Add RSB for LLVM targeting RTEMS

[Joel Sherrill]

LLVM support for RTEMS has been included in the tools shipped by Gaisler (e.g. their RCC). RTEMS itself should now have all modifications needed to not be tied to GCC. This project is to add an RSB recipe to build an LLVM toolchain.

Information from Daniel Hellstrom:

RCC-1.3 has not converted into using the RSB. However I believe we have upstreamed every change to the RTEMS kernel tree which was related to LLVM/Clang support for SPARC. The RCC-1.3 section 2.2.4 includes some information what is compiled with GCC in our LLVM toolchain:

• ​https://www.gaisler.com/anonftp/rcc/rcc-1.3/1.3.1/rcc-1.3.1.pdf

• ​https://www.gaisler.com/anonftp/rcc/rcc-1.3/1.3.1/sparc-rtems-5-llvm-8.0.0-1.3.1-linux.txz

For NOEL-V/RISC-V we only provide GCC toolchain today.

After building the tools with the RSB, those tools must be used to build at least one BSP from an architecture which is not currently supported by LLVM. ARM, RISC-V, or x86_64 are likely first architectures to consider with testing on a BSP which can run on a simulator. Test results are necessary for GCC (baseline) and LLVM (objective).

There are multiple tickets related to llvm. Finishing this project may close one or more of them. Any work on this ticket should see if the others need updating or closing.

Possible Mentors: Gedare Bloom, Daniel Hellstrom, Chris Johns, Joel Sherrill
Skills: C
Difficulty: Medium

[Hesham Almatary]

To completely remove the reliance on GCC, this should also include building:

• newlib
• compiler-rt
• libc++

Clang might still need a driver for linking RTEMS targets