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gh-139922: Tail calling for MSVC (VS 2026) #139962

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82d1259
Brandt's changes
Fidget-Spinner Aug 21, 2025
3248658
Hulon's and mine changes
Fidget-Spinner Aug 21, 2025
9ac430d
push stackrefs into tstate
Fidget-Spinner Aug 21, 2025
085c1d7
Fix last few remaining problems
Fidget-Spinner Aug 21, 2025
0b12f2e
fix handling of recursion
Fidget-Spinner Aug 24, 2025
acf48f5
Changes by Chris to support clang-cl
Fidget-Spinner Oct 11, 2025
35e96c1
Make the restrict MSVC only for now
Fidget-Spinner Oct 11, 2025
d7737e9
Merge remote-tracking branch 'upstream/main' into msvc-tailcall-take-two
Fidget-Spinner Oct 11, 2025
86f19cf
Change restricts to MSVC only
Fidget-Spinner Oct 11, 2025
40013cc
📜🤖 Added by blurb_it.
blurb-it[bot] Oct 11, 2025
48db59e
Reduce diff
Fidget-Spinner Oct 11, 2025
bc9d23c
Merge branch 'msvc-tailcall-take-two' of github.com:Fidget-Spinner/cp…
Fidget-Spinner Oct 11, 2025
19e02c2
Work around specialization
Fidget-Spinner Oct 11, 2025
66ec774
Fix VS 2026
Fidget-Spinner Oct 11, 2025
50f8ff7
fix a typo
Fidget-Spinner Oct 11, 2025
5d908b4
Move to macros to internal header
Fidget-Spinner Oct 11, 2025
0786133
Merge branch 'msvc-tailcall-take-two' of github.com:Fidget-Spinner/cp…
Fidget-Spinner Oct 11, 2025
e699d40
Reduce number of restricts
Fidget-Spinner Oct 13, 2025
66d6c39
Merge branch 'msvc-tailcall-take-two' of https://github.com/Fidget-Sp…
Fidget-Spinner Oct 13, 2025
5584fec
Reduce restrict use even more, reduce usage
Fidget-Spinner Oct 13, 2025
7eeeaa8
Apply Chris' changes
Fidget-Spinner Oct 23, 2025
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4 changes: 2 additions & 2 deletions Include/internal/pycore_ceval.h
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Original file line number Diff line number Diff line change
Expand Up @@ -303,9 +303,9 @@ PyAPI_FUNC(PyObject *)_PyEval_MatchKeys(PyThreadState *tstate, PyObject *map, Py
PyAPI_FUNC(void) _PyEval_MonitorRaise(PyThreadState *tstate, _PyInterpreterFrame *frame, _Py_CODEUNIT *instr);
PyAPI_FUNC(int) _PyEval_UnpackIterableStackRef(PyThreadState *tstate, PyObject *v, int argcnt, int argcntafter, _PyStackRef *sp);
PyAPI_FUNC(void) _PyEval_FrameClearAndPop(PyThreadState *tstate, _PyInterpreterFrame *frame);
PyAPI_FUNC(PyObject **) _PyObjectArray_FromStackRefArray(_PyStackRef *input, Py_ssize_t nargs, PyObject **scratch);
PyAPI_FUNC(PyObject **) _PyObjectArray_FromStackRefArray(_PyThreadStateImpl *_tstate, _PyStackRef *input, Py_ssize_t nargs);

PyAPI_FUNC(void) _PyObjectArray_Free(PyObject **array, PyObject **scratch);
PyAPI_FUNC(void) _PyObjectArray_Free(_PyThreadStateImpl *_tstate, PyObject **array, Py_ssize_t nargs, PyObject **temp_arr);

PyAPI_FUNC(PyObject *) _PyEval_GetANext(PyObject *aiter);
PyAPI_FUNC(void) _PyEval_LoadGlobalStackRef(PyObject *globals, PyObject *builtins, PyObject *name, _PyStackRef *writeto);
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5 changes: 5 additions & 0 deletions Include/internal/pycore_tstate.h
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Expand Up @@ -21,6 +21,9 @@ struct _gc_thread_state {
};
#endif

/* How much scratch space to give stackref to PyObject* conversion. */
#define MAX_STACKREF_SCRATCH 1024
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How arbitrary is this amount? What is the usual amount of space required for conversions?

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We used to arbitrarily always consume 10 PyObject/PyStackRef per call. Meaning even if you had a single item, the whole 10 slot space would be used.


// Every PyThreadState is actually allocated as a _PyThreadStateImpl. The
// PyThreadState fields are exposed as part of the C API, although most fields
// are intended to be private. The _PyThreadStateImpl fields not exposed.
Expand All @@ -47,6 +50,8 @@ typedef struct _PyThreadStateImpl {
struct _qsbr_thread_state *qsbr; // only used by free-threaded build
struct llist_node mem_free_queue; // delayed free queue

PyObject *stackref_scratch[MAX_STACKREF_SCRATCH];
int n_stackref_scratch_used;
#ifdef Py_GIL_DISABLED
// Stack references for the current thread that exist on the C stack
struct _PyCStackRef *c_stack_refs;
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1 change: 1 addition & 0 deletions Misc/NEWS.d/next/Core_and_Builtins/2025-10-11-17-01-21.gh-issue-139922.RUkXyd.rst
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@@ -0,0 +1 @@
Allow building CPython with the tail calling interpreter on Visual Studio 2026 MSVC. This provides a performance gain over the prior interpreter for MSVC. Patch by Ken Jin, Brandt Bucher, and Chris Eibl. With help from the MSVC team including Hulon Jenkins.
8 changes: 7 additions & 1 deletion Objects/abstract.c
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Expand Up @@ -205,8 +205,14 @@ PyObject_GetItem(PyObject *o, PyObject *key)
return type_error("'%.200s' object is not subscriptable", o);
}

// MSVC fails during a tail call release build with loads of
// error C4737: Unable to perform required tail call.
// without using Py_NO_INLINE here, but PGO works fine.
#if defined(_MSC_VER) && !defined(__clang__) && _Py_TAIL_CALL_INTERP && !defined(_Py_USING_PGO)
Py_NO_INLINE
#endif
int
PyMapping_GetOptionalItem(PyObject *obj, PyObject *key, PyObject **result)
PyMapping_GetOptionalItem(PyObject *obj, PyObject *key, PyObject **restrict result)
{
if (PyDict_CheckExact(obj)) {
return PyDict_GetItemRef(obj, key, result);
Expand Down
4 changes: 3 additions & 1 deletion PCbuild/pythoncore.vcxproj
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Original file line number Diff line number Diff line change
Expand Up @@ -595,7 +595,9 @@
<ClCompile Include="..\Python\bltinmodule.c" />
<ClCompile Include="..\Python\bootstrap_hash.c" />
<ClCompile Include="..\Python\brc.c" />
<ClCompile Include="..\Python\ceval.c" />
<ClCompile Include="..\Python\ceval.c">
<AdditionalOptions Condition="'$(UseTailCallInterp)' == 'true' and $(PlatformToolset) != 'ClangCL'">/std:clatest %(AdditionalOptions)</AdditionalOptions>
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What happens if we pass this option to clang-cl? Does it break?

Any possibility of passing a specific /std:c rather than "latest"?

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So I've worked with Chris on this and apparently it builds with clang-cl still the last time I checked with him? @chris-eibl

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@chris-eibl chris-eibl Oct 13, 2025 •
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What happens if we pass this option to clang-cl? Does it break?

Yupp, clang-cl doesn't like /std:clatest, hence I omitted it, because it doesn't need it.
Seems the clang-cl versions we are using (>=19)

  • support by default at least C99 (which is would be needed for restrict)
  • and support must tail, etc, in the "clang specific notation"

But I assume MSVC needs /std:clatest for [[msvc::musttail]] - @Fidget-Spinner correct?
And at least MSVC definitely needs at least /std:c11 for restrict (https://learn.microsoft.com/en-us/cpp/build/reference/std-specify-language-standard-version?view=msvc-170).

Any possibility of passing a specific /std:c rather than "latest"?

Yeah, we could explicitely enforce /std:c11for clang-cl, but we didn't need it in the past and AFAICT this PR didn't change anything that clang-cl sees, due to carefully crafted combinations of _MSC_VER and __clang__ like

#if defined(_MSC_VER) && !defined(__clang__)

and apparently it builds with clang-cl still the last time I checked with him?

Yes, it still does. Having done a lot of builds recently and still playing around.
And tail calling CI is green as well: https://github.com/python/cpython/actions/runs/18435075900/job/52527447202

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I'm under the impression we need clatest for msvc musttail as well.

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I've just verified again:

  • MSVC needs /std:clatest for [[msvc::musttail]]
  • using /std:c17 fails with error C2143: syntax error: missing ';' before '['
  • clang-cl doesn't accept /std:clatest

So atm we're stuck with this construct until MSVC introduces a new "fixed" option (or maybe accepts [[msvc::musttail]] in /std:c17).

I agree with Steve #135927 (comment)

For the record, I don't like building with Clatest and if we're considering making that change upstream, we should pick a version.

and we should revisit this once we know a fixed version option for it.

</ClCompile>
<ClCompile Include="..\Python\codecs.c" />
<ClCompile Include="..\Python\codegen.c" />
<ClCompile Include="..\Python\compile.c" />
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48 changes: 29 additions & 19 deletions Python/bytecodes.c
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Original file line number Diff line number Diff line change
Expand Up @@ -1983,7 +1983,7 @@ dummy_func(
ERROR_IF(true);
}
PyObject *str_o = _PyUnicode_JoinArray(&_Py_STR(empty), pieces_o, oparg);
STACKREFS_TO_PYOBJECTS_CLEANUP(pieces_o);
STACKREFS_TO_PYOBJECTS_CLEANUP(pieces_o, oparg);
DECREF_INPUTS();
ERROR_IF(str_o == NULL);
str = PyStackRef_FromPyObjectSteal(str_o);
Expand Down Expand Up @@ -2108,7 +2108,7 @@ dummy_func(
values_o, 2,
values_o+1, 2,
oparg);
STACKREFS_TO_PYOBJECTS_CLEANUP(values_o);
STACKREFS_TO_PYOBJECTS_CLEANUP(values_o, oparg*2);
DECREF_INPUTS();
ERROR_IF(map_o == NULL);
map = PyStackRef_FromPyObjectStealMortal(map_o);
Expand Down Expand Up @@ -2226,8 +2226,11 @@ dummy_func(
}
// we make no attempt to optimize here; specializations should
// handle any case whose performance we care about
PyObject *stack[] = {class, self};
PyObject *super = PyObject_Vectorcall(global_super, stack, oparg & 2, NULL);
PyObject *super;
{
PyObject *stack[] = {class, self};
super = PyObject_Vectorcall(global_super, stack, oparg & 2, NULL);
}
if (opcode == INSTRUMENTED_LOAD_SUPER_ATTR) {
PyObject *arg = oparg & 2 ? class : &_PyInstrumentation_MISSING;
if (super == NULL) {
Expand Down Expand Up @@ -2286,8 +2289,12 @@ dummy_func(
PyObject *name = GETITEM(FRAME_CO_NAMES, oparg >> 2);
PyTypeObject *cls = (PyTypeObject *)class;
int method_found = 0;
PyObject *attr_o = _PySuper_Lookup(cls, self, name,
Py_TYPE(self)->tp_getattro == PyObject_GenericGetAttr ? &method_found : NULL);
PyObject *attr_o;
{
int *method_found_ptr = &method_found;
attr_o = _PySuper_Lookup(cls, self, name,
Py_TYPE(self)->tp_getattro == PyObject_GenericGetAttr ? method_found_ptr : NULL);
}
if (attr_o == NULL) {
ERROR_NO_POP();
}
Expand Down Expand Up @@ -3510,10 +3517,13 @@ dummy_func(
}
assert(PyStackRef_IsTaggedInt(lasti));
(void)lasti; // Shut up compiler warning if asserts are off
PyObject *stack[5] = {NULL, PyStackRef_AsPyObjectBorrow(exit_self), exc, val_o, tb};
int has_self = !PyStackRef_IsNull(exit_self);
PyObject *res_o = PyObject_Vectorcall(exit_func_o, stack + 2 - has_self,
(3 + has_self) | PY_VECTORCALL_ARGUMENTS_OFFSET, NULL);
PyObject* res_o;
{
PyObject *stack[5] = {NULL, PyStackRef_AsPyObjectBorrow(exit_self), exc, val_o, tb};
int has_self = !PyStackRef_IsNull(exit_self);
res_o = PyObject_Vectorcall(exit_func_o, stack + 2 - has_self,
(3 + has_self) | PY_VECTORCALL_ARGUMENTS_OFFSET, NULL);
}
Py_XDECREF(original_tb);
ERROR_IF(res_o == NULL);
res = PyStackRef_FromPyObjectSteal(res_o);
Expand Down Expand Up @@ -3755,7 +3765,7 @@ dummy_func(
callable_o, args_o,
total_args | PY_VECTORCALL_ARGUMENTS_OFFSET,
NULL);
STACKREFS_TO_PYOBJECTS_CLEANUP(args_o);
STACKREFS_TO_PYOBJECTS_CLEANUP(args_o, total_args);
if (opcode == INSTRUMENTED_CALL) {
PyObject *arg = total_args == 0 ?
&_PyInstrumentation_MISSING : PyStackRef_AsPyObjectBorrow(arguments[0]);
Expand Down Expand Up @@ -3910,7 +3920,7 @@ dummy_func(
callable_o, args_o,
total_args | PY_VECTORCALL_ARGUMENTS_OFFSET,
NULL);
STACKREFS_TO_PYOBJECTS_CLEANUP(args_o);
STACKREFS_TO_PYOBJECTS_CLEANUP(args_o, total_args);
assert((res_o != NULL) ^ (_PyErr_Occurred(tstate) != NULL));
DECREF_INPUTS();
ERROR_IF(res_o == NULL);
Expand Down Expand Up @@ -4193,7 +4203,7 @@ dummy_func(
ERROR_IF(true);
}
PyObject *res_o = tp->tp_vectorcall((PyObject *)tp, args_o, total_args, NULL);
STACKREFS_TO_PYOBJECTS_CLEANUP(args_o);
STACKREFS_TO_PYOBJECTS_CLEANUP(args_o, total_args);
DECREF_INPUTS();
ERROR_IF(res_o == NULL);
res = PyStackRef_FromPyObjectSteal(res_o);
Expand Down Expand Up @@ -4264,7 +4274,7 @@ dummy_func(
PyCFunction_GET_SELF(callable_o),
args_o,
total_args);
STACKREFS_TO_PYOBJECTS_CLEANUP(args_o);
STACKREFS_TO_PYOBJECTS_CLEANUP(args_o, total_args);
assert((res_o != NULL) ^ (_PyErr_Occurred(tstate) != NULL));
DECREF_INPUTS();
ERROR_IF(res_o == NULL);
Expand Down Expand Up @@ -4300,7 +4310,7 @@ dummy_func(
ERROR_IF(true);
}
PyObject *res_o = cfunc(PyCFunction_GET_SELF(callable_o), args_o, total_args, NULL);
STACKREFS_TO_PYOBJECTS_CLEANUP(args_o);
STACKREFS_TO_PYOBJECTS_CLEANUP(args_o, total_args);
assert((res_o != NULL) ^ (_PyErr_Occurred(tstate) != NULL));
DECREF_INPUTS();
ERROR_IF(res_o == NULL);
Expand Down Expand Up @@ -4480,7 +4490,7 @@ dummy_func(
PyCFunctionFastWithKeywords cfunc =
_PyCFunctionFastWithKeywords_CAST(meth->ml_meth);
PyObject *res_o = cfunc(self, (args_o + 1), nargs, NULL);
STACKREFS_TO_PYOBJECTS_CLEANUP(args_o);
STACKREFS_TO_PYOBJECTS_CLEANUP(args_o, total_args);
assert((res_o != NULL) ^ (_PyErr_Occurred(tstate) != NULL));
DECREF_INPUTS();
ERROR_IF(res_o == NULL);
Expand Down Expand Up @@ -4559,7 +4569,7 @@ dummy_func(
}
PyCFunctionFast cfunc = _PyCFunctionFast_CAST(meth->ml_meth);
PyObject *res_o = cfunc(self, (args_o + 1), nargs);
STACKREFS_TO_PYOBJECTS_CLEANUP(args_o);
STACKREFS_TO_PYOBJECTS_CLEANUP(args_o, total_args);
assert((res_o != NULL) ^ (_PyErr_Occurred(tstate) != NULL));
DECREF_INPUTS();
ERROR_IF(res_o == NULL);
Expand Down Expand Up @@ -4658,7 +4668,7 @@ dummy_func(
callable_o, args_o,
positional_args | PY_VECTORCALL_ARGUMENTS_OFFSET,
kwnames_o);
STACKREFS_TO_PYOBJECTS_CLEANUP(args_o);
STACKREFS_TO_PYOBJECTS_CLEANUP(args_o, total_args);
if (opcode == INSTRUMENTED_CALL_KW) {
PyObject *arg = total_args == 0 ?
&_PyInstrumentation_MISSING : PyStackRef_AsPyObjectBorrow(arguments[0]);
Expand Down Expand Up @@ -4815,7 +4825,7 @@ dummy_func(
positional_args | PY_VECTORCALL_ARGUMENTS_OFFSET,
kwnames_o);
PyStackRef_CLOSE(kwnames);
STACKREFS_TO_PYOBJECTS_CLEANUP(args_o);
STACKREFS_TO_PYOBJECTS_CLEANUP(args_o, total_args);
assert((res_o != NULL) ^ (_PyErr_Occurred(tstate) != NULL));
DECREF_INPUTS();
ERROR_IF(res_o == NULL);
Expand Down
21 changes: 15 additions & 6 deletions Python/ceval.c
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Original file line number Diff line number Diff line change
Expand Up @@ -956,10 +956,12 @@ extern void _PyUOpPrint(const _PyUOpInstruction *uop);


PyObject **
_PyObjectArray_FromStackRefArray(_PyStackRef *input, Py_ssize_t nargs, PyObject **scratch)
_PyObjectArray_FromStackRefArray(_PyThreadStateImpl *_tstate, _PyStackRef *restrict input, Py_ssize_t nargs)
{
PyObject **result;
if (nargs > MAX_STACKREF_SCRATCH) {
/* +1 because vectorcall might use -1 to write self */
PyObject **target = _tstate->stackref_scratch + _tstate->n_stackref_scratch_used + nargs + 1;
if (target > _tstate->stackref_scratch + MAX_STACKREF_SCRATCH) {
// +1 in case PY_VECTORCALL_ARGUMENTS_OFFSET is set.
result = PyMem_Malloc((nargs + 1) * sizeof(PyObject *));
if (result == NULL) {
Expand All @@ -968,7 +970,9 @@ _PyObjectArray_FromStackRefArray(_PyStackRef *input, Py_ssize_t nargs, PyObject
result++;
}
else {
result = scratch;
result = _tstate->stackref_scratch + _tstate->n_stackref_scratch_used;
_tstate->n_stackref_scratch_used += (int)nargs + 1;
assert(_tstate->n_stackref_scratch_used < MAX_STACKREF_SCRATCH);
}
for (int i = 0; i < nargs; i++) {
result[i] = PyStackRef_AsPyObjectBorrow(input[i]);
Expand All @@ -977,10 +981,15 @@ _PyObjectArray_FromStackRefArray(_PyStackRef *input, Py_ssize_t nargs, PyObject
}

void
_PyObjectArray_Free(PyObject **array, PyObject **scratch)
_PyObjectArray_Free(_PyThreadStateImpl *_tstate, PyObject **array, Py_ssize_t nargs, PyObject **temp_arr)
{
if (array != scratch) {
PyMem_Free(array);
/* -1 because we +1 previously */
if (array == temp_arr) {
_tstate->n_stackref_scratch_used -= ((int)nargs + 1);
assert(_tstate->n_stackref_scratch_used >= 0);
}
else {
PyMem_Free(array-1);
}
}

Expand Down
28 changes: 14 additions & 14 deletions Python/ceval_macros.h
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Original file line number Diff line number Diff line change
Expand Up @@ -79,12 +79,15 @@
#endif

#if _Py_TAIL_CALL_INTERP
// Note: [[clang::musttail]] works for GCC 15, but not __attribute__((musttail)) at the moment.
# define Py_MUSTTAIL [[clang::musttail]]
# define Py_PRESERVE_NONE_CC __attribute__((preserve_none))
Py_PRESERVE_NONE_CC typedef PyObject* (*py_tail_call_funcptr)(TAIL_CALL_PARAMS);

# define TARGET(op) Py_PRESERVE_NONE_CC PyObject *_TAIL_CALL_##op(TAIL_CALL_PARAMS)
# if defined(_MSC_VER) && !defined(__clang__)
# define Py_MUSTTAIL [[msvc::musttail]]
# define Py_PRESERVE_NONE_CC __preserve_none
# else
# define Py_MUSTTAIL __attribute__((musttail))
# define Py_PRESERVE_NONE_CC __attribute__((preserve_none))
# endif
typedef PyObject *(Py_PRESERVE_NONE_CC *py_tail_call_funcptr)(TAIL_CALL_PARAMS);
# define TARGET(op) Py_NO_INLINE PyObject *Py_PRESERVE_NONE_CC _TAIL_CALL_##op(TAIL_CALL_PARAMS)
# define DISPATCH_GOTO() \
do { \
Py_MUSTTAIL return (((py_tail_call_funcptr *)instruction_funcptr_table)[opcode])(TAIL_CALL_ARGS); \
Expand Down Expand Up @@ -390,17 +393,14 @@ do { \

/* Stackref macros */

/* How much scratch space to give stackref to PyObject* conversion. */
#define MAX_STACKREF_SCRATCH 10

#define STACKREFS_TO_PYOBJECTS(ARGS, ARG_COUNT, NAME) \
/* +1 because vectorcall might use -1 to write self */ \
PyObject *NAME##_temp[MAX_STACKREF_SCRATCH+1]; \
PyObject **NAME = _PyObjectArray_FromStackRefArray(ARGS, ARG_COUNT, NAME##_temp + 1);
_PyThreadStateImpl *_tstate = (_PyThreadStateImpl *)tstate; \
PyObject **NAME##_temp = _tstate->stackref_scratch + _tstate->n_stackref_scratch_used; \
PyObject **NAME = _PyObjectArray_FromStackRefArray(_tstate, ARGS, ARG_COUNT);

#define STACKREFS_TO_PYOBJECTS_CLEANUP(NAME) \
/* +1 because we +1 previously */ \
_PyObjectArray_Free(NAME - 1, NAME##_temp);
#define STACKREFS_TO_PYOBJECTS_CLEANUP(NAME, ARG_COUNT) \
_PyObjectArray_Free(_tstate, NAME, ARG_COUNT, NAME##_temp);

#define CONVERSION_FAILED(NAME) ((NAME) == NULL)

Expand Down
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