One thing I did not see mentioned in the article, or in these comments (according to ctrl-f anyway) is the use of feenableexcept()[1] to track down the source of NaNs in your code.

    feenableexcept(FE_DIVBYZERO | FE_INVALID | FE_OVERFLOW);

will cause your code to get a SIGFPE whenever a NaN crawls out from under a rock. Of course it doesn't work with fast-math enabled, but if you're unknowingly getting NaNs without fast-math enabled, you obviously need to fix those before even trying fast-math, and they can be hard to find, and feenableexcept() makes finding them a lot easier.

[1] https://linux.die.net/man/3/feenableexcept