Allow embedders to exclude Cranelift's Pulley backends on hosts with a native backend

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Valutazione

Difficoltà
4/5
Tempo stimato
3-5 giorni
Idoneità per principianti
46/100
Tipo di issue
Funzionalità
Chiarezza
Abbastanza chiara
Stato di attività
Attiva
Stack tecnologico
rust
Ambito
backend, build-system

Direzione di ricerca

Start with the pulley feature in crates/wasmtime/Cargo.toml and the existing #[cfg(feature = "pulley")] handling in wasmtime-cranelift; then inspect the relevant build.rs checks. Determine how to forward the feature while preserving Pulley support for hosts without a native backend. Done means embedders can disable Pulley on supported hosts and receive a clear build error when it is required.

Scritto dal modello di indicizzazione a partire dal testo della issue.

Descrizione

Feature

wasmtime currently unconditionally enables the pulley feature of wasmtime-cranelift, which turns on cranelift-codegen/pulley. As a result every embedder using the cranelift feature links the pulley32 and pulley64 backends even with default-features = false, even when nothing ever targets Pulley.

Benefit

wasmtime 49.0.4, rustc 1.99.1, aarch64-apple-darwin, release profile with strip = true:

[dependencies]
wasmtime = { version = "=49.0.4", default-features = false, features = ["cranelift", "runtime", "std"] }

fn main() -> wasmtime::Result<()> {
    let engine = wasmtime::Engine::default();
    let module = wasmtime::Module::new(&engine, b"\0asm\x01\0\0\0")?;
    wasmtime::Instance::new(&mut wasmtime::Store::new(&engine, ()), &module, &[])?;
    Ok(())
}
file __text
as published 7,896,992 5,996,012
features = ["pulley"] removed via [patch] 6,121,152 4,522,352
delta -1,775,840 (-22.5%) -1,473,660 (-24.6%)

The patched build compiles and runs unchanged; wasmtime-cranelift already handles the feature's absence (#[cfg(feature = "pulley")]).

In the case of a large application like Aqueous, symbol attribution gives 1.54 MiB to the Pulley backends against 0.55 MiB for the aarch64 backend actually used.

Implementation

Ideally the feature has to be forwarded instead: pulley = ["wasmtime-cranelift?/pulley"] with pulley added to default. For hosts without a native backend, build.rs could then fail with a message asking for the pulley feature (when cranelift is on and pulley is off), rather than silently enabling it.

Alternatives

Alternatively keep the current behaviour and let build.rs accept an opt-out, e.g. a --cfg wasmtime_no_pulley_compiler (only allowed on hosts with a native backend).

Lingua principale
Rust
Stelle
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Fork
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Merge medio
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PR unite (30g)
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