Serhiy Storchaka <storchaka+cpyt...@gmail.com> added the comment:
> What does __builtin_unreachable()? Does it nothing? Call abort()? It is more than does nothing. It gives a hint to the compiler, so it can optimize out checks that lead to this code. For example, in switch (k) { case 0: ... case 1: ... case 2: ... case 3: ... default: __builtin_unreachable(); } the compiler can remove the check that k is in the range from 0 to 3 and use direct jump table. Or it can keep tests for k == 0, k == 1, k == 2 and remove the test for k == 3. Py_UNREACHABLE() replaces assert(0), but additionally silences compiler warning about unreachable code. It was the purpose of introducing Py_UNREACHABLE() (see issue14656). > For me, it's not an issue with the compiler, but more about handling corner > cases. Py_UNREACHABLE() is used in cases which "should not occur" but can > technically occur if you pass invalid data, or in "very unlikely case". It is bad if Py_UNREACHABLE() is now used in both cases: for assert(0) and for abort(). Using Py_UNREACHABLE() for "very unlikely case" is technically incorrect. Raise an exception or call Py_FatalError() in that case. We need to fix incorrect uses of Py_UNREACHABLE() or add Py_TRUE_UNREACHABLE() for true unreachable code. > Here it's unclear to me if this case can be reached or not. This is an infinite loop without "break". What can be the doubts? > _PyLong_Format() calls Py_UNREACHABLE() if base is not in (2, 8, 16). There is assert(base == 2 || base == 8 || base == 16) above in the code. Technically it is a bug. _PyLong_Format() can be used from public PyNumber_ToBase(). Either PyNumber_ToBase() or _PyLong_Format() should check that the base is 2, 8, 10 or 16, and raise an exception otherwise, but not abort the program. ---------- _______________________________________ Python tracker <rep...@bugs.python.org> <https://bugs.python.org/issue38249> _______________________________________ _______________________________________________ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com