[Perf] Linux/arm64: 1 Regression on 6/29/2026 9:44:34 PM +00:00

### Run Information

Name | Value
-- | --
Architecture | arm64
OS | azurelinux 3.0
Queue | CobaltAzureLinux
Baseline | [f5a9853b34aeeec46e5729715a62ef2095607f13](https://github.com/dotnet/runtime/commit/f5a9853b34aeeec46e5729715a62ef2095607f13)
Compare | [8b16f838b402a42d4edd4d6ed299ed2d4508a2a2](https://github.com/dotnet/runtime/commit/8b16f838b402a42d4edd4d6ed299ed2d4508a2a2)
Diff | [Diff](https://github.com/dotnet/runtime/compare/f5a9853b34aeeec46e5729715a62ef2095607f13...8b16f838b402a42d4edd4d6ed299ed2d4508a2a2)
Configs | CompilationMode:tiered, R2RType:r2r_interpreter, RunKind:micro

### Regressions in System.Linq.Tests.Perf_Enumerable

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

• [WhereLast_LastElementMatches - 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.Linq.Tests.Perf_Enumerable*'
```

### System.Linq.Tests.Perf_Enumerable.WhereLast_LastElementMatches(input: Array)

#### 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 System.Linq.Tests.Perf_Enumerable benchmark source at performance/src/benchmarks/micro/libraries/System.Linq/Perf.Enumerable.cs#L66-L68 and run the provided benchmarks_ci.py command. Compare the baseline and test commits and use the linked test report or profiling workflow to investigate the WhereLast_LastElementMatches regression on arm64 AzureLinux. Done means the regression is explained and the benchmark no longer shows the reported slowdown.

Written by the indexing model from the issue text.