C API: Implement Py_MIN(), Py_MAX() and Py_ABS() macros using typeof()

[vstinner]

Currently, arguments of Py_MIN(), Py_MAX() and Py_ABS() macros are evaluated twice. It's inefficient. We can do better using typeof(). I propose adding a typeof() implementation for these macros.

See also:

• GCC typeof: https://gcc.gnu.org/onlinedocs/gcc/Typeof.html
• n3543 "Add type-safe minimum and maximum type-generic macros" proposition for the C standard.

Linked PRs

• gh-157495: Implement Py_MIN() and Py_MAX() using typeof() #157496
• gh-157495: Avoid Py_MIN()/Py_MAX() side effects #157591

[serhiy-storchaka]

How would it work in C++?

[vstinner]

How would it work in C++?

The current implement (without typeof) is used for C++.

Statement expressions are not allowed in C++. My PR adds explicit tests on Py_MIN() and Py_MAX() in test_cppext: so the macros are tested in C++ with different compiler flags, with and without the limited C API.

typeof() cannot be used on MSVC neither because statement expression is a GNU extension which is only supported by GCC and clang (not by MSVC).

[skirpichev]

Currently, arguments of Py_MIN(), Py_MAX() and Py_ABS() macros are evaluated twice. It's inefficient.

Does it matter for default builds, when optimizations turned on?

[vstinner]

The documentation of Py_MIN(), Py_MAX() and Py_ABS() macros says:

The argument may be evaluated more than once. Consequently, do not pass an expression with side-effects directly to this macro.

min=Py_MIN(var, expensive_func()) should be replaced with result=expensive_func(); min=Py_MIN(var, result) for example.

Since my proposal change is only available on some compilers (GCC and clang, not on MSVC), Python code base should continue to follow the documentation advices.

I didn't check the 221 calls to these macros (git grep -E '(Py_MIN|Py_MAX|Py_ABS)\('|wc -l).

[vstinner]

I didn't check the 221 calls to these macros (git grep -E '(Py_MIN|Py_MAX|Py_ABS)('|wc -l).

Ok, I checked Py_MIN/MAX/ABS calls in the Python code base and I found a few calls which are inefficient. I wrote this PR gh-157591 to fix them.