[eigenspace]

This isn't really as valid a comparison as you might think it is. The results of operations varying is not the problem with 'fast-math', the problem is that can negatively impact accuracy in catastrophic ways (among other things).

Sure, automatic FMA can change the result, but to my knowledge it always gives a more accurate result, not a less accurate one, and the way in which the results may differ is bounded.

[adgjlsfhk1]

fma can give less accurate results, and can give very large differences. for example 1.5floatmax(Float64)-floatmax(Float64) is Inf, while the fma version will give .5floatmax(Float64)