New submission from Sergey Fedoseev <fedoseev.ser...@gmail.com>:
GCC (along with Clang and ICC) has __builtin_unreachable() and MSVC has __assume() builtins, that can be used to optimize out unreachable conditions. So we could add macro like this: #ifdef Py_DEBUG # define Py_ASSUME(cond) (assert(cond)) #else # if defined(_MSC_VER) # define Py_ASSUME(cond) (__assume(cond)) # elif defined(__GNUC__) # define Py_ASSUME(cond) (cond? (void)0: __builtin_unreachable()) # else # define Py_ASSUME(cond) ((void)0); # endif #endif Here's a pair of really simple examples showing how it can optimize code: https://godbolt.org/z/g9LYXF. Real world example. _PyLong_Copy() [1] calls _PyLong_New() [2]. _PyLong_New() checks the size, so that overflow does not occur. This check is redundant when _PyLong_New() is called from _PyLong_Copy(). We could add a function that bypass that check, but in LTO build PyObject_MALLOC() is inlined into _PyLong_New() and it also checks the size. Adding Py_ASSUME((size_t)size <= MAX_LONG_DIGITS) allows to bypass both checks. [1] https://github.com/python/cpython/blob/3a4f66707e824ef3a8384827590ebaa6ca463dc0/Objects/longobject.c#L287-L309 [2] https://github.com/python/cpython/blob/3a4f66707e824ef3a8384827590ebaa6ca463dc0/Objects/longobject.c#L264-L283 ---------- messages: 352228 nosy: sir-sigurd priority: normal severity: normal status: open title: add macro for __builtin_unreachable _______________________________________ Python tracker <rep...@bugs.python.org> <https://bugs.python.org/issue38147> _______________________________________ _______________________________________________ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
