Stream: git-wasmtime

Topic: wasmtime / PR #6667 Shepherd along type information in li...

Wasmtime GitHub notifications bot (Jun 28 2023 at 22:20):

alexcrichton opened PR #6667 from alexcrichton:lift-lower-types to bytecodealliance:main:

This commit is a large refactor to the component runtime of Wasmtime to shepherd along type information when lifting and lowering values. Previously lifting and lowering would assume type information given context such as "surely lowering a u32 must lower into the type InterfaceType::U32" or "lowering a Val works as it knows its own type". This is still true, and this commit isn't changing these features. The rationale for this commit instead stems from the upcoming implementation of resources in Wasmtime.

Resources are trickier than all existing types in Wasmtime because what exactly is the type of a resource depends on who you're asking. For example the host might have one type called http::Headers but a component could import it as two distinct types:

(component
  (import "headers1" (type $h1 (sub resource)))
  (import "headers2" (type $h2 (sub resource)))

  ;; ...
)

in the above component the $h1 and $h2 types will each get their own table at runtime for managing their state. This means that if the host instantiates the component with http::Headers supplied as both types then the same type on the outside maps to two different types inside. This means that the lowering of a host-defined type into the component is now dependent on the "name" that the component has for the type, basically if the function used $h1 or $h2. This overall means that the type that the component assigned for a function is significant as part of lifting and lowering. Hence the rationale for this commit, threading around this type information.

The major change in this commit is updates to the Lift and Lower traits. Previously they took a mishmash of parameters and now they needed to take more parameters, so I've updated them with:

• Lift operations take a &LiftContext<'_> and an InterfaceType as contextual information. The context stores the store, the options, and type information. The InterfaceType is the type that's being lifted, which would indicate which resource table to load from for example.

• Lower operations now take a &mut LowerContext<'_, T> and an InterfaceType. The LowerContext is similar to its lift cousin where it stores the store, options, and type information.

The different context passed in to lift and load, for example, is no longer distinguished and both simply take a &LiftContext<'_> which simplifies things a bit.

This refactoring was pretty far reaching and touches quite a bit of the component model implementation. This is because basically everything deals with type information as types can be recursively nested in one another. I've taken the liberty to make code continue to be ergonomic/understandable where appropriate so some "shapes" of code are now different to continue to accommodate readability and maintainability.

Finally it's worth noting that this should not have any actual function impact on components running today (or tomorrow). User-facing APIs haven't changed at all here and it's just the guts that are refactored. One unfortunate aspect, though, is that this is going to be a small perf hit on lifting/lowering due to the fact that type information essentially needs to be "iterated" over during the lifting/lowering process. This iteration involves index lookups in &ComponentTypes along with assertions that when you lower Vec<T> that the type is InterfaceType::List(i). These assertions should always succeed, and in theory could become some sort of unreachable_unchecked in the future, but for now it's all left as safe checks/panics for us to optimize at a later date if necessary.

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Wasmtime GitHub notifications bot (Jun 28 2023 at 22:20):

alexcrichton requested wasmtime-core-reviewers for a review on PR #6667.

Wasmtime GitHub notifications bot (Jun 28 2023 at 22:20):

alexcrichton requested fitzgen for a review on PR #6667.

Wasmtime GitHub notifications bot (Jun 28 2023 at 22:33):

alexcrichton updated PR #6667.

Wasmtime GitHub notifications bot (Jun 28 2023 at 22:47):

alexcrichton updated PR #6667.

Wasmtime GitHub notifications bot (Jun 29 2023 at 15:46):

fitzgen submitted PR review:

I like it!

Wasmtime GitHub notifications bot (Jun 29 2023 at 16:29):

fitzgen merged PR #6667.

Last updated: Dec 13 2025 at 19:03 UTC