Hacker News new | ask | show | jobs
by iainmerrick 3151 days ago
Sure, I meant “relatively easily reproducible in portable C code”.

I may be wrong, but it’s my understanding that even given the precise rounding modes, fusing etc, the results could still differ as implementations are allowed to use varying extended precision internally (and do).

For example, as far as I know it’s difficult to guarantee identical FPU results on x86 and ARM.

[Edit to add: I guess I'm complaining about the popular implementations rather than the IEEE spec itself, but for ordinary users like me it amounts to the same thing. Overall IEEE 754 is wonderful, so it's exciting to see a proposal for something even better!]
1 comments
wolfgke 3151 days ago
> For example, as far as I know it’s difficult to guarantee identical FPU results on x86 and ARM.

Can you give details/ressources on how it is difficult to obtain identical FPU results on x86 and ARM?

Does this even hold if you program in assembly using either

- only the primitives that are defined exactly in IEEE 754:2008 (i.e. not some functions defined in some, say, C library)

or

- using "identical" implementations of more complex functions (i.e. not the IEEE 754 primitives; think of cos, erf, gamma, ...)?

link
iainmerrick 3148 days ago
Can you give details/ressources on how it is difficult to obtain identical FPU results on x86 and ARM?

Not personally -- having already run into problems getting reproducible results on a single x86 machine with different compilers, I haven't even tried getting ARM to match!

Here's a long list of links on various issues: https://gafferongames.com/post/floating_point_determinism/

And here's a blog post that goes into detail on how to get reproducible results on x86: http://yosefk.com/blog/consistency-how-to-defeat-the-purpose... It's not too bad, but it sounds a bit fragile and I have no idea how well it would translate to other architectures. [Edit to add: hmm, actually, that post does say this is mostly just an x86 problem, or rather x87]

using "identical" implementations of more complex functions

That's a rather onerous requirement! Especially for multi-platform work. In almost all cases I'd like to be able to use the platform's math library, which is presumably well-optimized. Is there a good, reasonably efficient, highly portable implementation of math.h that gives fully reproducible results?

It seems to me (but I'd love to be convinced otherwise!) that if you really want reproducible results, fixed-point is a much better road to go down. Ints are just a lot more consistent than floats on pretty much every platform with a C compiler.

link