[wasm] Pass --experimental-wasm-exnref to V8 for perf runs#5247
Open
DrewScoggins wants to merge 1 commit into
Open
[wasm] Pass --experimental-wasm-exnref to V8 for perf runs#5247DrewScoggins wants to merge 1 commit into
DrewScoggins wants to merge 1 commit into
Conversation
The runtime switched WASM exception handling to the standardized exnref proposal (dotnet/runtime#129851), which V8 keeps behind the --experimental-wasm-exnref flag. The perf wasm v8 leg installs V8 via jsvu and runs benchmarks through BenchmarkDotNet, which was not passing the flag, so MicroBenchmarks aborts at startup with 'This browser/engine doesn't support WASM exception handling' / 'Top-level await promise never resolved'. Pass the flag through BDN's --wasmArgs for the v8 engine on both the wasm and wasm_coreclr legs. Gated on v8 so JavaScriptCore is unaffected; harmless (warning only) on V8 builds that enable exnref by default. See dotnet/runtime#129849. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Contributor
There was a problem hiding this comment.
Pull request overview
This PR fixes failing WASM performance runs under V8 by ensuring the V8 engine is launched with support for the standardized exnref WASM exception-handling proposal (currently gated in V8 behind --experimental-wasm-exnref). This aligns the perf harness with the runtime change in dotnet/runtime#129851 so benchmark startup no longer aborts due to missing exception-handling support.
Changes:
- Add
--experimental-wasm-exnrefto V8 launches by passing it through BenchmarkDotNet’s--wasmArgsoption. - Apply the change to both
wasmandwasm_coreclrruntime types. - Gate the behavior on
javascript_engine == "v8"to avoid affecting JavaScriptCore legs.
💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.
Open
3 tasks
pavelsavara
approved these changes
Jun 27, 2026
This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
Add this suggestion to a batch that can be applied as a single commit.This suggestion is invalid because no changes were made to the code.Suggestions cannot be applied while the pull request is closed.Suggestions cannot be applied while viewing a subset of changes.Only one suggestion per line can be applied in a batch.Add this suggestion to a batch that can be applied as a single commit.Applying suggestions on deleted lines is not supported.You must change the existing code in this line in order to create a valid suggestion.Outdated suggestions cannot be applied.This suggestion has been applied or marked resolved.Suggestions cannot be applied from pending reviews.Suggestions cannot be applied on multi-line comments.Suggestions cannot be applied while the pull request is queued to merge.Suggestion cannot be applied right now. Please check back later.
Problem
WASM dotnet performance runs began failing at startup:
Root cause
dotnet/runtime#129851 (
[browser] WASM exception handling to use the standardized exnref) switched the runtime to the standardized exnref exception-handling proposal. V8 keeps exnref behind the--experimental-wasm-exnrefflag.The perf wasm v8 leg installs V8 via
jsvu(version read from the runtime'sBrowserVersions.props, already at15.1.103) and runs benchmarks through BenchmarkDotNet. BDN was not passing--experimental-wasm-exnref, so the runtime's startup feature check (exceptionsFinal) fails and the benchmark process aborts.Fix
Pass the flag through BDN's existing
--wasmArgsoption for thev8engine, on both thewasmandwasm_coreclrlegs. This mirrors the previous--wasmArgspattern removed in #5134.javascript_engine == "v8"so JavaScriptCore is unaffected.No BenchmarkDotNet change is required (
--wasmArgsalready exists in the consumed nightly), and no separate jsvu pin change is needed (the version is authoritative fromBrowserVersions.props).Refs dotnet/runtime#129849, dotnet/runtime#129851.