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.
-fasm flag.
This is an alternative backend that uses the
LLVM compiler to produce
executable code. It generally produces code as 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. Currently LLVM 2.8 and
later are supported.
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 optand 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.
To install LLVM and Clang:
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.
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.
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 build in unregisterised mode only the LLVM and C code generators will be available. The native code generator won't be. LLVM usually offers a substantial performance benefit over the C backend in unregisterised mode.
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 --info.