[issue38147] add macro for __builtin_unreachable

[Sergey Fedoseev]

Sergey Fedoseev <fedoseev.sergey at gmail.com> added the comment:

> If you care of _PyLong_Copy() performance, you should somehow manually inline _PyLong_New() inside _PyLong_Copy().

It doesn't solve this:

> 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.

Here's example: https://github.com/sir-sigurd/cpython/commit/c8699d0c614a18d558216ae7d432107147c95c28.

I attach some disassembly from this example compiled with LTO, to demonstrate how the proposed macro affects generated code.

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Added file: https://bugs.python.org/file48614/disasm.txt

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