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Define _Py_TYPEOF on MSVC and other compilers supporting __typeof__ #121647

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fuhsnn opened this issue Jul 12, 2024 · 0 comments
Open

Define _Py_TYPEOF on MSVC and other compilers supporting __typeof__ #121647

fuhsnn opened this issue Jul 12, 2024 · 0 comments
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interpreter-core (Objects, Python, Grammar, and Parser dirs) OS-windows type-feature A feature request or enhancement

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@fuhsnn
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fuhsnn commented Jul 12, 2024 •
edited by bedevere-app bot
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Feature or enhancement

Proposal:

Recently, MSVC implemented __typeof__, which currently is defined as _Py_TYPEOF only on GCC and Clang.

cpython/Include/pyport.h

Lines 553 to 560 in 65feded

// _Py_TYPEOF(expr) gets the type of an expression.
//
// Example: _Py_TYPEOF(x) x_copy = (x);
//
// The macro is only defined if GCC or clang compiler is used.
#if defined(__GNUC__) || defined(__clang__)
# define _Py_TYPEOF(expr) __typeof__(expr)
#endif

The only users of _Py_TYPEOF are macros Py_CLEAR(), Py_SETREF() and Py_XSETREF().
If _Py_TYPEOF is not defined, these macros fall back to slightly uglier implementations with memcpy calls.

cpython/Include/refcount.h

Lines 421 to 442 in 65feded

#ifdef _Py_TYPEOF
#define Py_CLEAR(op) \
do { \
_Py_TYPEOF(op)* _tmp_op_ptr = &(op); \
_Py_TYPEOF(op) _tmp_old_op = (*_tmp_op_ptr); \
if (_tmp_old_op != NULL) { \
*_tmp_op_ptr = _Py_NULL; \
Py_DECREF(_tmp_old_op); \
} \
} while (0)
#else
#define Py_CLEAR(op) \
do { \
PyObject **_tmp_op_ptr = _Py_CAST(PyObject**, &(op)); \
PyObject *_tmp_old_op = (*_tmp_op_ptr); \
if (_tmp_old_op != NULL) { \
PyObject *_null_ptr = _Py_NULL; \
memcpy(_tmp_op_ptr, &_null_ptr, sizeof(PyObject*)); \
Py_DECREF(_tmp_old_op); \
} \
} while (0)
#endif

The proposal is to define _Py_TYPEOF on:

  • latest MSVC versions that support __typeof__
  • arbitrary compilers that support __typeof__
  • arbitrary compilers that support C23 typeof or C++11 decltype

By defining _Py_TYPEOF, uses of Py_CLEAR(), Py_SETREF() and Py_XSETREF() are switched to the _Py_TYPEOF implementation; on these compilers, the benefits would be:

Additionally, this removes a differing detail between MSVC and GCC/Clang builds, which should be generally desirable.

Has this already been discussed elsewhere?

No response given

Links to previous discussion of this feature:

Linked PRs
@fuhsnn fuhsnn added the type-feature A feature request or enhancement label Jul 12, 2024
fuhsnn added a commit to fuhsnn/cpython that referenced this issue Jul 12, 2024
@fuhsnn
pythongh-121647: Define _Py_TYPEOF macro on more compilers
85e5cbd 
Extend the _Py_TYPEOF macro to more implementations of __typeof__ and standardized equivalents on C23 and C++11.
On MSVC, __typeof__ is enabled through _MSC_VER check; for others, an autoconf probe is implemented.
@bedevere-app bedevere-app bot mentioned this issue Jul 12, 2024
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@Eclips4 Eclips4 added OS-windows interpreter-core (Objects, Python, Grammar, and Parser dirs) labels Jul 12, 2024
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