wasmtime / PR #13214 Enable inlining by default · git-wasmtime

Stream: git-wasmtime

Topic: wasmtime / PR #13214 Enable inlining by default

Wasmtime GitHub notifications bot (Apr 27 2026 at 20:53):

alexcrichton opened PR #13214 from alexcrichton:inlining to bytecodealliance:main:

This commit switches the default configuration of Wasmtime to enable cross-module inlining by default. This means that components with multiple modules will, by default, inline small functions across these modules. Additionally modules which use the GC proposal will have inlining performed by default. Intra-module inlining without GC, however, will continue to not happen.

This required two minor updates in tests. One is to disable inlining when the exact call stack is expected and another is to disable inlining when the stack-switching proposal is enabled because it currently falls over in Cranelift on the stack_switch instruction.

Wasmtime GitHub notifications bot (Apr 27 2026 at 20:55):

alexcrichton commented on PR #13214:

cc @fitzgen and @cfallin, this is initially a draft since I wanted to have somewhere to ask the question of "does anyone know a reason to not do this?" and figured that a minimal PR is good enough. So, to follow-through with that, do y'all know of a reason to not enable this by default? My main motivation would be optimizations like https://github.com/bytecodealliance/wasmtime/pull/13194 where for WASIp3 we'll want to have that on-by-default.

Wasmtime GitHub notifications bot (Apr 27 2026 at 20:59):

cfallin commented on PR #13214:

do y'all know of a reason to not enable this by default?

At an algorithmic level we'd want to make sure our heuristics are robust enough that we don't have blowups (in compilation time or in size of binary); but I guess we've been fuzzing this for a long time now and haven't seen any such issues. That would have been my only concern, otherwise :+1:

Wasmtime GitHub notifications bot (Apr 27 2026 at 21:43):

fitzgen commented on PR #13214:

Yeah I feel fine as far as our heuristics and fuzzing go. Ofc the heuristics need tweaks to be Good but that can happen at any time as separate work.

The one thing I would be tentative about here is the impact on compile times. I remember seeing some cases where they were quite a bit slower with inlining (although I never systematically investigated this) and I'd want to make sure that (a) compilation times for single core modules is not affected and (b) that we are okay with whatever slow down we actually see for compiling components.

Wasmtime GitHub notifications bot (Apr 27 2026 at 22:01):

github-actions[bot] added the label wasmtime:api on PR #13214.

Wasmtime GitHub notifications bot (Apr 27 2026 at 22:01):

github-actions[bot] added the label wasmtime:config on PR #13214.

Wasmtime GitHub notifications bot (Apr 27 2026 at 23:02):

github-actions[bot] commented on PR #13214:

Label Messager: wasmtime:config

It looks like you are changing Wasmtime's configuration options. Make sure to
complete this check list:

[ ] If you added a new Config method, you wrote extensive documentation for
it.

<details>

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Can this method panic? If so, when does it panic?

Example

Optional example here.
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[fuzzing-config]: https://github.com/bytecodealliance/wasmtime/blob/ca0e8d0a1d8cefc0496dba2f77a670571d8fdcab/crates/fuzzing/src/generators.rs#L182-L194
[fuzzing-docs]: https://docs.wasmtime.dev/contributing-fuzzing.html

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Wasmtime GitHub notifications bot (Apr 30 2026 at 00:46):

alexcrichton commented on PR #13214:

Discussion at the Cranelift meeting today concluded:

One avenue is that it should be confirmed that the quite-different inlining compilation path, when no inlining actually happens, should be double-checked to have little-to-no overehead.

Another avenue is that we should strive to, by default, not tamper with wasm backtraces. While it's possible to use frame tables built for debugging to recover inlining decisions after-the-fact that's not currently implemented. Given that we should probably change the defaults to only allowing inlining of intrinsics, for example, and have a dedicated mode for "inline some wasm things, but beware backtraces will be weird"

Wasmtime GitHub notifications bot (May 01 2026 at 23:59):

:cross_mark: alexcrichton closed without merge PR #13214.

Wasmtime GitHub notifications bot (May 01 2026 at 23:59):

alexcrichton commented on PR #13214:

I've morphed this PR into https://github.com/bytecodealliance/wasmtime/issues/13254 and https://github.com/bytecodealliance/wasmtime/pull/13250, so I'll close this in favor of those.

Last updated: May 03 2026 at 22:13 UTC