How is deterministic execution of WebAssembly ensured?

ulan · August 6, 2021, 4:48pm

Hi there. I am an engineer at DFINITY and want to share an answer to one of frequently asked questions: “How do we ensure that the WebAssembly code in canisters runs deterministically?”

The sources of nondeterminism in WebAssembly are known and listed here:

github.com

WebAssembly/design/blob/main/Nondeterminism.md

# Nondeterminism in WebAssembly

WebAssembly is a [portable](Portability.md) [sandboxed](Security.md) platform
with limited, local, nondeterminism.

  * *Limited*: nondeterministic execution can only occur in a small number of
    well-defined cases (described below) and, in those cases, the implementation
    may select from a limited set of possible behaviors.
  * *Local*: when nondeterministic execution occurs, the effect is local,
    there is no "spooky action at a distance".

The [rationale](Rationale.md) document details why WebAssembly is designed as
detailed in this document.

The following is a list of the places where the WebAssembly specification
currently admits nondeterminism:

 * New features will be added to WebAssembly, which means different implementations
   will have different support for each feature. This can be detected with
   `has_feature`, but is still a source of differences between executions.

This file has been truncated. show original

We disable features that may lead to nondeterminism. Specifically:

Threads are disabled.
SIMD instructions are disabled. In the future we may enable deterministic parts of SIMD.
Floating-point NaN values are canonicalized.

skilesare · October 22, 2021, 5:09pm

We were discussing Testing floats and it seemed like this topic might fit with this thread. Are floats deterministic in motoko? Can we test them for exactness? Or should we be using ranges?

nomeata · October 24, 2021, 8:02am

The semantics of floats is fully determined by the Wasm standard, with the exception of the bit-wise representation of NaNs, and to address this, the replica instruments your wasm code before execution to normalize the NaNs, so go ahead using them

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How is deterministic execution of WebAssembly ensured?

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