Wasmtime GitHub notifications bot (Sep 16 2026 at 21:20): Wasmtime GitHub notifications bot (Sep 16 2026 at 21:20):fitzgen opened PR #14351 from fitzgen:gc-type-alias-regions to bytecodealliance:main:
We have to take care to name the same region for a field that is shared between sub- and supertypes.
This is a ~0.3% speed up (in terms of instructions retired; my laptop is too noisy to get stable wall time numbers) on execution for the GC-using sightglass benchmarks. Note that we don't do dead-store elimination for GC object accesses because they are marked as trapping accesses, so this is just enabling more redundant-load elimination and store-to-load forwarding.
This is also (surprisingly) a ~2-3% speed up for compile times for the GC-using sightglass benchmarks, presumably because we eliminate more loads and therefore have less instructions to process through the rest of compilation.
This commit does not introduce alias regions for the different fields inside a GC object header or the array length; that is left for follow up commits.
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Wasmtime GitHub notifications bot (Sep 16 2026 at 21:20):fitzgen requested wasmtime-compiler-reviewers for a review on PR #14351.
Wasmtime GitHub notifications bot (Sep 16 2026 at 21:20):fitzgen requested alexcrichton for a review on PR #14351.
Wasmtime GitHub notifications bot (Sep 16 2026 at 21:20):fitzgen requested wasmtime-core-reviewers for a review on PR #14351.
Wasmtime GitHub notifications bot (Sep 16 2026 at 21:51)::thumbs_up: alexcrichton submitted PR review.
Wasmtime GitHub notifications bot (Sep 16 2026 at 21:51)::speech_balloon: alexcrichton created PR review comment:
With the current split/interning of types, is this split necessary? I'd have thought that
struct_tyis already unique across exceptions/structs of the same structure. For example above the internalcomposite_typefield is already different across exceptions/structs.
Wasmtime GitHub notifications bot (Sep 16 2026 at 21:51)::speech_balloon: alexcrichton created PR review comment:
Perhaps rename this to
gc_header_memflagsto avoid mis-use?
Wasmtime GitHub notifications bot (Sep 16 2026 at 21:51)::speech_balloon: alexcrichton created PR review comment:
Is it worth it trying to perhaps future-proof against the https://github.com/WebAssembly/multibyte-array-access proposal here? In that world I think we'll have to canonical all arrays-of-scalars to one
GcAccessregion, right?We probably won't implement that proposal for some time, but if we were to implement that in ~6 months I'd be worried that we wouldn't have any reason to come back and handle this.
Wasmtime GitHub notifications bot (Sep 17 2026 at 16:02)::memo: fitzgen submitted PR review.
Wasmtime GitHub notifications bot (Sep 17 2026 at 16:02)::speech_balloon: fitzgen created PR review comment:
Is it worth it trying to perhaps future-proof against the https://github.com/WebAssembly/multibyte-array-access proposal here? In that world I think we'll have to canonical all arrays-of-scalars to one
GcAccessregion, right?I don't believe so, because we will still know the type of the array object even if we are accessing its elements as
u64instead ofu8or whatever, and we choose the alias region based on the array object's type, not the array's elements' type: with this PR today, we already use different alias regions for two different(array i8)(as long as they really are two different types in two different subtyping hierarchies after canonicalization, e.g. due to rec groups). So we should already be future proof, afaik.
Wasmtime GitHub notifications bot (Sep 17 2026 at 16:08)::memo: fitzgen submitted PR review.
Wasmtime GitHub notifications bot (Sep 17 2026 at 16:08)::speech_balloon: fitzgen created PR review comment:
For structs we need to walk up the subtyping chain to find the type that introduced the field. For exceptions, there is no subtyping chain to walk. This split lets us funnel each type to the appropriate code path. I suppose we could try to unify them, and exceptions would just stop after one iteration of the subtype chain walk, since they never have a supertype, but I don't think we really gain anything from doing that other than removing just this single
if, but then also we'd muddy the water a bit for exactly what we're accessing (which we also aren't gaining a ton from right now, admittedly, but I do appreciate not losing precision when there is no reason to, so I think changing anything here is a bit of a wash on either side, so I'd ultimately prefer to leave it as-is unless you feel strongly about this).
Wasmtime GitHub notifications bot (Sep 17 2026 at 16:09):fitzgen updated PR #14351.
Wasmtime GitHub notifications bot (Sep 17 2026 at 16:10):fitzgen has enabled auto merge for PR #14351.
Wasmtime GitHub notifications bot (Sep 17 2026 at 17:41):fitzgen updated PR #14351.
Wasmtime GitHub notifications bot (Sep 17 2026 at 17:54):fitzgen added PR #14351 Give each GC struct field and array element its own alias region to the merge queue.
Wasmtime GitHub notifications bot (Sep 17 2026 at 17:58)::memo: alexcrichton submitted PR review.
Wasmtime GitHub notifications bot (Sep 17 2026 at 17:58)::speech_balloon: alexcrichton created PR review comment:
Definitely don't feel strong, it just felt weird to single out exceptions here when the desired properties otherwise fall out of not specializing around them.
Wasmtime GitHub notifications bot (Sep 17 2026 at 17:59)::memo: alexcrichton submitted PR review.
Wasmtime GitHub notifications bot (Sep 17 2026 at 17:59)::speech_balloon: alexcrichton created PR review comment:
Oh right, good point!
Wasmtime GitHub notifications bot (Sep 17 2026 at 18:19):github-merge-queue[bot] removed PR #14351 Give each GC struct field and array element its own alias region from the merge queue.
Wasmtime GitHub notifications bot (Sep 17 2026 at 18:34):alexcrichton added PR #14351 Give each GC struct field and array element its own alias region to the merge queue.
Wasmtime GitHub notifications bot (Sep 17 2026 at 19:01)::check: alexcrichton merged PR #14351.
Wasmtime GitHub notifications bot (Sep 17 2026 at 19:02):alexcrichton removed PR #14351 Give each GC struct field and array element its own alias region from the merge queue.
Last updated: Sep 20 2026 at 18:08 UTC