Definitely don't take an HN comment as a serious suggestion. Enable fast-math for your code, run your objective evaluation that's suitable for your domain, and if it passes the test, enjoy the added speed.
FWIW I have oodles of numerical C++ code where fast-math doesn't change the output.
For a very long time `-funsafe-math-optimizations` (and thus `-ffast-math`) had been infectious [1], so a responsible library should never have used `-ffast-math` anyway.
You are right in that the final binary is free to turn `-ffast-math` on if you can verify that everything went okay. But almost no one would actually verify that. It's like an advice that you shouldn't write your own crypto code---it's fine if you know what you are doing, but almost no one does, so the advice is technically false but still worthwhile.
That sounds like trying to run a program to check if it has underground behavior. How do you make a test that's comprehensive and future-compiler-safe?
FWIW I have oodles of numerical C++ code where fast-math doesn't change the output.