Stream: git-wasmtime

Topic: wasmtime / PR #13194 Inline `context.{get,set}` in compon...

Wasmtime GitHub notifications bot (Apr 24 2026 at 22:33):

alexcrichton opened PR #13194 from alexcrichton:inline-context-intrinsics to bytecodealliance:main:

This commit reimplements the context.{get,set} intrinsics in the component model, introduced in the component-model-async and component-model-threading proposals. The intent of these intrinsics in WASIp3, for example, are intended to replace the globals used for the stack pointer and TLS base in previous modules, for example. The implementation of loading from a global is a single load instruction, whereas the previous implementation of context.get was a full libcall, which is significantly more expensive. The goal of this PR is to ensure that the transition to using context.get and context.set for high-performance uses retains the same performance as the WASIp2 constructs.

Specifically the storage for context.{get,set} slots have been moved into the VMStoreContext structure which has a known layout to compiled code. There still remains storage within each GuestThread because there's only one store, and the idea is that whenever threads are switched between the switch operation is slightly more expensive now where it has to update and maintain the state in the store. The rationale for this is that it'll be far more often that these values are accessed rather than threads being swapped between.

The implementation chosen in this commit is to model the context.{get,set} intrinsics as UnsafeIntrinsics. This is a bit of a shoehorn where they're not actually unsafe, but all of the plumbing and support for UnsafeIntrinsic is effectively exactly what these want. To avoid duplicating lots of infrastructure that's where these now reside.

The concurrent.rs implementation has been updated to save/restore context from the store, and this additionally updates a few other switch points to ensure that the store never switches away or to a deleted thread. This niche situation happened in a few scenarios with no impact from before, but with the switching implementation having to access threads it became load-bearing that these must be valid.

The end result is that with -Cinlining the context.{get,set} instructions are two instructions instead of a full libcall. One instruction is loading VMStoreContext, which is GVN-able and hoist-able, while the other is the actual load/store. This is the same as the performance of the stack pointer being in an imported global, for example.

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Wasmtime GitHub notifications bot (Apr 24 2026 at 22:33):

alexcrichton requested fitzgen for a review on PR #13194.

Wasmtime GitHub notifications bot (Apr 24 2026 at 22:33):

alexcrichton requested wasmtime-compiler-reviewers for a review on PR #13194.

Wasmtime GitHub notifications bot (Apr 24 2026 at 22:33):

alexcrichton requested wasmtime-core-reviewers for a review on PR #13194.

Wasmtime GitHub notifications bot (Apr 24 2026 at 23:11):

alexcrichton updated PR #13194.

Wasmtime GitHub notifications bot (Apr 25 2026 at 01:07):

github-actions[bot] added the label wasmtime:api on PR #13194.

Wasmtime GitHub notifications bot (Apr 27 2026 at 17:26):

:thumbs_up: fitzgen submitted PR review.

Wasmtime GitHub notifications bot (Apr 27 2026 at 17:26):

:speech_balloon: fitzgen created PR review comment:

Should this be ty.bytes() or something to future proof for memory64?

Wasmtime GitHub notifications bot (Apr 27 2026 at 17:26):

:speech_balloon: fitzgen created PR review comment:

Unless you set the can move flag, we won't actually hoist these out of loops, which you imply we will do in the commit message.

Wasmtime GitHub notifications bot (Apr 27 2026 at 20:12):

:memo: alexcrichton submitted PR review.

Wasmtime GitHub notifications bot (Apr 27 2026 at 20:12):

:speech_balloon: alexcrichton created PR review comment:

Eventually, yes, but not yet unfortunately. The width of the type being loaded (the above match) and the width of the slot are independent and will need to be configured separately. The width above will be generalized to i64 eventually once the type is plumbed through, and the width of the slot will need to be increased when memory64 support is added. I'll add some comments and defensive assertions too.

Wasmtime GitHub notifications bot (Apr 27 2026 at 20:12):

:memo: alexcrichton submitted PR review.

Wasmtime GitHub notifications bot (Apr 27 2026 at 20:12):

:speech_balloon: alexcrichton created PR review comment:

Oh the hoistable load was the *mut VMStoreContext pointer load, not this one.

Wasmtime GitHub notifications bot (Apr 27 2026 at 20:24):

alexcrichton updated PR #13194.

Wasmtime GitHub notifications bot (Apr 27 2026 at 20:24):

alexcrichton has enabled auto merge for PR #13194.

Wasmtime GitHub notifications bot (Apr 27 2026 at 20:37):

alexcrichton added PR #13194 Inline context.{get,set} in components to the merge queue.

Wasmtime GitHub notifications bot (Apr 27 2026 at 21:02):

:check: alexcrichton merged PR #13194.

Wasmtime GitHub notifications bot (Apr 27 2026 at 21:02):

alexcrichton removed PR #13194 Inline context.{get,set} in components from the merge queue.

Last updated: May 03 2026 at 22:13 UTC