Message 352228 - Python tracker

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