICAL_SECTION(mp);
+    ix = _Py_dict_lookup(mp, key, hash, &value);
+    *value_addr = value == NULL ? PyStackRef_NULL : PyStackRef_FromPyObjectNew(value);
+    Py_END_CRITICAL_SECTION();
+    return ix;
+}
+
 #else   // Py_GIL_DISABLED
 
 Py_ssize_t
@@ -1506,6 +1570,15 @@ _Py_dict_lookup_threadsafe(PyDictObject *mp, PyObject *key, Py_hash_t hash, PyOb
     return ix;
 }
 
+Py_ssize_t
+_Py_dict_lookup_threadsafe_stackref(PyDictObject *mp, PyObject *key, Py_hash_t hash, _PyStackRef *value_addr)
+{
+    PyObject *val;
+    Py_ssize_t ix = _Py_dict_lookup(mp, key, hash, &val);
+	*value_addr = val == NULL ? PyStackRef_NULL : PyStackRef_FromPyObjectNew(val);
+    return ix;
+}
+
 #endif
 
 int
@@ -2418,6 +2491,31 @@ _PyDict_LoadGlobal(PyDictObject *globals, PyDictObject *builtins, PyObject *key)
     return value;
 }
 
+void
+_PyDict_LoadGlobalStackRef(PyDictObject *globals, PyDictObject *builtins, PyObject *key, _PyStackRef *res)
+{
+    Py_ssize_t ix;
+    Py_hash_t hash;
+
+    hash = _PyObject_HashFast(key);
+    if (hash == -1) {
+        *res = PyStackRef_NULL;
+    }
+
+    /* namespace 1: globals */
+    ix = _Py_dict_lookup_threadsafe_stackref(globals, key, hash, res);
+    if (ix == DKIX_ERROR) {
+        *res = PyStackRef_NULL;
+    }
+    if (ix != DKIX_EMPTY && !PyStackRef_IsNull(*res)) {
+        return;
+    }
+
+    /* namespace 2: builtins */
+    ix = _Py_dict_lookup_threadsafe_stackref(builtins, key, hash, res);
+    assert(ix >= 0 || PyStackRef_IsNull(*res));
+}
+
 /* Consumes references to key and value */
 static int
 setitem_take2_lock_held(PyDictObject *mp, PyObject *key, PyObject *value)

Python/bytecodes.c

@@ -1448,8 +1448,8 @@ dummy_func(
                     && PyDict_CheckExact(BUILTINS()))
                 {
                     v_o = _PyDict_LoadGlobal((PyDictObject *)GLOBALS(),
-                                            (PyDictObject *)BUILTINS(),
-                                            name);
+                                             (PyDictObject *)BUILTINS(),
+                                             name);
                     if (v_o == NULL) {
                         if (!_PyErr_Occurred(tstate)) {
                             /* _PyDict_LoadGlobal() returns NULL without raising
@@ -1507,10 +1507,9 @@ dummy_func(
 
         op(_LOAD_GLOBAL, ( -- res, null if (oparg & 1))) {
             PyObject *name = GETITEM(FRAME_CO_NAMES, oparg>>1);
-            PyObject *res_o = _PyEval_LoadGlobal(GLOBALS(), BUILTINS(), name);
-            ERROR_IF(res_o == NULL, error);
+            _PyEval_LoadGlobalStackRef(GLOBALS(), BUILTINS(), name, STACK_ENTRY(res));
+            ERROR_IF(PyStackRef_IsNull(res), error);
             null = PyStackRef_NULL;
-            res = PyStackRef_FromPyObjectSteal(res_o);
         }
 
         macro(LOAD_GLOBAL) =

Python/ceval.c

@@ -3072,15 +3072,14 @@ _PyEval_GetANext(PyObject *aiter)
     return awaitable;
 }
 
-PyObject *
-_PyEval_LoadGlobal(PyObject *globals, PyObject *builtins, PyObject *name)
+void
+_PyEval_LoadGlobalStackRef(PyObject *globals, PyObject *builtins, PyObject *name, _PyStackRef *writeto)
 {
-    PyObject *res;
     if (PyDict_CheckExact(globals) && PyDict_CheckExact(builtins)) {
-        res = _PyDict_LoadGlobal((PyDictObject *)globals,
+        _PyDict_LoadGlobalStackRef((PyDictObject *)globals,
                                     (PyDictObject *)builtins,
-                                    name);
-        if (res == NULL && !PyErr_Occurred()) {
+                                    name, writeto);
+        if (PyStackRef_IsNull(*writeto) && !PyErr_Occurred()) {
             /* _PyDict_LoadGlobal() returns NULL without raising
                 * an exception if the key doesn't exist */
             _PyEval_FormatExcCheckArg(PyThreadState_GET(), PyExc_NameError,
@@ -3090,22 +3089,25 @@ _PyEval_LoadGlobal(PyObject *globals, PyObject *builtins, PyObject *name)
     else {
         /* Slow-path if globals or builtins is not a dict */
         /* namespace 1: globals */
+        PyObject *res;
         if (PyMapping_GetOptionalItem(globals, name, &res) < 0) {
-            return NULL;
+            *writeto = PyStackRef_NULL;
+            return;
         }
         if (res == NULL) {
             /* namespace 2: builtins */
             if (PyMapping_GetOptionalItem(builtins, name, &res) < 0) {
-                return NULL;
+                *writeto = PyStackRef_NULL;
+                return;
             }
             if (res == NULL) {
                 _PyEval_FormatExcCheckArg(
                             PyThreadState_GET(), PyExc_NameError,
                             NAME_ERROR_MSG, name);
             }
         }
+        *writeto = PyStackRef_FromPyObjectSteal(res);
     }
-    return res;
 }
 
 PyObject *

Tools/cases_generator/generators_common.py

@@ -47,10 +47,10 @@ def write_header(
     )
 
 
-def emit_to(out: CWriter, tkn_iter: Iterator[Token], end: str) -> None:
+def emit_to(out: CWriter, tkn_iter: Iterator[Token], end: str, *, allow_unbalanced_parens: bool = False) -> None:
     parens = 0
     for tkn in tkn_iter:
-        if tkn.kind == end and parens == 0:
+        if tkn.kind == end and (parens == 0 or allow_unbalanced_parens):
             return
         if tkn.kind == "LPAREN":
             parens += 1
@@ -77,6 +77,7 @@ def __init__(self, out: CWriter):
             "DECREF_INPUTS": self.decref_inputs,
             "SYNC_SP": self.sync_sp,
             "PyStackRef_FromPyObjectNew": self.py_stack_ref_from_py_object_new,
+            "STACK_ENTRY": self.stack_entry,
         }
         self.out = out
 
@@ -211,6 +212,33 @@ def py_stack_ref_from_py_object_new(
         # unused portions of the stack to NULL.
         stack.flush_single_var(self.out, target, uop.stack.outputs)
 
+    def stack_entry(
+        self,
+        tkn: Token,
+        tkn_iter: Iterator[Token],
+        uop: Uop,
+        stack: Stack,
+        inst: Instruction | None,
+    ) -> None:
+        emit_to(self.out, tkn_iter, "LPAREN")
+        target = next(tkn_iter)
+        size = "0"
+        for output in uop.stack.inputs:
+            size += f" - {output.size or 1}"
+        for output in uop.stack.outputs:
+            if output.name == target.text:
+                self.out.emit(f" &stack_pointer[{size}]")
+                break
+            size += f" + {output.size or 1}"
+        else:
+            raise analysis_error("STACK_ENTRY operand is not a stack output", target)
+
+        next(tkn_iter)  # Consume )
+        emit_to(self.out, tkn_iter, "SEMI", allow_unbalanced_parens=True)
+        self.emit(";\n")
+        # Update the variable
+        self.out.emit(f"{target.text} = stack_pointer[{size}];\n")
+
     def emit_tokens(
         self,
         uop: Uop,