5 ms·

Does this determinism extend to floating point computations? This has historically been a pain point with multiplayer games where the client state has to be per

Does this determinism extend to floating point computations? This has historically been a pain point with multiplayer games where the client state has to be periodically re-synced with the server state due to slowly accumulating drift in floating point calculations.

MuffinFlavored 3y ago
is one option to sync “bignum” structs instead of f64 or is that not realistic in terms of complexity/size?
kodablah 3y ago
When I implemented a WASM compiler, the only source of float-based non-determinism I found was in the exact byte representation of NaN. Floating point math is deterministic. See https://webassembly.org/docs/faq/#why-is-there-no-fast-math-mode-with-relaxed-floating-point-semantics https://webassembly.org/docs/faq/#why-is-there-no-fast-math-... and https://github.com/WebAssembly/design/blob/main/Nondeterminism.md https://github.com/WebAssembly/design/blob/main/Nondetermini....
- adgjlsfhk1 3y ago
  The other common one is elementary functions from libm (i.e. exp/log etc)
  - jcranmer 3y ago
    Do note that sqrt should return the same results on all libm implementations, as well as functions like nextafter or scalbn.
- ajb 3y ago
  It may also be thread nondeterminism, eg relying on the commutativity of addition etc.
- aseipp 3y ago
  On that note, Wasmtime has an option to enable NaN canonicalization, which should patch that hole, too: https://docs.rs/wasmtime/latest/wasmtime/struct.Config.html#method.cranelift_nan_canonicalization https://docs.rs/wasmtime/latest/wasmtime/struct.Config.html#...
- nextaccountic 3y ago
  > Floating point math is deterministic. It is deterministic in a single machine, but it unfortunately is not deterministic across machines (and specially not across architectures) ... however, if you restrict yourself to a subset of floating point, it's straightforward to provide cross-platform determinism. That's what's the Rapier physics engine does https://rapier.rs/docs/user_guides/rust/determinism/ https://rapier.rs/docs/user_guides/rust/determinism/
yencabulator 3y ago
That's probably more about GPUs cheating to be faster[1]. https://asawicki.info/news_1741_myths_about_floating-point_numbers https://asawicki.info/news_1741_myths_about_floating-point_n... [1]: Back in the day, CPU results could also vary based on path of execution; see tweet in article.

MuffinFlavored 3y ago

is one option to sync “bignum” structs instead of f64 or is that not realistic in terms of complexity/size?

kodablah 3y ago

When I implemented a WASM compiler, the only source of float-based non-determinism I found was in the exact byte representation of NaN. Floating point math is deterministic. See https://webassembly.org/docs/faq/#why-is-there-no-fast-math-mode-with-relaxed-floating-point-semantics https://webassembly.org/docs/faq/#why-is-there-no-fast-math-... and https://github.com/WebAssembly/design/blob/main/Nondeterminism.md https://github.com/WebAssembly/design/blob/main/Nondetermini....

adgjlsfhk1 3y ago
The other common one is elementary functions from libm (i.e. exp/log etc)
- jcranmer 3y ago
  Do note that sqrt should return the same results on all libm implementations, as well as functions like nextafter or scalbn.
ajb 3y ago
It may also be thread nondeterminism, eg relying on the commutativity of addition etc.
aseipp 3y ago
On that note, Wasmtime has an option to enable NaN canonicalization, which should patch that hole, too: https://docs.rs/wasmtime/latest/wasmtime/struct.Config.html#method.cranelift_nan_canonicalization https://docs.rs/wasmtime/latest/wasmtime/struct.Config.html#...
nextaccountic 3y ago
> Floating point math is deterministic. It is deterministic in a single machine, but it unfortunately is not deterministic across machines (and specially not across architectures) ... however, if you restrict yourself to a subset of floating point, it's straightforward to provide cross-platform determinism. That's what's the Rapier physics engine does https://rapier.rs/docs/user_guides/rust/determinism/ https://rapier.rs/docs/user_guides/rust/determinism/

yencabulator 3y ago

That's probably more about GPUs cheating to be faster[1]. https://asawicki.info/news_1741_myths_about_floating-point_numbers https://asawicki.info/news_1741_myths_about_floating-point_n... [1]: Back in the day, CPU results could also vary based on path of execution; see tweet in article.