> There is always going to be a degree of un-reproducibility just due to the nature of math.
Fundamentally the math is deterministic (reproducible): if you do the same sequence of mathematical operations you get the same result. I gather you're getting at the non-associativity of floating point (eg additions), which is a fair point, but if you arrange to do your floating point operations in the same sequence then it will be reproducible.
This requires engineering on the part of compiler writers, but ultimately is solvable, given enough funding.
(About the compiler writers part: I'm thinking about GHC Haskell and the like... the Clangs/GCCs of the world will have no problems because of Translation Units. Things may change when Modules are a practical choice.)
Until then, those issues will continue to be lumped together with all the software reasons it wasn't reproducible.
My first Pentium could usually but not always math correctly until Intel replaced it during a recall.
Sitting in the middle results with additional downsides from modifying pipeline without core upsides of reproducible builds.
There is always going to be a degree of un-reproducibility just due to the nature of math. If you don't have the same system, same compiler version (down to the minor or patch level), same dependency versions, same build flags, filesystem ordering, OS handling etc. . .you're going to get differences.
The RB project has readily disclosed that there is a degree of "significantly reproducible" sussing that each end user is going to have to do. The fact that the Debian maintainers chose not to display the degree of reproducibility is probably because showing low reproducibility scores undermines the efforts to evangelize the movement.
I think that's understandable, but also is a bit of a two edged sword. If we don't disclose scores, we allow for the misrepresentation that "this is safe because it has the word reproducible in it". If we disclose scores, we get articles like this saying "wow, thats a really low score, wtf" and short lived paranoia gives way to ambivalence about the whole thing.
It's difficult to capture the nuance in this in pithy tidbits, hence blog post on HN with me explaining this :).