Llvm-mos
From F256 Foenix
The llvm-mos project is a suite of development tools aimed at 65xx processors. It comes with optimizing C and C++ compilers that are mostly compatible with the C99 and C++11 standards, allowing for modern programming styles.
Foenix support is currently not integrated into the main repository (see discussion at the official llvm-mos GitHub), but there is fork available with basic support for the F256K.