Struct sc_executor_wasmtime::Semantics

pub struct Semantics {
    pub fast_instance_reuse: bool,
    pub deterministic_stack_limit: Option<DeterministicStackLimit>,
    pub canonicalize_nans: bool,
}

Fields

fast_instance_reuse: bool

Enabling this will lead to some optimization shenanigans that make calling WasmInstance extermely fast.

Primarily this is achieved by not recreating the instance for each call and performing a bare minimum clean up: reapplying the data segments and restoring the values for global variables. The vast majority of the linear memory is not restored, meaning that effects of previous executions on the same WasmInstance can be observed there.

This is not a problem for a standard substrate runtime execution because it’s up to the runtime itself to make sure that it doesn’t involve any non-determinism.

Since this feature depends on instrumentation, it can be set only if runtime is instantiated using the runtime blob, e.g. using create_runtime.

deterministic_stack_limit: Option<DeterministicStackLimit>

Specifiying Some will enable deterministic stack height. That is, all executor invocations will reach stack overflow at the exactly same point across different wasmtime versions and architectures.

This is achieved by a combination of running an instrumentation pass on input code and configuring wasmtime accordingly.

Since this feature depends on instrumentation, it can be set only if runtime is instantiated using the runtime blob, e.g. using create_runtime.

canonicalize_nans: bool

Controls whether wasmtime should compile floating point in a way that doesn’t allow for non-determinism.

By default, the wasm spec allows some local non-determinism wrt. certain floating point operations. Specifically, those operations that are not defined to operate on bits (e.g. fneg) can produce NaN values. The exact bit pattern for those is not specified and may depend on the particular machine that executes wasmtime generated JITed machine code. That is a source of non-deterministic values.

The classical runtime environment for Substrate allowed it and punted this on the runtime developers. For PVFs, we want to ensure that execution is deterministic though. Therefore, for PVF execution this flag is meant to be turned on.