> if f and g are algebraically equivalent programs then FPSan(f) and FPSan(g) produce identical results when given identical inputs
Ok, but we want the other direction. If FPSan(f) & FPSan(g) produce identical results for identical inputs then we want to conclude that f & g are also equivalent. If g is an "optimized" version of f then this would allow checking equivalence but that's not what they are proving or maybe they are but it looks like the converse is contingent on an unproven conjecture.
Wow, that's pretty cool. Translating (almost) arbitrary floating point programs into weird integer programs while also preserving equivalence under non-strict floating point semantics? Mathematics can be surprisingly wonderful.