Stream: git-wasmtime

Topic: wasmtime / PR #14785 Add `--preload` support for components

Wasmtime GitHub notifications bot (Oct 16 2026 at 17:57):

fitzgen opened PR #14785 from fitzgen:component-pre-load to bytecodealliance:main:

This required the ability to wrap a live instance into a wasmtime::component::Linker as well.

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Wasmtime GitHub notifications bot (Oct 16 2026 at 17:57):

fitzgen requested dicej for a review on PR #14785.

Wasmtime GitHub notifications bot (Oct 16 2026 at 17:57):

fitzgen requested wasmtime-core-reviewers for a review on PR #14785.

Wasmtime GitHub notifications bot (Oct 16 2026 at 20:47):

github-actions[bot] added the label wasmtime:api on PR #14785.

Wasmtime GitHub notifications bot (Oct 16 2026 at 20:47):

fitzgen updated PR #14785.

Wasmtime GitHub notifications bot (Oct 16 2026 at 22:31):

fitzgen updated PR #14785.

Wasmtime GitHub notifications bot (Oct 19 2026 at 15:02):

:memo: alexcrichton submitted PR review:

Do you have an immediate use case for this at the moment, and if so could it use wasm-tools compose or wac in the meantime?

I'm hesitant myself to start committing to something here becuase there's a lot that's possible in this space and I'm not sure that this is the best approach. Some thoughts I'd have are:

• Technically --preload should be able to work for core wasm modules loaded into a component, that'd satisfy a component importing a core module for example.
• Val is so horrendously slow that I'd rather not feature it so prominently within the implementation of an option that I suspect everyone expects to be fast.
• Instead of using Val our thinking historically has been that something performs a composition and Wasmtime sees the composition, so for example wasmtime --preload could compose on-the-fly like it does with unsafe builtins or perhaps something similar to that.
• Overall repurposing --preload as-is from core modules may not be the best design here. In theory this could support behavior like -L on linkers where the idea is you can point wasmtime to various directories to automatically find and load *.wasms as appropriate.

Basically I'm not confident that we should take --preload as-is and support it within the CLI, and I'm also not confident in the implementation using Val. That's what leads me to ask if a composition works for your use case if you have one. Otherwise though I think it's fine to work through these questions/ideas, but I'm not sure if that's what you were expecting to do here.

Wasmtime GitHub notifications bot (Oct 19 2026 at 15:25):

:thumbs_up: dicej submitted PR review:

LGTM, thanks!

It occurred to me that we could precalculate the parameters which have resource borrows to drop based on the types rather than visit them all at runtime, but I suppose we're not trying to optimize this given that composition remains the fast path.

Wasmtime GitHub notifications bot (Oct 19 2026 at 15:27):

dicej commented on PR #14785:

Oops, my review crossed with Alex's; he makes some good points about e.g. using WAC just-in-time.

Wasmtime GitHub notifications bot (Oct 19 2026 at 16:07):

fitzgen commented on PR #14785:

My immediate goal is to be able to easily run+profile sightglass benchmarks without needing to stub out the bench.start/bench.end imports. I could use wac or whatever, but that is a bit more annoying and less it-just-works-out-of-the-box compared to core wasm, but I guess that is fine as a temporary workaround.

Wasmtime GitHub notifications bot (Oct 19 2026 at 16:07):

:cross_mark: fitzgen closed without merge PR #14785.

Last updated: Oct 31 2026 at 23:03 UTC