[Perf] Windows/x64: 1 Regression on 9/3/2026 2:13:40 AM +00:00

### Run Information

Name | Value
-- | --
Architecture | x64
OS | Windows 10.0.22631
Queue | ViperWindows
Baseline | [399c3eb8ba823f00f9a250de4a3cdbdfb5d18f33](https://github.com/dotnet/runtime/commit/399c3eb8ba823f00f9a250de4a3cdbdfb5d18f33)
Compare | [a4ec83ce5eb429ae06d11ac13b1881eaf6651b1c](https://github.com/dotnet/runtime/commit/a4ec83ce5eb429ae06d11ac13b1881eaf6651b1c)
Diff | [Diff](https://github.com/dotnet/runtime/compare/399c3eb8ba823f00f9a250de4a3cdbdfb5d18f33...a4ec83ce5eb429ae06d11ac13b1881eaf6651b1c)
Configs | CompilationMode:tiered, RunKind:micro

### Regressions in System.Numerics.Tests.Perf_BitOperations

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

• [PopCount_uint - 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.Numerics.Tests.Perf_BitOperations*'
```

### System.Numerics.Tests.Perf_BitOperations.PopCount_uint

#### 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 linked Perf_BitOperations.cs benchmark source, then run the provided benchmarks_ci.py command using the documented workflow. Compare the baseline and test commits and inspect the PopCount_uint performance report. Done means identifying and addressing the cause of the Windows/x64 regression so the benchmark no longer shows the reported slowdown.

Written by the indexing model from the issue text.