Struct sc_executor_wasmtime::Semantics

pub struct Semantics {
    pub instantiation_strategy: InstantiationStrategy,
    pub deterministic_stack_limit: Option<DeterministicStackLimit>,
    pub canonicalize_nans: bool,
    pub parallel_compilation: bool,
    pub extra_heap_pages: u64,
    pub max_memory_size: Option<usize>,
}

Fields

instantiation_strategy: InstantiationStrategy

The instantiation strategy to use.

deterministic_stack_limit: Option<DeterministicStackLimit>

Specifying 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.

parallel_compilation: bool

Configures wasmtime to use multiple threads for compiling.

extra_heap_pages: u64

The number of extra WASM pages which will be allocated on top of what is requested by the WASM blob itself.

max_memory_size: Option<usize>

The total amount of memory in bytes an instance can request.

If specified, the runtime will be able to allocate only that much of wasm memory. This is the total number and therefore the Semantics::extra_heap_pages is accounted for.

That means that the initial number of pages of a linear memory plus the Semantics::extra_heap_pages multiplied by the wasm page size (64KiB) should be less than or equal to max_memory_size, otherwise the instance won’t be created.

Moreover, memory.grow will fail (return -1) if the sum of sizes of currently mounted and additional pages exceeds max_memory_size.

The default is None.

Trait Implementations

impl Clone for Semantics

fn clone(&self) -> Semantics

Returns a copy of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.