5.10. GHC Backends

GHC supports multiple backend code generators. This is the part of the compiler responsible for taking the last intermediate representation that GHC uses (a form called Cmm that is a simple, C like language) and compiling it to executable code. The backends that GHC support are described below.

5.10.1. Native Code Generator (-fasm)

The default backend for GHC. It is a native code generator, compiling Cmm all the way to assembly code. It is the fastest backend and generally produces good performance code. It has the best support for compiling shared libraries. Select it with the -fasm flag.

5.10.2. LLVM Code Generator (-fllvm)

This is an alternative backend that uses the LLVM compiler to produce executable code. It generally produces code with performance as good as the native code generator but for some cases can produce much faster code. This is especially true for numeric, array heavy code using packages like vector. The penalty is a significant increase in compilation times. Select the LLVM backend with the -fllvm flag.

You must install and have LLVM available on your PATH for the LLVM code generator to work. Specifically GHC needs to be able to call the opt and llc tools. Secondly, if you are running Mac OS X with LLVM 3.0 or greater then you also need the Clang C compiler compiler available on your PATH.


Note that this GHC release expects an LLVM version in the 13 up to 16 (not inclusive) release series.

To install LLVM and Clang:

  • Linux: Use your package management tool.
  • Mac OS X: Clang is included by default on recent OS X machines when Xcode is installed (from 10.6 and later). LLVM is not included. In order to use the LLVM based code generator, you should install the Homebrew package manager for OS X. Alternatively you can download binaries for LLVM and Clang from here.
  • Windows: You should download binaries for LLVM and clang from here.

5.10.3. C Code Generator (-fvia-C)


Use the C code generator. Only supposed in unregisterised GHC builds.

This is the oldest code generator in GHC and is generally not included any more having been deprecated around GHC 7.0. Select it with the -fvia-C flag.

The C code generator is only supported when GHC is built in unregisterised mode, a mode where GHC produces “portable” C code as output to facilitate porting GHC itself to a new platform. This mode produces much slower code though so it’s unlikely your version of GHC was built this way. If it has then the native code generator probably won’t be available. You can check this information by calling ghc --info (see --info).

5.10.4. JavaScript Code Generator

This is an alternative code generator included in GHC 9.6 and above. It generates ECMA-262 compliant JavaScript and is included as a technical preview. At time of writing, it is being actively developed but is not suitable for serious projects and production environments. The JavaScript backend is not distributed in the GHC bindist and requires a manual build. See building the JavaScript backend page on the GHC wiki for build instructions.

A JavaScript cross-compiling GHC produces an executable script, and a directory of the same name suffixed with .jsexe. For example, compiling a file named Foo.hs will produce an executable script Foo and a Foo.jsexe directory. The script is a thin wrapper that calls Node.js on the payload of the compiled Haskell code and can be run in the usual way, e.g., ./Foo, as long as node is in your environment . The actual payload is in <ModuleName>.jsexe/all.js, for example Foo.jsexe/all.js. This file is the Haskell program cross-compiled to JavaScript concrete syntax and can be wrapped in a <script> HTML tag. For a breakdown of the rest of the build artifacts see the compiler output section in the wiki.

5.10.5. Unregisterised compilation

The term “unregisterised” really means “compile via vanilla C”, disabling some of the platform-specific tricks that GHC normally uses to make programs go faster. When compiling unregisterised, GHC simply generates a C file which is compiled via gcc.

When GHC is built in unregisterised mode only the C code generator is available. Neither the LLVM nor native code generator can be used by an unregisterised build.

Unregisterised compilation can be useful when porting GHC to a new machine, since it reduces the prerequisite tools to gcc, as, and ld and nothing more, and furthermore the amount of platform-specific code that needs to be written in order to get unregisterised compilation going is usually fairly small.

Unregisterised compilation cannot be selected at compile-time; you have to build GHC with the appropriate options set. Consult the GHC Building Guide for details.

You can check if your GHC is unregisterised by calling ghc --print-unregisterised (see --print-unregisterised) or ghc --info (see --info).