[Perf] Windows/x64: 1 Regression on 9/14/2026 6:37:21 AM +00:00

### Run Information

Name | Value
-- | --
Architecture | x64
OS | Windows 10.0.22631
Queue | ViperWindows
Baseline | [edd70ca597f37a95a3ccc04c127fb8e883cc4139](https://github.com/dotnet/runtime/commit/edd70ca597f37a95a3ccc04c127fb8e883cc4139)
Compare | [a652cdfed564e5b96265a52d8d9d9d001efaeb7e](https://github.com/dotnet/runtime/commit/a652cdfed564e5b96265a52d8d9d9d001efaeb7e)
Diff | [Diff](https://github.com/dotnet/runtime/compare/edd70ca597f37a95a3ccc04c127fb8e883cc4139...a652cdfed564e5b96265a52d8d9d9d001efaeb7e)
Configs | CompilationMode:tiered, RunKind:micro

### Regressions in System.IO.Tests.Perf_StreamWriter

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

• [WriteString - 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
py .\performance\scripts\benchmarks_ci.py -f net8.0 --filter 'System.IO.Tests.Perf_StreamWriter*'
```

### System.IO.Tests.Perf_StreamWriter.WriteString(writeLength: 100)

#### 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.Runtime.Extensions/Perf.StreamWriter.cs and the reported baseline and compare commits. Run benchmarks_ci.py with the provided System.IO.Tests.Perf_StreamWriter filter, then use the profiling and benchmarking workflow documentation to investigate the WriteString regression. Done means the regression is explained and the benchmark no longer shows the reported slowdown.

Written by the indexing model from the issue text.