Stream: git-wasmtime

Topic: wasmtime / PR #12933 Add GC zeal assertions

Wasmtime GitHub notifications bot (Apr 01 2026 at 21:17):

fitzgen opened PR #12933 from fitzgen:add-gc-zeal-integrity-checks to bytecodealliance:main:

• Poison freed GC objects and new heap memory
• Assert newly-allocated objects are filled with poison pattern
• Add gc_assert! checks for valid VMGcKind on GcHeap::index[_mut]
• Add gc_assert! checks for valid VMGcKind during tracing
• Add VMGcKind::try_from_u32() for fallible kind validation
• Add over-approximated stack roots list integrity checks, called before and after trace and sweep. Validates kind, in-list bit, ref count, and that the list is not cyclic.
• Add assertion that all free blocks of memory contain the poison pattern, before and after trace and sweep

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Wasmtime GitHub notifications bot (Apr 01 2026 at 21:17):

fitzgen requested wasmtime-core-reviewers for a review on PR #12933.

Wasmtime GitHub notifications bot (Apr 01 2026 at 21:17):

fitzgen requested alexcrichton for a review on PR #12933.

Wasmtime GitHub notifications bot (Apr 01 2026 at 21:17):

fitzgen requested wasmtime-compiler-reviewers for a review on PR #12933.

Wasmtime GitHub notifications bot (Apr 01 2026 at 22:57):

github-actions[bot] added the label wasmtime:api on PR #12933.

Wasmtime GitHub notifications bot (Apr 01 2026 at 22:57):

github-actions[bot] added the label wasmtime:ref-types on PR #12933.

Wasmtime GitHub notifications bot (Apr 01 2026 at 22:57):

github-actions[bot] commented on PR #12933:

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Wasmtime GitHub notifications bot (Apr 01 2026 at 23:05):

alexcrichton submitted PR review.

Wasmtime GitHub notifications bot (Apr 01 2026 at 23:05):

alexcrichton created PR review comment:

Also, I'd recommend unconditional calls below and a conditional early-return in the function itself here.

Wasmtime GitHub notifications bot (Apr 01 2026 at 23:05):

alexcrichton created PR review comment:

if cfg!(gc_zeal) instead of #[cfg]

Wasmtime GitHub notifications bot (Apr 01 2026 at 23:05):

alexcrichton created PR review comment:

I'm a bit confused by how this works. If the counter is positive, alloc_raw is runn, wouldn't this execute here too? I figured that the None case up above would reset gc_zeal_alloc_counter and otherwise this block wouldn't be needed here.

Wasmtime GitHub notifications bot (Apr 01 2026 at 23:05):

alexcrichton created PR review comment:

And one final thought: with a #[cfg] this means that it's not factored into the caching logic for example and isn't visible in Tunables (e.g. compile with-zeal and then load into without-zeal or vice-versa). Not a huge concern necessarily but figured I'd write down anyway

Wasmtime GitHub notifications bot (Apr 01 2026 at 23:05):

alexcrichton created PR review comment:

This is probably me just not being used to "zeal", but to me the naming here is pretty disconnected from the purpose. IIUC this env var is "return OOM everything nth allocation" which to me would be better named something like WASMTIME_GC_ZEAL_OOM_AFTER or something like that. Maybe I just don't like the name "zeal" since I have no idea what it's connected to...

Wasmtime GitHub notifications bot (Apr 02 2026 at 19:19):

fitzgen submitted PR review.

Wasmtime GitHub notifications bot (Apr 02 2026 at 19:19):

fitzgen created PR review comment:

Yeah, I'd like to also add it to tunables eventually, so we can use it during fuzzing. Will cross that bridge when we get to it though.

Wasmtime GitHub notifications bot (Apr 02 2026 at 19:26):

fitzgen submitted PR review.

Wasmtime GitHub notifications bot (Apr 02 2026 at 19:26):

fitzgen created PR review comment:

Yeah, I futzed around with this and made it more complicated than needed. Will simplify.

Wasmtime GitHub notifications bot (Apr 02 2026 at 19:27):

fitzgen submitted PR review.

Wasmtime GitHub notifications bot (Apr 02 2026 at 19:27):

fitzgen created PR review comment:

The OOM is just a mechanism, what its real purpose is to force GC after every N allocations.

Wasmtime GitHub notifications bot (Apr 02 2026 at 19:28):

fitzgen updated PR #12933.

Wasmtime GitHub notifications bot (Apr 02 2026 at 19:28):

fitzgen has enabled auto merge for PR #12933.

Wasmtime GitHub notifications bot (Apr 02 2026 at 19:41):

fitzgen added PR #12933 Add GC zeal assertions to the merge queue.

Wasmtime GitHub notifications bot (Apr 02 2026 at 20:04):

fitzgen merged PR #12933.

Wasmtime GitHub notifications bot (Apr 02 2026 at 20:04):

fitzgen removed PR #12933 Add GC zeal assertions from the merge queue.

Last updated: Apr 13 2026 at 00:25 UTC