PyLong_As*() functions computes result for large longs like this:
size_t x, prev;
x = 0;
while (--i >= 0) {
prev = x;
x = (x << PyLong_SHIFT) | v->ob_digit[i];
if ((x >> PyLong_SHIFT) != prev) {
*overflow = sign;
goto exit;
}
}
It can be rewritten like this:
size_t x = 0;
while (--i >= 0) {
if (x > (size_t)-1 >> PyLong_SHIFT) {
goto overflow;
}
x = (x << PyLong_SHIFT) | v->ob_digit[i];
}
This provides some speed-up:
PyLong_AsSsize_t()
$ python -m perf timeit -s "from struct import Struct; N = 1000; pack = Struct('n'*N).pack; values = (2**30,)*N" "pack(*values)" --compare-to=../cpython-master/venv/bin/python
/home/sergey/tmp/cpython-master/venv/bin/python: ..................... 9.69 us +- 0.02 us
/home/sergey/tmp/cpython-dev/venv/bin/python: ..................... 8.61 us +- 0.07 us
Mean +- std dev: [/home/sergey/tmp/cpython-master/venv/bin/python] 9.69 us +- 0.02 us -> [/home/sergey/tmp/cpython-dev/venv/bin/python] 8.61 us +- 0.07 us: 1.12x faster (-11%)
PyLong_AsSize_t()
$ python -m perf timeit -s "from struct import Struct; N = 1000; pack = Struct('N'*N).pack; values = (2**30,)*N" "pack(*values)" --compare-to=../cpython-master/venv/bin/python
/home/sergey/tmp/cpython-master/venv/bin/python: ..................... 10.5 us +- 0.1 us
/home/sergey/tmp/cpython-dev/venv/bin/python: ..................... 8.19 us +- 0.17 us
Mean +- std dev: [/home/sergey/tmp/cpython-master/venv/bin/python] 10.5 us +- 0.1 us -> [/home/sergey/tmp/cpython-dev/venv/bin/python] 8.19 us +- 0.17 us: 1.29x faster (-22%)
PyLong_AsLong()
$ python -m perf timeit -s "from struct import Struct; N = 1000; pack = Struct('l'*N).pack; values = (2**30,)*N" "pack(*values)" --compare-to=../cpython-master/venv/bin/python
/home/sergey/tmp/cpython-master/venv/bin/python: ..................... 9.68 us +- 0.02 us
/home/sergey/tmp/cpython-dev/venv/bin/python: ..................... 8.48 us +- 0.22 us
Mean +- std dev: [/home/sergey/tmp/cpython-master/venv/bin/python] 9.68 us +- 0.02 us -> [/home/sergey/tmp/cpython-dev/venv/bin/python] 8.48 us +- 0.22 us: 1.14x faster (-12%)
PyLong_AsUnsignedLong()
$ python -m perf timeit -s "from struct import Struct; N = 1000; pack = Struct('L'*N).pack; values = (2**30,)*N" "pack(*values)" --compare-to=../cpython-master/venv/bin/python
/home/sergey/tmp/cpython-master/venv/bin/python: ..................... 10.5 us +- 0.1 us
/home/sergey/tmp/cpython-dev/venv/bin/python: ..................... 8.41 us +- 0.26 us
Mean +- std dev: [/home/sergey/tmp/cpython-master/venv/bin/python] 10.5 us +- 0.1 us -> [/home/sergey/tmp/cpython-dev/venv/bin/python] 8.41 us +- 0.26 us: 1.25x faster (-20%)
The mentioned pattern is also used in PyLong_AsLongLongAndOverflow(), but I left it untouched since the proposed change doesn't seem to affect its performance.
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