[issue39521] reversed(mylist) much slower on Python 3.8 32-bit for Windows

[Raymond Hettinger]

Raymond Hettinger <raymond.hettinger at gmail.com> added the comment:

The code for listiter_next() and listreviter_next() is almost the same. 

The main difference that the code for reversed saves the index to Py_ssize_t variable.  Maybe that causes a 32-bit to 64-bit conversion and back.  The change was made on 30 March 2016 in fbb1c5ee068d209e33f6e15ecb4821d5d8b107fa

Another possible cause is that this is just a random build outcome due to PGO or incidental branch mis-prediction from aliasing (as described in https://stackoverflow.com/a/17906589/1001643 ).

On the python.org mac 64-bit build, I'm not seeing any difference, so this is unique to the Windows 32-bit build.

$ python3.7 -m timeit -r11 -s "a = list(range(250)) * 4000" "list(iter(a))"
100 loops, best of 11: 3.08 msec per loop
$ python3.7 -m timeit -r11 -s "a = list(range(250)) * 4000" "list(reversed(a))"
100 loops, best of 11: 3.04 msec per loop
$ python3.8 -m timeit -r11 -s "a = list(range(250)) * 4000" "list(iter(a))"
100 loops, best of 11: 3.07 msec per loop
$ python3.8 -m timeit -r11 -s "a = list(range(250)) * 4000" "list(reversed(a))"
100 loops, best of 11: 3.01 msec per loop

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nosy: +rhettinger

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