Author vstinner
Recipients Aaron Hall, benjamin.peterson, eric.snow, mark.dickinson, miss-islington, pablogsal, serhiy.storchaka, thatiparthy, vstinner
Date 2018-11-23.16:48:40
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Message-id <1542991721.13.0.788709270274.issue35059@psf.upfronthosting.co.za>
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Advantages of inline functions over C macros:

https://mail.python.org/pipermail/python-dev/2018-November/155805.html


There are multiple advantages:

* Better development and debugging experience: tools understand
inlined functions much better than C macros: gdb, Linux perf, etc.

* Better API: arguments now have a type and the function has a return
type. In practice, some macros still cast their argument to PyObject*
to not introduce new compiler warnings in Python 3.8. For example,
even if Py_INCREF() is documented (*) as a function expecting
PyObject*, it accepts any pointer type (PyTupleObject*,
PyUnicodeObject*, etc.). Technically, it also accepts PyObject** which
is a bug, but that's a different story ;-)

* Much better code, just plain regular C. C macros are ugly: "do { ...
} while (0)" workaround, additional parenthesis around each argument,
strange "expr1, expr2" syntax of "macro expression" which returns a
value (inline function just uses regular "return" and ";" at the end
of instructions), strange indentation, etc.

* No more "macro pitfals":
https://gcc.gnu.org/onlinedocs/cpp/Macro-Pitfalls.html

* Local variables no longer need a magic name to avoid risk of name
conflict, and have a clearly defined scope. Py_DECREF() and
_Py_XINCREF() no longer need a local variable since it's argument
already has a clearly defined type: PyObject*. I introduced a new
variable in _Py_Dealloc() to fix a possible race condition.
Previously, the variable was probably avoided because it's tricky use
variables in macros.

* #ifdef can now be used inside the inline function: it makes the code
easier to understand.

* etc.


Are you aware that Python had macros like:

#define _Py_REF_DEBUG_COMMA ,
#define _Py_CHECK_REFCNT(OP) /* a semicolon */;

I let you judge the quality of this macro:

#define _Py_NewReference(op) (                          \
    _Py_INC_TPALLOCS(op) _Py_COUNT_ALLOCS_COMMA         \
    _Py_INC_REFTOTAL  _Py_REF_DEBUG_COMMA               \
    Py_REFCNT(op) = 1)

Is it an expression? Can it be used in "if (test)
_Py_NewReference(op);"? It doesn't use the "do { ... } while (0)"
protection against macro pitfals.

(*) Py_INCREF doc:
https://docs.python.org/dev/c-api/refcounting.html#c.Py_INCREF
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Date User Action Args
2018-11-23 16:48:41vstinnersetrecipients: + vstinner, mark.dickinson, benjamin.peterson, eric.snow, serhiy.storchaka, thatiparthy, Aaron Hall, pablogsal, miss-islington
2018-11-23 16:48:41vstinnersetmessageid: <1542991721.13.0.788709270274.issue35059@psf.upfronthosting.co.za>
2018-11-23 16:48:41vstinnerlinkissue35059 messages
2018-11-23 16:48:40vstinnercreate