Reading struct's code I find some checks seem outdated. They seemly exists because there are two kinds of integer in 2.x. I wrote a patch to remove them but maybe they exist for other reasons I don't see. Sorry if I misunderstand them. :-(

Could you please run microbenchmarks for affected code to be sure that changes don't hit performance (if for example PyLong_FromLongLong is significantly slower than PyLong_FromLong)?

There seems no significant regression in performance. A microbenchmark against nu_longlong:

./python3 -m perf timeit --duplicate=1000 --compare-to /home/angwer/repos/cpython/python -s 'import struct; p = struct.Struct("@L")' 'p.unpack(b"d\x00\x00\x00\x00\x00\x00\x00")'
python: ..................... 156 ns +- 10 ns
python3: ..................... 157 ns +- 9 ns

Median +- std dev: [python] 156 ns +- 10 ns -> [python3] 157 ns +- 9 ns: 1.01x slower
Not significant!

I see a tiny 3% regression.

$ ./python -m timeit -s 'import struct; u = struct.Struct("100000Q").unpack; b = b"abcd\0\0\0\0"*100000' 'u(b)'
Unpatched:  50 loops, best of 5: 9.34 msec per loop
Patched:    50 loops, best of 5: 9.66 msec per loop

$ ./python -m timeit -s 'import struct; u = struct.Struct("<100000Q").unpack; b = b"abcd\0\0\0\0"*100000' 'u(b)'
Unpatched:  50 loops, best of 5: 9.37 msec per loop
Patched:    50 loops, best of 5: 9.71 msec per loop

$ ./python -m timeit -s 'import struct; u = struct.Struct(">100000Q").unpack; b = b"\0\0\0\0dcba"*100000' 'u(b)'
Unpatched:  20 loops, best of 5: 11.6 msec per loop
Patched:    20 loops, best of 5: 12 msec per loop

The test on my box is reverse and stable, python3 is unpatched and python is patched. Anyway, it can't be called significant.

 ./python3 -m perf timeit --compare-to /home/angwer/repos/cpython/python -s 'import struct; u = struct.Struct("100000Q").unpack; b=b"abcd\x00\x00\x00\x00"*100000' 'u(b)'
python: ..................... 2.96 ms +- 0.05 ms
python3: ..................... 3.13 ms +- 0.03 ms

Median +- std dev: [python] 2.96 ms +- 0.05 ms -> [python3] 3.13 ms +- 0.03 ms: 1.06x slower

./python3 -m perf timeit --compare-to /home/angwer/repos/cpython/python -s 'import struct; u = struct.Struct("<100000Q").unpack; b=b"abcd\x00\x00\x00\x00"*100000' 'u(b)'
python: ..................... 2.96 ms +- 0.05 ms
python3: ..................... 3.13 ms +- 0.14 ms

Median +- std dev: [python] 2.96 ms +- 0.05 ms -> [python3] 3.13 ms +- 0.14 ms: 1.06x slower

./python3 -m perf timeit --compare-to /home/angwer/repos/cpython/python -s 'import struct; u = struct.Struct(">100000Q").unpack; b=b"\x00\x00\x00\x00dcba"*100000' 'u(b)'
python: ..................... 3.51 ms +- 0.09 ms
python3: ..................... 3.67 ms +- 0.06 ms

Median +- std dev: [python] 3.51 ms +- 0.09 ms -> [python3] 3.67 ms +- 0.06 ms: 1.05x slower

Your patch LGTM. Current code in _struct.c is ugly. I just wonder if it is worth to optimize PyLong_FromLongLong and PyLong_FromUnsignedLongLong if they are slower than PyLong_FromLong and PyLong_FromUnsignedLong.

New changeset 96f502856796f9310fed7161dc540201a4afc1ee by Xiang Zhang in branch 'master':
bpo-30224: remove outdated checks in struct (#1374)
https://github.com/python/cpython/commit/96f502856796f9310fed7161dc540201a4afc1ee