[New-bugs-announce] [issue34152] performance of some list slice assignment can be improved

[Sergey Fedoseev]

New submission from Sergey Fedoseev <fedoseev.sergey at gmail.com>:

Script for benchmarks:

NAME=list-slice.json

python -m perf timeit --name "l[len(l):] = reversed(l)" -l 1 -s "l = [None] * 1000000" "l[len(l):] = reversed(l)" --append $NAME
python -m perf timeit --name "l[len(l):] = l" -l 1 -s "l = [None] * 1000000" "l[len(l):] = l" --append $NAME
python -m perf timeit --name "l[::-1] = l" -s "l = [None] * 1000000" "l[::-1] = l" --append $NAME
python -m perf timeit --name "l[:] = l" -s "l = [None] * 1000000" "l[:] = l" --append $NAME
python -m perf timeit --name "l[len(l)//2:] = l" -l 1 -s "l = [None] * 1000000" "l[len(l)//2:] = l"  --append $NAME
python -m perf timeit --name "l[:len(l)//2] = l" -l 1 -s "l = [None] * 1000000" "l[:len(l)//2] = l" --append $NAME
python -m perf timeit --name "l[len(l)//4:len(l)*3//4] = l" -l 1 -s "l = [None] * 1000000" "l[len(l)//4:len(l)*3//4] = l" --append $NAME

Results table:

+------------------------------+-------------------+------------------------------------+
| Benchmark                    | list-slice-master | list-slice                         |
+==============================+===================+====================================+
| l[len(l):] = reversed(l)     | 8.44 ms           | 5.18 ms: 1.63x faster (-39%)       |
+------------------------------+-------------------+------------------------------------+
| l[len(l):] = l               | 7.88 ms           | 3.37 ms: 2.34x faster (-57%)       |
+------------------------------+-------------------+------------------------------------+
| l[::-1] = l                  | 10.4 ms           | 582 us: 17.85x faster (-94%)       |
+------------------------------+-------------------+------------------------------------+
| l[:] = l                     | 10.6 ms           | 86.1 ns: 123128.46x faster (-100%) |
+------------------------------+-------------------+------------------------------------+
| l[len(l)//2:] = l            | 11.1 ms           | 2.08 ms: 5.33x faster (-81%)       |
+------------------------------+-------------------+------------------------------------+
| l[:len(l)//2] = l            | 11.5 ms           | 1.76 ms: 6.53x faster (-85%)       |
+------------------------------+-------------------+------------------------------------+
| l[len(l)//4:len(l)*3//4] = l | 11.3 ms           | 2.27 ms: 4.98x faster (-80%)       |
+------------------------------+-------------------+------------------------------------+

----------
components: Interpreter Core
messages: 321908
nosy: sir-sigurd
priority: normal
severity: normal
status: open
title: performance of some list slice assignment can be improved
type: performance

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue34152>
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