micro-optimization of PyLong_FromSize_t()

[sir-sigurd]

BPO	37802
Nosy	@gpshead, @vstinner, @gnprice, @sir-sigurd
PRs	bpo-37802: Slightly improve perfomance of PyLong_FromSize_t() #15192 bpo-37802: Fix a compiler warning in longobject.c #16517

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2019-09-12.14:42:39.846>
created_at = <Date 2019-08-09.13:27:40.684>
labels = ['interpreter-core', '3.9', 'performance']
title = 'micro-optimization of PyLong_FromSize_t()'
updated_at = <Date 2019-10-01.11:29:56.281>
user = 'https://github.com/sir-sigurd'

bugs.python.org fields:

activity = <Date 2019-10-01.11:29:56.281>
actor = 'vstinner'
assignee = 'none'
closed = True
closed_date = <Date 2019-09-12.14:42:39.846>
closer = 'gregory.p.smith'
components = ['Interpreter Core']
creation = <Date 2019-08-09.13:27:40.684>
creator = 'sir-sigurd'
dependencies = []
files = []
hgrepos = []
issue_num = 37802
keywords = ['patch']
message_count = 4.0
messages = ['349285', '350446', '352184', '353675']
nosy_count = 4.0
nosy_names = ['gregory.p.smith', 'vstinner', 'Greg Price', 'sir-sigurd']
pr_nums = ['15192', '16517']
priority = 'normal'
resolution = 'fixed'
stage = 'commit review'
status = 'closed'
superseder = None
type = 'performance'
url = 'https://bugs.python.org/issue37802'
versions = ['Python 3.9']

[sir-sigurd]

Currently PyLong_FromSize_t() uses PyLong_FromLong() for values < PyLong_BASE. It's suboptimal because PyLong_FromLong() needs to handle the sign. Removing PyLong_FromLong() call and handling small ints directly in PyLong_FromSize_t() makes it faster:

$ python -m perf timeit -s "from itertools import repeat; _len = repeat(None, 2).__length_hint__" "_len()" --compare-to=../cpython-master/venv/bin/python --duplicate=10000
/home/sergey/tmp/cpython-master/venv/bin/python: ..................... 18.7 ns +- 0.3 ns
/home/sergey/tmp/cpython-dev/venv/bin/python: ..................... 16.7 ns +- 0.1 ns
Mean +- std dev: [/home/sergey/tmp/cpython-master/venv/bin/python] 18.7 ns +- 0.3 ns -> [/home/sergey/tmp/cpython-dev/venv/bin/python] 16.7 ns +- 0.1 ns: 1.12x faster (-10%)

$ python -m perf timeit -s "from itertools import repeat; _len = repeat(None, 2**10).__length_hint__" "_len()" --compare-to=../cpython-master/venv/bin/python --duplicate=10000
/home/sergey/tmp/cpython-master/venv/bin/python: ..................... 26.2 ns +- 0.0 ns
/home/sergey/tmp/cpython-dev/venv/bin/python: ..................... 25.0 ns +- 0.7 ns
Mean +- std dev: [/home/sergey/tmp/cpython-master/venv/bin/python] 26.2 ns +- 0.0 ns -> [/home/sergey/tmp/cpython-dev/venv/bin/python] 25.0 ns +- 0.7 ns: 1.05x faster (-5%)

$ python -m perf timeit -s "from itertools import repeat; _len = repeat(None, 2**30).__length_hint__" "_len()" --compare-to=../cpython-master/venv/bin/python --duplicate=10000
/home/sergey/tmp/cpython-master/venv/bin/python: ..................... 25.6 ns +- 0.1 ns
/home/sergey/tmp/cpython-dev/venv/bin/python: ..................... 25.6 ns +- 0.0 ns
Mean +- std dev: [/home/sergey/tmp/cpython-master/venv/bin/python] 25.6 ns +- 0.1 ns -> [/home/sergey/tmp/cpython-dev/venv/bin/python] 25.6 ns +- 0.0 ns: 1.00x faster (-0%)

This change makes PyLong_FromSize_t() consistently faster than PyLong_FromSsize_t(). So it might make sense to replace PyLong_FromSsize_t() with PyLong_FromSize_t() in __length_hint__() implementations and other similar cases. For example:

$ python -m perf timeit -s "_len = iter(bytes(2)).__length_hint__" "_len()" --compare-to=../cpython-master/venv/bin/python --duplicate=10000
/home/sergey/tmp/cpython-master/venv/bin/python: ..................... 19.4 ns +- 0.3 ns
/home/sergey/tmp/cpython-dev/venv/bin/python: ..................... 17.3 ns +- 0.1 ns
Mean +- std dev: [/home/sergey/tmp/cpython-master/venv/bin/python] 19.4 ns +- 0.3 ns -> [/home/sergey/tmp/cpython-dev/venv/bin/python] 17.3 ns +- 0.1 ns: 1.12x faster (-11%)

$ python -m perf timeit -s "_len = iter(bytes(2**10)).__length_hint__" "_len()" --compare-to=../cpython-master/venv/bin/python --duplicate=10000
/home/sergey/tmp/cpython-master/venv/bin/python: ..................... 26.3 ns +- 0.1 ns
/home/sergey/tmp/cpython-dev/venv/bin/python: ..................... 25.3 ns +- 0.2 ns
Mean +- std dev: [/home/sergey/tmp/cpython-master/venv/bin/python] 26.3 ns +- 0.1 ns -> [/home/sergey/tmp/cpython-dev/venv/bin/python] 25.3 ns +- 0.2 ns: 1.04x faster (-4%)

$ python -m perf timeit -s "_len = iter(bytes(2**30)).__length_hint__" "_len()" --compare-to=../cpython-master/venv/bin/python --duplicate=10000
/home/sergey/tmp/cpython-master/venv/bin/python: ..................... 27.6 ns +- 0.1 ns
/home/sergey/tmp/cpython-dev/venv/bin/python: ..................... 26.0 ns +- 0.1 ns
Mean +- std dev: [/home/sergey/tmp/cpython-master/venv/bin/python] 27.6 ns +- 0.1 ns -> [/home/sergey/tmp/cpython-dev/venv/bin/python] 26.0 ns +- 0.1 ns: 1.06x faster (-6%)

[sir-sigurd]

Previous benchmarks results were obtained with non-LTO build.

Here are results for LTO build:

$ python -m perf timeit -s "from itertools import repeat; _len = repeat(None, 0).__length_hint__" "_len()" --compare-to=../cpython-master/venv/bin/python --duplicate=1000
/home/sergey/tmp/cpython-master/venv/bin/python: ..................... 14.9 ns +- 0.2 ns
/home/sergey/tmp/cpython-dev/venv/bin/python: ..................... 13.1 ns +- 0.5 ns
Mean +- std dev: [/home/sergey/tmp/cpython-master/venv/bin/python] 14.9 ns +- 0.2 ns -> [/home/sergey/tmp/cpython-dev/venv/bin/python] 13.1 ns +- 0.5 ns: 1.13x faster (-12%)

$ python -m perf timeit -s "from itertools import repeat; _len = repeat(None, 2**10).__length_hint__" "_len()" --compare-to=../cpython-master/venv/bin/python --duplicate=1000
/home/sergey/tmp/cpython-master/venv/bin/python: ..................... 22.1 ns +- 0.1 ns
/home/sergey/tmp/cpython-dev/venv/bin/python: ..................... 20.9 ns +- 0.4 ns
Mean +- std dev: [/home/sergey/tmp/cpython-master/venv/bin/python] 22.1 ns +- 0.1 ns -> [/home/sergey/tmp/cpython-dev/venv/bin/python] 20.9 ns +- 0.4 ns: 1.05x faster (-5%)

$ python -m perf timeit -s "from itertools import repeat; _len = repeat(None, 2**30).__length_hint__" "_len()" --compare-to=../cpython-master/venv/bin/python --duplicate=1000
/home/sergey/tmp/cpython-master/venv/bin/python: ..................... 23.3 ns +- 0.0 ns
/home/sergey/tmp/cpython-dev/venv/bin/python: ..................... 21.6 ns +- 0.1 ns
Mean +- std dev: [/home/sergey/tmp/cpython-master/venv/bin/python] 23.3 ns +- 0.0 ns -> [/home/sergey/tmp/cpython-dev/venv/bin/python] 21.6 ns +- 0.1 ns: 1.08x faster (-8%)

$ python -m perf timeit -s "from itertools import repeat; _len = repeat(None, 2**60).__length_hint__" "_len()" --compare-to=../cpython-master/venv/bin/python --duplicate=1000
/home/sergey/tmp/cpython-master/venv/bin/python: ..................... 24.4 ns +- 0.1 ns
/home/sergey/tmp/cpython-dev/venv/bin/python: ..................... 22.7 ns +- 0.1 ns
Mean +- std dev: [/home/sergey/tmp/cpython-master/venv/bin/python] 24.4 ns +- 0.1 ns -> [/home/sergey/tmp/cpython-dev/venv/bin/python] 22.7 ns +- 0.1 ns: 1.08x faster (-7%)

[gpshead]

New changeset c6734ee by Gregory P. Smith (Sergey Fedoseev) in branch 'master':
bpo-37802: Slightly improve perfomance of PyLong_FromUnsigned*() (GH-15192)
c6734ee

[vstinner]

New changeset 6314abc by Victor Stinner in branch 'master':
bpo-37802: Fix a compiler warning in longobject.c (GH-16517)
6314abc