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Using CrossGen to Create Native Images

Introduction

When you create a .NET assembly using C# compiler, your assembly contains only MSIL code. When the app runs, the JIT compiler translates the MSIL code into native code, before the CPU can execute them. This execution model has some advantages. For example, your assembly code can be portable across all platforms and architectures that support .NET Core. However, this portability comes with a performance cost. Your app starts up more slowly, because the JIT compiler has to spend time to translate the code.

To help make your app start up faster, CoreCLR includes a tool called CrossGen, which can pre-compile the MSIL code into native code.

Getting CrossGen

If you build CoreCLR yourself, the CrossGen tool (crossgen.exe on Windows, or crossgen on other platforms) is created as part of the build, and stored in the same output directory as other CoreCLR binaries. If you install CoreCLR using a NuGet package, you can find CrossGen in the tools folder of the NuGet package.

Regardless of how you obtain CrossGen, it is very important that it must match other CoreCLR binaries.

  • If you build CrossGen yourself, you should use it with runtime and System.Private.CoreLib.dll generated from the same build. Do not attempt to mix CrossGen from one build with binaries generated from another build.
  • If you install CrossGen from NuGet, make sure you use CrossGen from exactly the same NuGet package as the rest of your CoreCLR binaries. Do not attempt to mix binaries from multiple NuGet packages.

If you do not follow the above rules, you are likely to encounter errors while running CrossGen.

Using CrossGen

In most cases, the build script automatically runs CrossGen to create the native image for System.Private.CoreLib.dll and mscorlib.dll. When this happens, you will find System.Private.CoreLib.ni.dll and mscorlib.ni.dll in your output directory. System.Private.CoreLib.dll and mscorlib.dll are the MSIL assemblies created by the C# compiler, while System.Private.CoreLib.ni.dll and mscorlib.ni.dll are the native images that contain CPU-specific code. Once the build is done, you only need System.Private.CoreLib.ni.dll and mscorlib.ni.dll to use CoreCLR. The original MSIL assemblies are no longer needed by the runtime.

If you installed CoreCLR from a NuGet package, System.Private.CoreLib.ni.dll and mscorlib.ni.dll are included in the package.

If for some reason you did not get System.Private.CoreLib.dll or mscorlib.ni.dll with the rest of your CoreCLR, you can easily create it yourself using CrossGen.

If your System.Private.CoreLib.dll, mscorlib.dll and JIT compiler (clrjit.dll on Windows or libclrjit.* on other platforms) are all in the same directory as CrossGen itself, you can compile System.Private.CoreLib.dll and mscorlib.dll with the following commands (first two commands for Windows, next two commands for other platforms):

.\crossgen.exe System.Private.CoreLib.dll
.\crossgen.exe mscorlib.dll
./crossgen System.Private.CoreLib.dll
./crossgen mscorlib.dll

If your files are scattered in different directories, or if you want to create native images for other assemblies, the command line is slightly more complex:

.\crossgen.exe /JITPath path\clrjit.dll /Platform_Assemblies_Paths "path1;path2;..." path\assemblyName.dll
./crossgen -JITPath path/libclrjit.so -Platform_Assemblies_Paths "path1:path2:..." path/assemblyName.dll

The /Platform_Assemblies_Paths is used to specify the locations of all the dependencies of the input assembly, including the input assembly itself. You should use full paths for these locations. Relative paths do not always work. If there are multiple paths, separate them with semicolons (;) on Windows, or colons (:) on non-Windows platforms. It is generally a good idea to enclose the path list in quotes to protect any special characters from the shell.

Using native images

Running CrossGen on an assembly creates a "native image" file, with the extension of .ni.dll or .ni.exe. You should include the native images in your app, either by replacing the original MSIL assmblies with the native images, or by putting the native images next to the MSIL assemblies. When the native images are present, CoreCLR runtime will automatically use it instead of the original MSIL assemblies.

Common errors

The following are some of the command errors while creating or using native images:

  • "Error: Could not load file or assembly '...' or one of its dependencies. The system cannot find the file specified. (Exception from HRESULT: 0x80070002)": CrossGen wasn't able to find a particular dependency that it needs. Verify that you have the assembly specified in the error message, and make sure its location is included in /Platform_Assemblies_Paths.
  • CoreCLR unable to initialize: While there are many possible causes of this error, one possibility is a mismatch between System.Private.CoreLib.ni.dll and coreclr.dll (or libcoreclr.so). Make sure they come from the same build or NuGet package.
  • "Unable to load Jit Compiler": Please get a copy of clrjit.dll (or libclrjit.so or libclrjit.dylib, depending on your platform), and place it in the same directory as CrossGen (or use /JITPath option). You can either build clrjit.dll yourself, or get it from Microsoft.NETCore.Jit NuGet package. To avoid possible issues, please use clrjit.dll from the same build as crossgen.exe if possible.