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pythongh-143732: Specialize __setitem__ dunder method for STORE_SUBSCR
Add STORE_SUBSCR_PY_DUNDER, which enters a Python `__setitem__` directly rather than calling it through PyObject_SetItem, following the template pythongh-143732 asks for. Unlike BINARY_OP_SUBSCR_GETITEM, the frame for the dunder cannot simply be pushed. _RETURN_VALUE is `(retval -- res)`, so a returning frame always pushes exactly one value onto the caller: BINARY_OP_SUBSCR_GETITEM is declared pops 2 / pushes 0 and the +1 arrives on return, netting -1 to match generic BINARY_OP. STORE_SUBSCR must net -3, so a frame-pushing member would pop 3, declare 0 pushed and gain +1 on return, netting -2. `__setitem__` returns a None that nothing consumes. So a shim frame (_Py_SetItemCleanup) is pushed underneath the `__setitem__` frame, in the same way CALL_ALLOC_AND_ENTER_INIT uses _Py_InitCleanup for `__init__`. The dunder returns into the shim, whose sole instruction EXIT_SETITEM discards that value and pops the shim without pushing a result, keeping the net stack effect at -3. The guard and the frame push are deliberately a single uop. Splitting them, mirroring _BINARY_OP_SUBSCR_CHECK_FUNC, puts the looked-up function on the caller's value stack as a temporary; STORE_SUBSCR already occupies three stack slots, so that extra slot overruns co_stacksize and trips ASSERT_WITHIN_STACK_BOUNDS. BINARY_OP_SUBSCR_GETITEM survives it because BINARY_OP occupies only two. The function is cached on the heap type (_spec_cache.setitem, mirroring _spec_cache.getitem) rather than in the inline cache, so STORE_SUBSCR's cache size is unchanged. The cost is paid in heap type size instead: the pointers in _specialization_cache are grouped ahead of the version fields so the two uint32_t versions share one word of padding, which holds the struct at 32 bytes rather than 40, i.e. 8 bytes per heap type rather than 16. Tier 1 only; tier 2 / JIT support is left for a follow-up. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
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‎Include/cpython/object.h‎

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@@ -266,9 +266,18 @@ struct _specialization_cache {
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// - If getitem->func_version == getitem_version, then getitem can be called
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// with two positional arguments and no keyword arguments, and has neither
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// *args nor **kwargs (as required by BINARY_OP_SUBSCR_GETITEM):
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// The same contract applies to setitem, except that it is the Python
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// function that PyType_Lookup(cls, "__setitem__") would return, and it is
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// called with three positional arguments (as required by
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// STORE_SUBSCR_PY_DUNDER).
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// The pointers are grouped ahead of the versions so that the two version
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// fields share one word of padding, which keeps this struct at 32 bytes
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// rather than 40.
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PyObject *getitem;
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uint32_t getitem_version;
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PyObject *setitem;
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PyObject *init;
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uint32_t getitem_version;
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uint32_t setitem_version;
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};
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/* The *real* layout of a type object when allocated on the heap */

‎Include/internal/pycore_code.h‎

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@@ -549,6 +549,7 @@ extern PyObject *_PyInstrumentation_BranchesIterator(PyCodeObject *code);
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struct _PyCode8 _PyCode_DEF(8);
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PyAPI_DATA(const struct _PyCode8) _Py_InitCleanup;
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PyAPI_DATA(const struct _PyCode8) _Py_SetItemCleanup;
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#ifdef Py_GIL_DISABLED
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‎Include/internal/pycore_magic_number.h‎

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@@ -303,6 +303,7 @@ Known values:
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Python 3.16a1 3703 (Replace DELETE_GLOBAL with PUSH_NULL; STORE_GLOBAL)
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Python 3.16a1 3704 (Replace DELETE_ATTR with PUSH_NULL; STORE_ATTR)
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Python 3.16a1 3705 (Add INTRINSIC_ADD_CONDITIONAL_ANNOTATION)
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Python 3.16a1 3706 (Add STORE_SUBSCR_PY_DUNDER and EXIT_SETITEM)
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Python 3.17 will start with 3750
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@@ -312,7 +313,7 @@ Known values:
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*/
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#define PYC_MAGIC_NUMBER 3705
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#define PYC_MAGIC_NUMBER 3706
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/* This is equivalent to converting PYC_MAGIC_NUMBER to 2 bytes
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(little-endian) and then appending b'\r\n'. */
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#define PYC_MAGIC_NUMBER_TOKEN \

‎Include/internal/pycore_opcode_metadata.h‎

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‎Include/internal/pycore_typeobject.h‎

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@@ -164,6 +164,7 @@ typedef int (*_py_validate_type)(PyTypeObject *);
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// Exported for external JIT support
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int _PyType_Validate(PyTypeObject *ty, _py_validate_type validate, unsigned int *tp_version);
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int _PyType_CacheGetItemForSpecialization(PyHeapTypeObject *ht, PyObject *descriptor, uint32_t tp_version);
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int _PyType_CacheSetItemForSpecialization(PyHeapTypeObject *ht, PyObject *descriptor, uint32_t tp_version);
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// Precalculates count of non-unique slots and fills wrapperbase.name_count.
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extern int _PyType_InitSlotDefs(PyInterpreterState *interp);

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