Require Two's complement to build Python.

Only very old machines (built in the 1960s?) like UNIVAC 1100/2200 series use signed number representations different than two's complement. Nowadays, all CPUs use the two's complement representation and CPython code base already relies on that in many places (especially Objects/longobject.c). For example, Python adds the -fwrapv compiler flag to GCC and clang.

The signed parameter of int.from_bytes() and int.to_bytes() indicates whether two's complement is used to represent the integer.

I propose to be more explicit on build requirements for integers:

• Two's complement
• No trap representation
• No padding bits

Mark Dickinson @mdickinson likes to repeat that CPython has these requirements :-)

• bpo-44946: Streamline operators and creation of ints for common case of single 'digit'. #27832 (review)

I think the -(x + 1) pattern came from days when we worried too much about alternative integer representations allowed by the C spec (sign-magnitude, ones' complement, two's complement with a trap representation, etc.). But since then other parts of the codebase have been happy to assume that all integer types are two's complement, no padding bits, no trap representation, so I think it's safe to do so here. (And your changes to the other bitwise operations already assume two's complement and no trap representation.)

• bpo-39277, pytime: Fix overflow check on double to int cast #17933 (comment)

(...) But making all our usual assumptions about integer representation (two's complement, no trap representation, no padding bits, etc.), -INT_MIN-1 is the same as INT_MAX (...)

I created this issue while reviewing PR #99762 which proposes to generalize a micro-optimization relying on a cast from an signed integer to an unsigned integer: replace 0 <= index && index < limit (Py_ssize_t) with (size_t)index < (size_t)limit. I'm not sure that two's complement is strictly required for this micro-optimization, but it reminded it to me :-)

cc @mdickinson

Linked PRs

• gh-100008: Document Python build requirements #100009
• gh-100008: Require Two's complement to build Python #100014
• gh-100008: VS 2017 is required since Python 3.11 #100045