I currently build deterministic multiplayer WebGL games in Unity, built via C#->IL2CPP->Emscripten->WASM. The server is the same code base running on Microsoft's .Net runtime.The chances of being able to run deterministic floating point calculations across this stack is basically zero (even leaving aside that the games are often run on ARM chips), and so we use this library when floats are absolutely necessary (but more often just plain longs):https://github.com/asik/FixedMath.NetIt is a little terrifying that e.g. normalizing a vector involves a while loop, but all things considered the whole thing runs surprisingly well.(I agree with everything in your post, just thought I could add a real world field report)

 We also built and shipped a deterministic multiplayer WebGL game[1], but using CoffeeScript[2] + C++ -> Emscripten/dylib/DLLs to run the game in the browser and on Windows and Mac.Our game would snapshot the entire game state every few seconds and send that back to server to detect desyncs and cheaters. Floating point math, to our astonishment, was not the source of any non-determinism.I'm 80% sure that only source of non-determinism we encountered were from trig functions, so we just hard-coded lookup tables.2: It was 2012 when we started.
 You use that library when you want fractional values right? That is, numbers with a binary point but not floats.
 For the most part, I use longs (for instance a FixedVec is a (long,long,long) struct where 1 = 1/1000 of a meter).However, complicated calculations or anything involving angles or other math functions quickly becomes more convenient when expressed as a Fix64, which is more or less a drop in replacement for float.I would ideally use Fix64 everywhere, but given the torturous route the C# takes to be transformed into something that's executed on the client machines, my faith in the compiler's ability to generate good code for that is basically zero. I mentally treat long + long as a single instruction, but Fix64 + Fix64 as a function call.
 That's rough, fortunately for my own projects I'm only doing Unity on desktop, so I haven't had to go this far.Even something simple like multiplying up and dividing down quickly adds a lot of overhead, and when running on mobiles you really need all the speed you can get.

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