Python/specialize.c (part 2)

cpython 3.14 @ ab2d84fe1023/Python/specialize.c ↗

This annotation covers the arithmetic, comparison, unpack, and iteration specializations in Python/specialize.c. For _Py_Specialize_LoadAttr / StoreAttr see the compile/specialize_c annotation.

Map

Lines	Symbol	Role
1-400	_Py_Specialize_BinaryOp	Replace BINARY_OP with type-specific variant
401-700	_Py_Specialize_CompareOp	Replace COMPARE_OP with type-specific variant
701-950	_Py_Specialize_UnpackSequence	Replace UNPACK_SEQUENCE for known-length sequences
951-1200	_Py_Specialize_ForIter	Replace FOR_ITER for known iterator types
1201-2200	Deopt and miss counters	_Py_BackoffCounter mechanics

Reading

_Py_Specialize_BinaryOp

BINARY_OP specializes to BINARY_OP_ADD_INT when both operands are int, to BINARY_OP_ADD_FLOAT for float, and to BINARY_OP_ADD_UNICODE for str. The specializer checks the types at the specialization point; on type mismatch during execution, the specialized variant falls back to the generic BINARY_OP.

// CPython: Python/specialize.c:1420 binary_op_fail_kind
static void
binary_op_fail_kind(int oparg, PyObject *lhs, PyObject *rhs)
{
    switch (oparg) {
    case NB_ADD:
        if (!PyLong_CheckExact(lhs)) {
            SPECIALIZATION_FAIL(BINARY_OP, SPEC_FAIL_BINARY_OP_ADD_DIFFERENT_TYPES);
            return;
        }
        ...
    }
}

_Py_Specialize_ForIter

FOR_ITER specializes to FOR_ITER_LIST, FOR_ITER_TUPLE, FOR_ITER_RANGE, or FOR_ITER_GEN based on the type of the iterator on the stack. These variants call the type's tp_iternext directly without the generic PyObject_GetIter overhead.

Deopt and backoff

If a specialised variant encounters a type mismatch at runtime, it deoptimizes back to the generic opcode and decrements a _Py_BackoffCounter. When the counter reaches zero, the specializer is allowed to try again. This prevents hot code paths from thrashing between specialized and generic variants.

gopy notes

gopy uses a generic dispatch switch with no specialization. The BINARY_OP variants map to type-switch code in vm/eval_simple.go. Adaptive specialization is not yet planned.