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0 pointsjcranmer2y ago0 comments

There are some things that end up "accidentally" demanding exact equality of floats. Comparing the reference output of a program that writes floating-point numbers comes to mind. A multiplayer game that relies on each client computing gamestate locally can get desyncs to happen if two clients compute floats that differ even by that last bit.

Another, admittedly niche, scenario is that numerics code can use lower-precision types to emulate higher-precision types (e.g., represent a number with a pair of doubles).

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nine_k2y ago

AFAICT any modern multiplayer games use a central node that forces a coherent state of the world on all clients, otherwise cheating through desync becomes a serious problem. (Trusting clients in adversarial transactions, like deathmatch, is hard in general: cheating through altered rendering also used to be a thing.)

yvdriess2y ago

Depends on the game and platform. RTS games tend to still have peer to peer netcode. Deterministic simulation is still a thing.

Here is an interesting GDC talk on building a fixed precision engine core to ensure determinism: https://youtu.be/wwLW6CjswxM

bee_rider2y ago

This is totally off the cuff, but if you were using a pair of doubles like that, you’d essentially need to represent these new operations with numerical algorithms on the pair of doubles, right? So the algorithms would presumably need to deal with the last bit being a bit dodgy at times, right?

pclmulqdq2y ago

With appropriate rounding modes, the basic operations (+, -, *) are fairly short code sequences. Things like exp and 1/x can become adventures, but they're not too bad.

stephencanon2y ago

Division is actually pretty straightforward: compute a residual using the multiply-add that you already have, divide _that_, and then add it to the quotient.

Or (roughly equivalently, but maybe easier to understand) do a native double division, then do a Newton-Raphson step, which requires only multiplication and addition (just like you would refine a reciprocal estimate).

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