Thank you! This point has been driving me mad for the last couple of years.

While answering a sibling comment, I found a paper analyzing various libms for their precision: https://homepages.loria.fr/pzimmermann/papers/glibc238-20230... (2023). I only skimmed it, but it looks like only LLVM's libm guarantees 0.5 ulp for every supported single-precision operation, and everyone else is wildly off. That's a pretty good result, though -- it means there's at least one reasonably compliant libm :)

Another sibling comments says that guaranteeing 0.5 ulp for double-precision operations is nigh impossible (and then another says it is after all). I don't have the knowledge to confirm which is true, but it's possible that this is the best we can get.