Bug report

Bug description:

In PEP 709, for the given example:

def f(lst):
    return [x for x in lst]

The bytecode is generated this way:

1           0 RESUME                   0

2           2 LOAD_FAST                0 (lst)
            4 GET_ITER
            6 LOAD_FAST_AND_CLEAR      1 (x)
            8 SWAP                     2
           10 BUILD_LIST               0
...

FWIK, the first SWAP after LOAD_FAST_AND_CLEAR is used to perform the original implicitly passed .0 in co_varnames (the iterated target passed from the MAKE_FUNCTION's caller before PEP 709).

And this seems to 100% be the needed case (I'm not sure, but I suspect it should be) when LOAD_FAST_AND_CLEAR is performed.

This makes me wonder if it's possible to merge the first SWAP operation under LOAD_FAST_AND_CLEAR, this should provide a negligible performance gain + bytecode operation simplification.

(I've verified this with under MacOS 15.5 (24F74) + Python 3.13.3, this should be something reproducible after 3.12.)

More background (an idea from a talk I gave in EuroPython 2025): https://github.com/note35/about/blob/dev/public/slides/2025-bytecode.pdf

CPython versions tested on:

3.12

Operating systems tested on:

macOS