gh-96735: Fix undefined behaviour in struct unpacking functions by mdickinson · Pull Request #96739 · python/cpython (original) (raw)

Some not very rigorous timings, on macOS/Intel, non-optimised non-debug build. (This PR is not primarily about performance, but it would be unfortunate if it caused a significant performance regression.)

On this branch:

mdickinson@lovelace cpython % ./python.exe -m timeit -s "import struct; s = struct.Struct('<hhh'); v=b'\x1b\xe0\xcfKSh'" "s.unpack(v)"
5000000 loops, best of 5: 92.6 nsec per loop
mdickinson@lovelace cpython % ./python.exe -m timeit -s "import struct; s = struct.Struct('>hhh'); v=b'\x1b\xe0\xcfKSh'" "s.unpack(v)"
5000000 loops, best of 5: 94.3 nsec per loop
mdickinson@lovelace cpython % ./python.exe -m timeit -s "import struct; s = struct.Struct('<iii'); v=b'\x1b\xe0\xcfKSh'*2" "s.unpack(v)"
5000000 loops, best of 5: 99.1 nsec per loop
mdickinson@lovelace cpython % ./python.exe -m timeit -s "import struct; s = struct.Struct('>iii'); v=b'\x1b\xe0\xcfKSh'*2" "s.unpack(v)"
5000000 loops, best of 5: 98.3 nsec per loop
mdickinson@lovelace cpython % ./python.exe -m timeit -s "import struct; s = struct.Struct('<qqq'); v=b'\x1b\xe0\xcfKSh'*4" "s.unpack(v)"
2000000 loops, best of 5: 99.1 nsec per loop
mdickinson@lovelace cpython % ./python.exe -m timeit -s "import struct; s = struct.Struct('>qqq'); v=b'\x1b\xe0\xcfKSh'*4" "s.unpack(v)"
2000000 loops, best of 5: 102 nsec per loop

On the main branch at commit 6281aff:

mdickinson@lovelace cpython % ./python.exe -m timeit -s "import struct; s = struct.Struct('<hhh'); v=b'\x1b\xe0\xcfKSh'" "s.unpack(v)"  
5000000 loops, best of 5: 94.9 nsec per loop
mdickinson@lovelace cpython % ./python.exe -m timeit -s "import struct; s = struct.Struct('>hhh'); v=b'\x1b\xe0\xcfKSh'" "s.unpack(v)"  
5000000 loops, best of 5: 98.2 nsec per loop
mdickinson@lovelace cpython % ./python.exe -m timeit -s "import struct; s = struct.Struct('<iii'); v=b'\x1b\xe0\xcfKSh'*2" "s.unpack(v)"
2000000 loops, best of 5: 99.1 nsec per loop
mdickinson@lovelace cpython % ./python.exe -m timeit -s "import struct; s = struct.Struct('>iii'); v=b'\x1b\xe0\xcfKSh'*2" "s.unpack(v)"
2000000 loops, best of 5: 104 nsec per loop
mdickinson@lovelace cpython % ./python.exe -m timeit -s "import struct; s = struct.Struct('<qqq'); v=b'\x1b\xe0\xcfKSh'*4" "s.unpack(v)"
2000000 loops, best of 5: 101 nsec per loop
mdickinson@lovelace cpython % ./python.exe -m timeit -s "import struct; s = struct.Struct('>qqq'); v=b'\x1b\xe0\xcfKSh'*4" "s.unpack(v)"
2000000 loops, best of 5: 112 nsec per loop