JIT Regression Test Extraction

🚨 Do NOT create a test when: the issue has no reproducible code and you cannot compose a minimal repro, the issue is a duplicate of an existing JIT regression test, or the bug is in libraries/runtime rather than the JIT compiler itself.

Extract a JIT regression test case from a GitHub issue into a properly structured test under src/tests/JIT/Regression_*/.

Step 1: Gather Information from the GitHub Issue

From the GitHub issue, extract:

1. Issue number → folder/file name (e.g., #99391 → Runtime_99391)
2. Reproduction code — if none provided, compose a minimal repro yourself
3. Environment variables — any DOTNET_* vars needed to reproduce
4. Expected behavior — correct output/behavior

Step 2: Choose the Test Location

For a simple test using optimized compilation without debug information, add the source file directly to:

src/tests/JIT/Regression_ro_2/Runtime_<issue_number>.cs

Regression_ro_2/Regression_ro_2.csproj recursively includes sources in its directory. Do not edit its source list or create a project for the test. Use a Runtime_<issue_number>/ subdirectory only when keeping multiple related files together.

If the test needs its own project (see Step 4), use a separate directory outside the globbed source directories:

src/tests/JIT/Regression_2/Runtime_<issue_number>/

Step 3: Create the Test File

Create a Runtime_<issue_number>.cs file following these conventions:

Example:

// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.

using System;
using System.Runtime.CompilerServices;
using Xunit;

public class Runtime_<issue_number>
{
    [Fact]
    public static void TestEntryPoint()
    {
        // Test code that exercises the bug
        // Use Assert.Equal, Assert.True, etc. for validation
    }
}

Key Conventions

• License header: Always include the standard .NET Foundation license header
• Class name: Match the file name exactly (Runtime_<issue_number>)
• Test method: [Fact] attribute, named TestEntryPoint()
• Minimize the reproduction: Strip to the minimal case that triggers the bug
• Use [MethodImpl(MethodImplOptions.NoInlining)] when preventing inlining is needed to reproduce

Example: Simple Test (from Runtime_99391)

// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.


namespace Runtime_99391;

using System;
using System.Runtime.CompilerServices;
using System.Numerics;
using Xunit;

public class Runtime_99391
{
    [Fact]
    public static void TestEntryPoint()
    {
        Vector2 result2a = Vector2.Normalize(Value2);
        Assert.Equal(new Vector2(0, 1), result2a);
    }

    private static Vector2 Value2
    {
        [MethodImpl(MethodImplOptions.NoInlining)]
        get => new Vector2(0, 2);
    }
}

Step 4: Create a .csproj File Only When Required

A custom .csproj file is only required when:

• Environment variables are needed to reproduce the bug (such as DOTNET_JitStressModeNames)
• Special compilation settings are required
• Separate assemblies, native dependencies, or process isolation are required

Otherwise, the source glob in Regression_ro_2.csproj includes the test automatically. Keep custom projects and their sources under src/tests/JIT/Regression_2/Runtime_<issue_number>/. Regression_2/Regression_2.csproj discovers those projects recursively without compiling their sources directly. Do not place them in source-glob directories such as src/tests/JIT/Regression_ro_2/, where they would also be compiled with the runner's settings.

If a custom .csproj file is needed, it should be located next to the test source file with the following name: Runtime_<issue_number>.csproj. Example:

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <Optimize>True</Optimize>
    <DebugType>None</DebugType>
    <!-- Needed for CLRTestEnvironmentVariable -->
    <RequiresProcessIsolation>true</RequiresProcessIsolation>
  </PropertyGroup>
  <ItemGroup>
    <Compile Include="$(MSBuildProjectName).cs" />

    <CLRTestEnvironmentVariable Include="DOTNET_TieredCompilation" Value="0" />
  </ItemGroup>
</Project>

Tips

• No .csproj edits needed for simple tests — add the .cs file under src/tests/JIT/Regression_ro_2/.
• Look at recent tests under src/tests/JIT/Regression_ro_2/ for simple tests, or src/tests/JIT/Regression_2/Runtime_* for tests with custom projects.