Improve perfomance of PyLong_FromDouble()

[sir-sigurd]

BPO	37986
Nosy	@mdickinson, @sir-sigurd
PRs	bpo-37986: Improve perfomance of PyLong_FromDouble() #15611
Files	bench-long-from-float.py

^{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 2020-05-10.09:21:34.511>
created_at = <Date 2019-08-30.09:00:41.312>
labels = ['interpreter-core', '3.9', 'performance']
title = 'Improve perfomance of PyLong_FromDouble()'
updated_at = <Date 2020-05-10.09:21:34.510>
user = 'https://github.com/sir-sigurd'

bugs.python.org fields:

activity = <Date 2020-05-10.09:21:34.510>
actor = 'mark.dickinson'
assignee = 'none'
closed = True
closed_date = <Date 2020-05-10.09:21:34.511>
closer = 'mark.dickinson'
components = ['Interpreter Core']
creation = <Date 2019-08-30.09:00:41.312>
creator = 'sir-sigurd'
dependencies = []
files = ['48573']
hgrepos = []
issue_num = 37986
keywords = []
message_count = 2.0
messages = ['350861', '368574']
nosy_count = 2.0
nosy_names = ['mark.dickinson', 'sir-sigurd']
pr_nums = ['15611']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'performance'
url = 'https://bugs.python.org/issue37986'
versions = ['Python 3.9']

[sir-sigurd]

This patch simplifies fast path for floats that fit into C long and moves it from float.__trunc__ to PyLong_FromDouble().

+---------------------+---------------------+------------------------------+
| Benchmark | long-from-float-ref | long-from-float |
+=====================+=====================+==============================+
| int(1.) | 39.5 ns | 37.3 ns: 1.06x faster (-6%) |
+---------------------+---------------------+------------------------------+
| int(2.**20) | 46.4 ns | 45.6 ns: 1.02x faster (-2%) |
+---------------------+---------------------+------------------------------+
| int(2.**30) | 52.5 ns | 49.0 ns: 1.07x faster (-7%) |
+---------------------+---------------------+------------------------------+
| int(2.**60) | 50.0 ns | 49.2 ns: 1.02x faster (-2%) |
+---------------------+---------------------+------------------------------+
| int(-2.**63) | 76.6 ns | 48.6 ns: 1.58x faster (-37%) |
+---------------------+---------------------+------------------------------+
| int(2.**80) | 77.1 ns | 72.5 ns: 1.06x faster (-6%) |
+---------------------+---------------------+------------------------------+
| int(2.**120) | 91.5 ns | 87.7 ns: 1.04x faster (-4%) |
+---------------------+---------------------+------------------------------+
| math.ceil(1.) | 57.4 ns | 32.9 ns: 1.74x faster (-43%) |
+---------------------+---------------------+------------------------------+
| math.ceil(2.**20) | 60.5 ns | 41.3 ns: 1.47x faster (-32%) |
+---------------------+---------------------+------------------------------+
| math.ceil(2.**30) | 64.2 ns | 43.9 ns: 1.46x faster (-32%) |
+---------------------+---------------------+------------------------------+
| math.ceil(2.**60) | 66.3 ns | 42.3 ns: 1.57x faster (-36%) |
+---------------------+---------------------+------------------------------+
| math.ceil(-2.**63) | 67.7 ns | 43.1 ns: 1.57x faster (-36%) |
+---------------------+---------------------+------------------------------+
| math.ceil(2.**80) | 66.6 ns | 65.6 ns: 1.01x faster (-1%) |
+---------------------+---------------------+------------------------------+
| math.ceil(2.**120) | 79.9 ns | 80.5 ns: 1.01x slower (+1%) |
+---------------------+---------------------+------------------------------+
| math.floor(1.) | 58.4 ns | 31.2 ns: 1.87x faster (-47%) |
+---------------------+---------------------+------------------------------+
| math.floor(2.**20) | 61.0 ns | 39.6 ns: 1.54x faster (-35%) |
+---------------------+---------------------+------------------------------+
| math.floor(2.**30) | 64.2 ns | 43.9 ns: 1.46x faster (-32%) |
+---------------------+---------------------+------------------------------+
| math.floor(2.**60) | 62.1 ns | 40.1 ns: 1.55x faster (-35%) |
+---------------------+---------------------+------------------------------+
| math.floor(-2.**63) | 64.1 ns | 39.9 ns: 1.61x faster (-38%) |
+---------------------+---------------------+------------------------------+
| math.floor(2.**80) | 62.2 ns | 62.7 ns: 1.01x slower (+1%) |
+---------------------+---------------------+------------------------------+
| math.floor(2.**120) | 77.0 ns | 77.8 ns: 1.01x slower (+1%) |
+---------------------+---------------------+------------------------------+

I'm going to speed-up conversion of larger floats in a follow-up PR.

[mdickinson]

New changeset 86a93fd by Sergey Fedoseev in branch 'master':
bpo-37986: Improve perfomance of PyLong_FromDouble() (GH-15611)
86a93fd