New submission from Cong Ma <m.c...@protonmail.ch>:
(This is a summarized form of the commit message in the attached patch. I'm submitting a patch instead of a PR over GitHub, because it seems that the ``autoreconf`` output files are part of the repository. In order for the changes to take effect in the repo, I may have to run ``autoreconf`` and add the clobbered output files to the repo, which I don't think is a good idea. Also on my system the ``autoreconf`` can only work correctly if I add a missing M4 file "ax_check_compile_flag.m4" from the Autoconf Archive <https://www.gnu.org/software/autoconf-archive/ax_check_compile_flag.html> for the ``AX_CHECK_COMPILE_FLAG`` macro used in the existing ``configure.ac``. I don't think it's wise for me to introduce so many changes at once if most developers don't need to run ``autoreconf`` often.) The problem ----------- Definition of the ``Py_UNREACHABLE()`` macro relied on testing compiler versions in preprocessor directives. This is unreliable chiefly because compilers masquerade as each other. The current implementation tests the ``__GNUC__`` and ``__GNUC_MINOR__`` macros as the logic (GCC version >= 4.5) for determining whether the compiler intrinsic ``__builtin_unreachable()`` is present (see commits eebaa9bf, 24ba3b0d). However, Clang defines these macros too and can cause confusion. Clang 11 pretends to be GCC 4.2.1 in its predefined macros. As a result, Clang won't use the intrinsic even if it's supported. This doesn't seem to match the intent behind the original implementation. The solution ------------ Test the presence of the compiler-builtin ``__builtin_unreachable()`` at configure-time using Autoconf, and conditionally define the ``Py_UNREACHABLE()`` macro depending on the configuration. The idea is based on the ``ax_gcc_builtin.m4`` code [0] by Gabriele Svelto. Alternative ideas ----------------- Recent versions of Clang and GCC support the ``__has_builtin()`` macro. However, this may be unreliable before Clang 10 [1], while GCC support is only available as of GCC 10 and its semantics may not be the same as Clang's [2]. Therefore ``__has_builtin()`` may not be as useful as it seems. We may attempt to improve the accuracy of version checking in ``#if`` directives, but this could be brittle and difficult to explain, verify, or maintain. Links ----- [0] https://www.gnu.org/software/autoconf-archive/ax_gcc_builtin.html [1] https://clang.llvm.org/docs/LanguageExtensions.html#has-builtin [2] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66970#c24 ---------- components: Build files: 0001-Properly-implement-Py_UNREACHABLE-macro-using-autoco.patch keywords: patch messages: 389454 nosy: congma priority: normal severity: normal status: open title: [PATCH] Properly implement Py_UNREACHABLE macro using autoconf. type: enhancement Added file: https://bugs.python.org/file49910/0001-Properly-implement-Py_UNREACHABLE-macro-using-autoco.patch _______________________________________ Python tracker <rep...@bugs.python.org> <https://bugs.python.org/issue43615> _______________________________________ _______________________________________________ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
