Mono LLVM

Mono support two compilation engines, a fast, JIT-friendly compilation engine which does not generate very fast code, and a slower compilation engine based on the LLVM optimizing compiler that produces superior code.

For example the SciMark score goes from 609 to 851.

This extra performance comes at a cost: It consumes more time and more memory to JIT compile using LLVM than using Mono’s built-in JIT, so it is not a solution for everyone.

Compiling LLVM support for Mono 5.18 and newer

Since Mono 5.18 the build system supports building with LLVM support out of the box by providing --enable-llvm to autogen.sh/configure when building from source.

Also note that most packages are shipped with an LLVM enabled Mono runtime.

Compiling LLVM support for older versions of Mono

You need to install both LLVM and Mono from git, as follows:

Get and install Mono and LLVM like this:

Alternatively, you can try our LLVM branch which removes some restrictions so more methods can be compiled with LLVM:

git clone https://github.com/mono/llvm.git
cd llvm
./configure --prefix=/mono --enable-optimized --enable-targets="x86 x86_64"
make && make install

Use ‘git checkout mono’ when compiling against mono HEAD.

Then get Mono as usual:

git clone https://github.com/mono/mono
cd mono

Since I am using the non-standard prefix for the LLVM installation (/mono), we need to make sure that we add /mono/bin to the PATH so that the configure script can detect the llvm installation:

export MONO_USE_LLVM=1
PATH=/mono/bin:$PATH

The next step is to run the configure script, the one flag that matters here is one of:

• --enable-llvm
• --enable-loadedllvm

You can either link LLVM into your mono executable, or you can split it in a separate shared library (for example for Linux distributions that might want to ship both a slim Mono by default).

NOTE, --enable-loadedllvm configuration is not supported for Windows hosted builds.

./autogen.sh --prefix=/mono --enable-llvm=yes
make && make install

Now you have an LLVM-powered Mono. By default, the mono JIT is used to compile code, to make the runtime use LLVM, pass the ‘–llvm’ command line option, or add it to the MONO_ENV_OPTIONS environment variable.

LLVM generates better code at runtime, but also takes longer to execute. This is why you need to explicitly tell Mono’s runtime to use the LLVM backend. This is achieved by either setting the environment variable MONO_USE_LLVM or by passing the –llvm command line option to Mono.

Limitations

LLVM is not able to support some of the features that Mono needs, so in those cases the JIT compiler will still fall back to Mono’s JIT engine (methods that contain try/catch clauses or methods that do interface calls).

When building LLVM for regular Windows builds (not building Windows hosted cross compilers), only x64 Visual Studio builds support LLVM powered Mono runtime. Mono runtime build using mingw only supports LLVM for Windows hosted cross compilers. For more details around regular Mono Windows LLVM builds (not building Windows hosted cross compilers), see Compiling Mono On Windows