non_sign_bits miscalculation in the function 'create_dynamic_map' in bitsandbytes/functional.py

[Talel-bm]

I found a potential bug in the following line of code:

non_sign_bits = total_bits - (1 if signed else 1)

in create_dynamic_map in bitsandbytes/functional.py

[TimDettmers]

Thank you! Yes, that looks wrong to me. I am surprised that the tests pass. I think it is because the non-signed data types are not used. They are usually only used for the Adam denominator. We need to check that this behavior is correct. The Adam optimizer is used frequently in software like Unsloth.

[ved1beta]

@matthewdouglas if you want can i investigate in this. and propose a fix?

[matthewdouglas]

This is behaving as expected, though it could be simplified.

non_sign_bits = total_bits - 1

@TimDettmers we discussed this on Slack: https://huggingface.slack.com/archives/C07BZPMKSNP/p1726533257085839?thread_ts=1725995060.515399&cid=C07BZPMKSNP