[3.13] gh-141004: Document remaining pyport.h utility macros (GH-144279) (GH-144478)

gh-141004: Document remaining `pyport.h` utility macros (GH-144279)
(cherry picked from commit 914fbec)

Co-authored-by: Peter Bierma <zintensitydev@gmail.com>
Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>
Co-authored-by: Victor Stinner <vstinner@python.org>

Author: 4 people

Doc/c-api/intro.rst

@@ -139,6 +139,35 @@ complete listing.
 
    .. versionadded:: 3.3
 
+.. c:macro:: Py_ALIGNED(num)
+
+   Specify alignment to *num* bytes on compilers that support it.
+
+   Consider using the C11 standard ``_Alignas`` specifier over this macro.
+
+.. c:macro:: Py_ARITHMETIC_RIGHT_SHIFT(type, integer, positions)
+
+   Similar to ``integer >> positions``, but forces sign extension, as the C
+   standard does not define whether a right-shift of a signed integer will
+   perform sign extension or a zero-fill.
+
+   *integer* should be any signed integer type.
+   *positions* is the number of positions to shift to the right.
+
+   Both *integer* and *positions* can be evaluated more than once;
+   consequently, avoid directly passing a function call or some other
+   operation with side-effects to this macro. Instead, store the result as a
+   variable and then pass it.
+
+   *type* is unused and only kept for backwards compatibility. Historically,
+   *type* was used to cast *integer*.
+
+   .. versionchanged:: 3.1
+
+      This macro is now valid for all signed integer types, not just those for
+      which ``unsigned type`` is legal. As a result, *type* is no longer
+      used.
+
 .. c:macro:: Py_ALWAYS_INLINE
 
    Ask the compiler to always inline a static inline function. The compiler can
@@ -161,6 +190,15 @@ complete listing.
 
    .. versionadded:: 3.11
 
+.. c:macro:: Py_CAN_START_THREADS
+
+   If this macro is defined, then the current system is able to start threads.
+
+   Currently, all systems supported by CPython (per :pep:`11`), with the
+   exception of some WebAssembly platforms, support starting threads.
+
+   .. versionadded:: 3.13
+
 .. c:macro:: Py_CHARMASK(c)
 
    Argument must be a character or an integer in the range [-128, 127] or [0,
@@ -178,11 +216,35 @@ complete listing.
    .. versionchanged:: 3.8
       MSVC support was added.
 
+.. c:macro:: Py_FORCE_EXPANSION(X)
+
+   This is equivalent to ``X``, which is useful for token-pasting in
+   macros, as macro expansions in *X* are forcefully evaluated by the
+   preprocessor.
+
+.. c:macro:: Py_GCC_ATTRIBUTE(name)
+
+   Use a GCC attribute *name*, hiding it from compilers that don't support GCC
+   attributes (such as MSVC).
+
+   This expands to ``__attribute__((name))`` on a GCC compiler, and expands
+   to nothing on compilers that don't support GCC attributes.
+
 .. c:macro:: Py_GETENV(s)
 
    Like ``getenv(s)``, but returns ``NULL`` if :option:`-E` was passed on the
    command line (see :c:member:`PyConfig.use_environment`).
 
+.. c:macro:: Py_LL(number)
+
+   Use *number* as a ``long long`` integer literal.
+
+   This usally expands to *number* followed by ``LL``, but will expand to some
+   compiler-specific suffixes (such as ``I64``) on older compilers.
+
+   In modern versions of Python, this macro is not very useful, as C99 and
+   later require the ``LL`` suffix to be valid for an integer.
+
 .. c:macro:: Py_LOCAL(type)
 
    Declare a function returning the specified *type* using a fast-calling
@@ -232,13 +294,37 @@ complete listing.
 
    .. versionadded:: 3.11
 
+.. c:macro:: Py_SAFE_DOWNCAST(value, larger, smaller)
+
+   Cast *value* to type *smaller* from type *larger*, validating that no
+   information was lost.
+
+   On release builds of Python, this is roughly equivalent to
+   ``(smaller) value`` (in C++, ``static_cast<smaller>(value)`` will be
+   used instead).
+
+   On debug builds (implying that :c:macro:`Py_DEBUG` is defined), this asserts
+   that no information was lost with the cast from *larger* to *smaller*.
+
+   *value*, *larger*, and *smaller* may all be evaluated more than once in the
+   expression; consequently, do not pass an expression with side-effects directly to
+   this macro.
+
 .. c:macro:: Py_STRINGIFY(x)
 
    Convert ``x`` to a C string.  E.g. ``Py_STRINGIFY(123)`` returns
    ``"123"``.
 
    .. versionadded:: 3.4
 
+.. c:macro:: Py_ULL(number)
+
+   Similar to :c:macro:`Py_LL`, but *number* will be an ``unsigned long long``
+   literal instead. This is done by appending ``U`` to the result of ``Py_LL``.
+
+   In modern versions of Python, this macro is not very useful, as C99 and
+   later require the ``ULL``/``LLU`` suffixes to be valid for an integer.
+
 .. c:macro:: Py_UNREACHABLE()
 
    Use this when you have a code path that cannot be reached by design.
@@ -379,6 +465,16 @@ complete listing.
    This macro is intended for defining CPython's C API itself;
    extension modules should not use it for their own symbols.
 
+.. c:macro:: Py_VA_COPY
+
+   This is a :term:`soft deprecated` alias to the C99-standard ``va_copy``
+   function.
+
+   Historically, this would use a compiler-specific method to copy a ``va_list``.
+
+   .. versionchanged:: 3.6
+      This is now an alias to ``va_copy``.
+
 
 .. _api-objects:
 

Tools/check-c-api-docs/ignored_c_api.txt

@@ -31,15 +31,6 @@ Py_TPFLAGS_IS_ABSTRACT
 PyExpat_CAPI_MAGIC
 PyExpat_CAPSULE_NAME
 # pyport.h
-Py_ALIGNED
-Py_ARITHMETIC_RIGHT_SHIFT
-Py_CAN_START_THREADS
-Py_FORCE_EXPANSION
-Py_GCC_ATTRIBUTE
-Py_LL
-Py_SAFE_DOWNCAST
-Py_ULL
-Py_VA_COPY
 PYLONG_BITS_IN_DIGIT
 PY_DWORD_MAX
 PY_FORMAT_SIZE_T