Floating-Point Determinism (2013)

https://randomascii.wordpress.com/2013/07/16/floating-point-determinism/

by signa11 • 5 years ago

24 15 5 years ago

randomascii.wordpress.com

... what floating point determinism? Sorry, couldn't help myself. Even in the cases where it's deterministic, the implementation differences across hardware AND software make such a prospect downright hilarious to me.

alkonaut5• 5 years ago

The determinism that ensures a game replay can use only initial conditions to recreate a whole sequence. There is tons of code relying on fp determinism.

erwincoumans2• 5 years ago

This works when running the game on the same machine and same software. Once you use a different CPU or different compiler, floating point determinism (and such replay) can easily fail.

gopalv• 5 years ago

FP correctness is getting a bit of "bicycle for the problem" for me with https://herbie.uwplse.org/

Though I don't actually do any professional work with floats more than whatever distributed SQL correctness issues I get with sum() of doubles in the right/wrong order.

The error I try to correct are for visual/game code I wrote for fun & push through SSE.

More recently have started looking at bfloat16 which doesn't seem to have a lot of work done for this area.

nextaccountic2• 5 years ago

Are you the author of https://herbie.uwplse.org/? That's pretty cool!

Anyway floating point numerical stability is a different issue: some expressions produce results with less precision than other, different expressions, even though they should be the same thing algebraically (if we were dealing with real numbers or at least rational numbers). The issue here is that FP has limited precision, and it makes it lose some algebraic properties like associativity; and this limited precision sometimes lead to catastrophic cancellation and other problems that sharply reduce the precision (this doesn't happen if you use something with arbitrary precision like rational numbers).

Floating point determinism on the other hand is about always giving you the same result when running the same code (even if you happen to run in a different machine; and that's sometimes challenging because some architectures may not support features like subnormal numbers). It's okay if different code gives different results as far as determinism is concerned.

Sniffnoy• 5 years ago

(2013)

slmjkdbtl• 5 years ago

Don't have experience with floating point indeterminism in game networking, but is it possible to only use integers for a video game?

Floating-Point Determinism (2013)

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