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GH-93143: Don't turn LOAD_FAST into LOAD_FAST_CHECK #99075

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Merged
merged 6 commits into from
Nov 8, 2022
Merged

GH-93143: Don't turn LOAD_FAST into LOAD_FAST_CHECK #99075

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e0f2459
Just assign None instead of changing co_code
brandtbucher Oct 20, 2022
6f68d32
blurb add
brandtbucher Oct 20, 2022
dd52da5
Feedback from code review
brandtbucher Oct 20, 2022
da07230
fixup
brandtbucher Oct 20, 2022
555d23e
Perform assignments *before* adjusting the stack
brandtbucher Oct 20, 2022
4e597b3
Address code review
brandtbucher Oct 21, 2022
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31 changes: 21 additions & 10 deletions Lib/test/test_peepholer.py
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Original file line number Diff line number Diff line change
Expand Up @@ -819,8 +819,8 @@ def test_setting_lineno_adds_check(self):
code = textwrap.dedent("""\
def f():
x = 2
L = 3
L = 4
if not x:
return 4
for i in range(55):
x + 6
del x
Expand All @@ -832,15 +832,22 @@ def f():
exec(code, ns)
f = ns['f']
self.assertInBytecode(f, "LOAD_FAST")
self.assertNotInBytecode(f, "LOAD_FAST_CHECK")
co_code = f.__code__.co_code
def trace(frame, event, arg):
if event == 'line' and frame.f_lineno == 9:
frame.f_lineno = 2
frame.f_lineno = 3
sys.settrace(None)
return None
return trace
sys.settrace(trace)
f()
self.assertNotInBytecode(f, "LOAD_FAST")
e = r"cannot leave local name 'x' unbound \(assigning None\)"
with self.assertWarnsRegex(RuntimeWarning, e):
sys.settrace(trace)
result = f()
self.assertEqual(result, 4)
self.assertInBytecode(f, "LOAD_FAST")
self.assertNotInBytecode(f, "LOAD_FAST_CHECK")
self.assertEqual(f.__code__.co_code, co_code)

def make_function_with_no_checks(self):
code = textwrap.dedent("""\
Expand All @@ -862,16 +869,20 @@ def f():

def test_deleting_local_adds_check(self):
f = self.make_function_with_no_checks()
co_code = f.__code__.co_code
def trace(frame, event, arg):
if event == 'line' and frame.f_lineno == 4:
del frame.f_locals["x"]
sys.settrace(None)
return None
return trace
sys.settrace(trace)
f()
self.assertNotInBytecode(f, "LOAD_FAST")
self.assertInBytecode(f, "LOAD_FAST_CHECK")
e = r"cannot leave local name 'x' unbound \(assigning None\)"
with self.assertWarnsRegex(RuntimeWarning, e):
sys.settrace(trace)
f()
self.assertInBytecode(f, "LOAD_FAST")
self.assertNotInBytecode(f, "LOAD_FAST_CHECK")
self.assertEqual(f.__code__.co_code, co_code)

def test_modifying_local_does_not_add_check(self):
f = self.make_function_with_no_checks()
Expand Down
4 changes: 4 additions & 0 deletions Misc/NEWS.d/next/Core and Builtins/2022-10-19-23-54-43.gh-issue-93143.1wCYub.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,4 @@
Rather than changing :attr:`~types.CodeType.co_code`, the interpreter will
now display a :exc:`RuntimeWarning` and assign :const:`None` to any fast
locals that are incorrectly left unbound after jumps or :keyword:`del`
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There aren't necessarily incorrect, so maybe just "locals that are unbound after ..."

statements executed while tracing.
111 changes: 47 additions & 64 deletions Objects/frameobject.c
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Original file line number Diff line number Diff line change
Expand Up @@ -620,39 +620,46 @@ _PyFrame_GetState(PyFrameObject *frame)
Py_UNREACHABLE();
}

static void
add_load_fast_null_checks(PyCodeObject *co)
static bool
op_exists(PyCodeObject *co, int opcode, int oparg, int start, int stop)
{
int changed = 0;
// This only works when opcode is a non-quickened form:
assert(_PyOpcode_Deopt[opcode] == opcode);
int check_oparg = 0;
_Py_CODEUNIT *instructions = _PyCode_CODE(co);
for (Py_ssize_t i = 0; i < Py_SIZE(co); i++) {
int opcode = _Py_OPCODE(instructions[i]);
switch (opcode) {
case LOAD_FAST:
case LOAD_FAST__LOAD_FAST:
case LOAD_FAST__LOAD_CONST:
changed = 1;
_Py_SET_OPCODE(instructions[i], LOAD_FAST_CHECK);
break;
case LOAD_CONST__LOAD_FAST:
changed = 1;
_Py_SET_OPCODE(instructions[i], LOAD_CONST);
break;
case STORE_FAST__LOAD_FAST:
changed = 1;
_Py_SET_OPCODE(instructions[i], STORE_FAST);
break;
while (start < stop) {
int check_opcode = _PyOpcode_Deopt[_Py_OPCODE(instructions[start])];
check_oparg |= _Py_OPARG(instructions[start]);
if (check_opcode == opcode && check_oparg == oparg) {
return true;
}
if (check_opcode == EXTENDED_ARG) {
check_oparg <<= 8;
}
i += _PyOpcode_Caches[_PyOpcode_Deopt[opcode]];
else {
check_oparg = 0;
}
start += 1 + _PyOpcode_Caches[check_opcode];
}
if (changed && co->_co_cached != NULL) {
// invalidate cached co_code object
Py_CLEAR(co->_co_cached->_co_code);
Py_CLEAR(co->_co_cached->_co_cellvars);
Py_CLEAR(co->_co_cached->_co_freevars);
Py_CLEAR(co->_co_cached->_co_varnames);
PyMem_Free(co->_co_cached);
co->_co_cached = NULL;
return false;
}

static void
maybe_bind_fast_local(_PyInterpreterFrame *f, int i)
{
PyCodeObject *co = f->f_code;
if (f->localsplus[i] == NULL && op_exists(co, LOAD_FAST, i, 0, Py_SIZE(co)))
{
// We're being overly-cautious here (it's too expensive to run a full
// flow analysis), but at least one instruction expects i to be bound.
// Rather than crashing (or changing co_code), just bind None instead:
const char *e = "cannot leave local name %R unbound (assigning None)";
PyObject *name = PyTuple_GET_ITEM(co->co_localsplusnames, i);
if (PyErr_WarnFormat(PyExc_RuntimeWarning, 0, e, name)) {
// It's okay if frame_obj is NULL, just try anyways:
PyErr_WriteUnraisable((PyObject *)f->frame_obj);
}
f->localsplus[i] = Py_NewRef(Py_None);
}
}

Expand Down Expand Up @@ -745,8 +752,6 @@ frame_setlineno(PyFrameObject *f, PyObject* p_new_lineno, void *Py_UNUSED(ignore
return -1;
}

add_load_fast_null_checks(f->f_frame->f_code);

/* PyCode_NewWithPosOnlyArgs limits co_code to be under INT_MAX so this
* should never overflow. */
int len = (int)Py_SIZE(f->f_frame->f_code);
Expand Down Expand Up @@ -826,6 +831,10 @@ frame_setlineno(PyFrameObject *f, PyObject* p_new_lineno, void *Py_UNUSED(ignore
}
start_stack = pop_value(start_stack);
}
// Populate any NULL locals that might be needed:
for (int i = 0; i < f->f_frame->f_code->co_nlocalsplus; i++) {
maybe_bind_fast_local(f->f_frame, i);
}
/* Finally set the new lasti and return OK. */
f->f_lineno = 0;
f->f_frame->prev_instr = _PyCode_CODE(f->f_frame->f_code) + best_addr;
Expand Down Expand Up @@ -1108,31 +1117,6 @@ PyFrame_New(PyThreadState *tstate, PyCodeObject *code,
return f;
}

static int
_PyFrame_OpAlreadyRan(_PyInterpreterFrame *frame, int opcode, int oparg)
{
// This only works when opcode is a non-quickened form:
assert(_PyOpcode_Deopt[opcode] == opcode);
int check_oparg = 0;
for (_Py_CODEUNIT *instruction = _PyCode_CODE(frame->f_code);
instruction < frame->prev_instr; instruction++)
{
int check_opcode = _PyOpcode_Deopt[_Py_OPCODE(*instruction)];
check_oparg |= _Py_OPARG(*instruction);
if (check_opcode == opcode && check_oparg == oparg) {
return 1;
}
if (check_opcode == EXTENDED_ARG) {
check_oparg <<= 8;
}
else {
check_oparg = 0;
}
instruction += _PyOpcode_Caches[check_opcode];
}
return 0;
}

int
_PyFrame_FastToLocalsWithError(_PyInterpreterFrame *frame) {
/* Merge fast locals into f->f_locals */
Expand Down Expand Up @@ -1195,7 +1179,8 @@ _PyFrame_FastToLocalsWithError(_PyInterpreterFrame *frame) {
// run yet.
if (value != NULL) {
if (PyCell_Check(value) &&
_PyFrame_OpAlreadyRan(frame, MAKE_CELL, i)) {
op_exists(co, MAKE_CELL, i, 0, lasti))
{
// (likely) MAKE_CELL must have executed already.
value = PyCell_GET(value);
}
Expand Down Expand Up @@ -1269,7 +1254,7 @@ _PyFrame_LocalsToFast(_PyInterpreterFrame *frame, int clear)
}
fast = _PyFrame_GetLocalsArray(frame);
co = frame->f_code;
bool added_null_checks = false;
int lasti = _PyInterpreterFrame_LASTI(frame);

PyErr_Fetch(&error_type, &error_value, &error_traceback);
for (int i = 0; i < co->co_nlocalsplus; i++) {
Expand All @@ -1289,10 +1274,6 @@ _PyFrame_LocalsToFast(_PyInterpreterFrame *frame, int clear)
}
}
PyObject *oldvalue = fast[i];
if (!added_null_checks && oldvalue != NULL && value == NULL) {
add_load_fast_null_checks(co);
added_null_checks = true;
}
PyObject *cell = NULL;
if (kind == CO_FAST_FREE) {
// The cell was set when the frame was created from
Expand All @@ -1302,8 +1283,8 @@ _PyFrame_LocalsToFast(_PyInterpreterFrame *frame, int clear)
}
else if (kind & CO_FAST_CELL && oldvalue != NULL) {
/* Same test as in PyFrame_FastToLocals() above. */
if (PyCell_Check(oldvalue) &&
_PyFrame_OpAlreadyRan(frame, MAKE_CELL, i)) {
if (PyCell_Check(oldvalue) && op_exists(co, MAKE_CELL, i, 0, lasti))
{
// (likely) MAKE_CELL must have executed already.
cell = oldvalue;
}
Expand All @@ -1321,6 +1302,8 @@ _PyFrame_LocalsToFast(_PyInterpreterFrame *frame, int clear)
else if (value != oldvalue) {
Py_XINCREF(value);
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Suggested change
Py_XINCREF(value);
Py_INCREF(value);

Py_XSETREF(fast[i], value);
// Populate any NULL locals that might be needed:
maybe_bind_fast_local(frame, i);
}
Py_XDECREF(value);
}
Expand Down