gh-144995: Optimize memoryview == memoryview

[vstinner]

• Issue: Optimize memoryview comparison: a memoryview is equal to itself #144995

[vstinner]

Results of the benchmark from the issue:

bytes 0.000122 seconds
mview 0.000146 seconds
⇒ 1.197965 time slower

memoryview comparison complexity is no longer O(n) but O(1): values are no longer compared.

[eendebakpt]

Does this cover the complex types?

import numpy as np
a = np.array([1+2j, 3+4j, float('nan')], dtype=np.complex128)
mv = memoryview(a)
mv == mv # False

[vstinner]

This memory format is Zd. Oh, my change doesn't work for this memoryview. I should replace the blocklist with an allowlist. I'm not a memoryview/buffer expert. I didn't know that 3rd party projects can have their own format.

[vstinner]

@eendebakpt: I updated the PR to allow formats known to be safe for pointer comparison (integer types), instead of blocking formats known to use floats.

I excluded the format P since I don't know well this format. Or can we allow it?

[eendebakpt]

@eendebakpt: I updated the PR to allow formats known to be safe for pointer comparison (integer types), instead of blocking formats known to use floats.

I excluded the format P since I don't know well this format. Or can we allow it?

I think adding the P is fine (but I am no expert either). Leaving it out is the safe option, we can reconsider if this turns out to be a performance bottleneck.