[New-bugs-announce] [issue39521] reversed(mylist) much slower on Python 3.8.1 32-bit for Windows

[Stefan Pochmann]

New submission from Stefan Pochmann <stefan.pochmann at gmail.com>:

Somehow `reversed` became much slower than `iter` on lists:

List with 1,000 elements:

> python -m timeit -s "a = list(range(1000))" "list(iter(a))"
50000 loops, best of 5: 5.73 usec per loop

> python -m timeit -s "a = list(range(1000))" "list(reversed(a))"
20000 loops, best of 5: 14.2 usec per loop

List with 1,000,000 elements:

> python -m timeit -s "a = list(range(250)) * 4000" "list(iter(a))"
50 loops, best of 5: 7.08 msec per loop

> python -m timeit -s "a = list(range(250)) * 4000" "list(reversed(a))"
20 loops, best of 5: 15.5 msec per loop

On another machine I tried ten different Python versions and found out it's only version 3.8.1 and only the 32-bit version:

                32-bit              64-bit
CPython     iter   reversed     iter   reversed
 3.5.4      19.8     19.9       22.4     22.7
 3.6.8      19.8     19.9       22.3     22.6
 3.7.6      19.9     19.9       22.3     22.5
 3.8.1      19.8     24.9       22.4     22.6

Another time with 3.8.0 instead of 3.8.1:

                32-bit              64-bit
CPython     iter   reversed     iter   reversed
 3.5.4      19.5     19.6       21.9     22.2
 3.6.8      19.5     19.7       21.8     22.1
 3.7.6      19.5     19.6       21.7     22.0
 3.8.0      19.4     24.5       21.7     22.1

I used the "Stable Releases" "executable installer"s from here:
https://www.python.org/downloads/windows/

More details here: https://stackoverflow.com/q/60005302/12671057

----------
components: Build
messages: 361195
nosy: Stefan Pochmann
priority: normal
severity: normal
status: open
title: reversed(mylist) much slower on Python 3.8.1 32-bit for Windows
type: performance
versions: Python 3.8

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<https://bugs.python.org/issue39521>
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