How deterministic is floating point inaccuracy?

Question

I understand that floating point calculations have accuracy issues and there are plenty of questions explaining why. My question is if I run the same calculation twice, can

Answer 1

This answer in the C++ FAQ probably describes it the best:

http://www.parashift.com/c++-faq-lite/newbie.html#faq-29.18

It is not only that different architectures or compiler might give you trouble, float pointing numbers already behave in weird ways within the same program. As the FAQ points out if y == x is true, that can still mean that cos(y) == cos(x) will be false. This is because the x86 CPU calculates the value with 80bit, while the value is stored as 64bit in memory, so you end up comparing a truncated 64bit value with a full 80bit value.

The calculation are still deterministic, in the sense that running the same compiled binary will give you the same result each time, but the moment you adjust the source a bit, the optimization flags or compile it with a different compiler all bets are off and anything can happen.

Practically speaking, I it is not quite that bad, I could reproduce simple float pointing math with different version of GCC on 32bit Linux bit for bit, but the moment I switched to 64bit Linux the result were no longer the same. Demos recordings created on 32bit wouldn't work on 64bit and vice versa, but would work fine when run on the same arch.