R Extensions Support

The GraalVM R runtime can run R extensions in two modes:

native: the native machine code is run directly on your CPU, the same way GNU R runs R extensions.
llvm: if the LLVM bitcode is available, it can be interpreted by the LLVM interpreter shipped with GraalVM.

The native mode is better suited for code that does not extensively interact with the R API, for example, plain C or Fortran numerical computations working on primitive arrays. The llvm mode provides significantly better performance for extensions that frequently call between R and the C/C++ code, because GraalVM’s LLVM runtime is also partially evaluated like the R code. Both can be inlined and optimized as one compilation unit. Moreover, GraalVM’s LLVM runtime is supported by GraalVM tools which allows users to, for instance, debug R and C code together.

In one GraalVM R process, any R package can be loaded in either mode. That is, GraalVM’s R runtime supports mixing packages loaded in the native mode with packages loaded in the llvm mode in one process.

Generating LLVM Bitcode

As of version 19.3.0, the GraalVM R runtime is configured to use the LLVM toolchain to compile R packages’ native code. This toolchain produces standard executable binaries for a given system, but it also embeds the corresponding LLVM bitcode into them. The binaries produced by the LLVM toolchain can be loaded in both modes: native or llvm.

The GraalVM R runtime can be reconfigured to use your system default compilers when installing R packages by running:

# use local installation of GGC:
R -e 'fastr.setToolchain("native")'
# to revert back to using the GraalVM's LLVM toolchain:
R -e 'fastr.setToolchain("llvm")'

Using the system default compilers may be more reliable, but you lose the ability to load the R packages built with the LLVM toolchain in the llvm mode, because they will not contain the embedded bitcode. Moreover, mixing packages built by the local system default compilers and packages built by the LLVM toolchain in one R process may cause linking issues.

Fortran Compiler

As of version 20.1.0, the GraalVM R runtime uses gfortran as the default Fortran compiler when installing R packages. Since gfortran cannot produce bitcode, packages that contain Fortran code will not work in the llvm mode.

The GraalVM R runtime contains the F2C tool, which can convert Fortran code to C and then compile it with the LLVM toolchain. Users can configure GraalVM’s R runtime to use this tool by editing the configuration file R_HOME/etc/Makeconf, variable FC.

Choosing the Running Mode

Starting with version 19.3.0, GraalVM’s R runtime uses the following defaults:

native mode to load the packages
llvm toolchain to build their sources

To enable the llvm mode for loading the packages, use --R.BackEnd=llvm. You can also enable each mode selectively for the given R packages by using:

--R.BackEndLLVM=package1,package2
--R.BackEndNative=package1,package2

Moreover, you can configure which packages will be always run in the native mode in file R_HOME/etc/native-packages. GraalVM’s R runtime comes with a default configuration that covers some popular R packages that are known to not work yet in the llvm mode.