[Perf] Linux/x64: 1 Regression on 6/24/2026 2:33:49 AM +00:00

### Run Information

Name | Value
-- | --
Architecture | x64
OS | ubuntu 22.04
Queue | ViperUbuntu
Baseline | [f4430a4ba762ef82841ff7a8c340ed8854b2bca9](https://github.com/dotnet/runtime/commit/f4430a4ba762ef82841ff7a8c340ed8854b2bca9)
Compare | [46b72c7c81592b429e062bacebe8d39f17362333](https://github.com/dotnet/runtime/commit/46b72c7c81592b429e062bacebe8d39f17362333)
Diff | [Diff](https://github.com/dotnet/runtime/compare/f4430a4ba762ef82841ff7a8c340ed8854b2bca9...46b72c7c81592b429e062bacebe8d39f17362333)
Configs | CompilationMode:wasm, RunKind:micro

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

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<Char>.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 and run the provided benchmarks_ci.py command for System.Memory.Span. Compare the baseline and compare commits, then use the linked test report and profiling workflow to investigate LastIndexOfValue. Done means the regression is explained and the benchmark result is validated against the reported 1.12 ratio.

Written by the indexing model from the issue text.