[Perf] Linux/x64: 1 Regression on 6/19/2026 6:38:08 AM +00:00

### Run Information

Name | Value
-- | --
Architecture | x64
OS | ubuntu 22.04
Queue | ViperUbuntu
Baseline | [ee2a8a9df2f6fc43c85e2aec1c8cae2e22d97e12](https://github.com/dotnet/runtime/commit/ee2a8a9df2f6fc43c85e2aec1c8cae2e22d97e12)
Compare | [18696adecd2e6e6528ba3d543c97b0013b961aa6](https://github.com/dotnet/runtime/commit/18696adecd2e6e6528ba3d543c97b0013b961aa6)
Diff | [Diff](https://github.com/dotnet/runtime/compare/ee2a8a9df2f6fc43c85e2aec1c8cae2e22d97e12...18696adecd2e6e6528ba3d543c97b0013b961aa6)
Configs | CompilationMode:wasm, RunKind:micro

### Regressions in System.Memory.Span<Byte>

Benchmark | Baseline | Test | Test/Base | Test Quality | Edge Detector | Baseline IR | Compare IR | IR Ratio
-- | -- | -- | -- | -- | -- | -- | -- | --
|

• [LastIndexOfValue - Duration of single invocation]()
• 📝 - [Benchmark Source]()
• [ADX - Test Multi Config Graph]()

![graph]()
[Test Report]()

### Repro
General Docs link: https://github.com/dotnet/performance/blob/main/docs/benchmarking-workflow-dotnet-runtime.md

```cmd
git clone https://github.com/dotnet/performance.git
python3 .\performance\scripts\benchmarks_ci.py -f net8.0 --filter 'System.Memory.Span*'
```

### System.Memory.Span<Byte>.LastIndexOfValue(Size: 3000)

#### ETL Files

#### Histogram

#### JIT Disasms

### Docs

[Profiling workflow for dotnet/runtime repository](https://github.com/dotnet/performance/blob/master/docs/profiling-workflow-dotnet-runtime.md)
[Benchmarking workflow for dotnet/runtime repository](https://github.com/dotnet/performance/blob/master/docs/benchmarking-workflow-dotnet-runtime.md)

Start with the benchmark source at src/benchmarks/micro/libraries/System.Memory/Span.cs lines 103-104, then run the provided benchmarks_ci.py command for System.Memory.Span. Compare the baseline and test commits, ee2a8a9df2f6fc43c85e2aec1c8cae2e22d97e12 and 18696adecd2e6e6528ba3d543c97b0013b961aa6. Done means identifying the cause of the LastIndexOfValue WASM regression and verifying the benchmark result.

Written by the indexing model from the issue text.