Add a self-comparison fast path for some built-ins types

Feature or enhancement

Proposal:

a = [0] * 1000000
b = [0] * 1000000
timeit.timeit("a == b", globals=globals(), number=1000)
Out: 0.5669927500130143
timeit.timeit("a == a", globals=globals(), number=1000) # should be faster?
Out: 0.5312646660022438

for strings:

s1 = "a" * 1000000 + "b"
s2 = "a" * 1000000 + "c"
timeit.timeit("s1 == s2", globals=globals(), number=1000)
Out: 0.017710374988382682
timeit.timeit("s1 == s1", globals=globals(), number=1000)
Out: 1.5166995581239462e-05

Why doesn't Cpython, in the example above, return True from the == simply based on the object being compared to itself, whereas this optimisation is there for strings?

I made my own branch with such optimization: https://github.com/dima179-fuse/cpython/pull/1
It appears to make some benchmarks faster, and tests pass.

result.txt

Also attached are pyperformance results for both my branches and main:
2025-11-21_21-36-main-92972aea0f0e.json.gz

self-list-comparison-optimisation-e4ebdcbb8b51.json.gz

standard-containers-comparison-optimisation-54518a64f037.json.gz

Has this already been discussed elsewhere?

No response given

Links to previous discussion of this feature:

No response

Linked PRs

• gh-142027

Start by reviewing linked PR gh-142027 and the issue's self-comparison benchmarks for lists and strings. Determine the agreed scope for built-in types and validate any proposal with the referenced tests and pyperformance results; done requires an accepted approach with supporting benchmark evidence.