Compiling for Cortex M3 bare metal

Question

Is there a guide somewhere that describes how to get LLVM to emit a binary for Cortex-M3 that I can massage into running bare metal? I've spent considerable time playing with LLVM on Windows and Ubuntu to no avail. I can get ARM-like assembly out. I can get bit code out, but what I really need is ELF, DWARF, Hobbit, Gandalf or any other Lord of the Rings critter that has a file format specification. Any and all help appreciated! I'm compiling LLVM 3.4 with CLANG on Ubuntu, Windows and/or OS X.

Answer 1

I created a firmware framework - PolyMCU https://github.com/labapart/polymcu - that is based on CMake that support GCC and LLVM. Because it is based on CMake you can build your firmware on Linux/Windows/MacOS. It also uses Newlib and supports Baremetal/CMSIS RTOS (RTX)/FreeRTOS.

The benefit of using PolyMCU is this framework does not add any software layer on top of the libc and the MCU vendor's SDKs. Another benefit is you can easily switch toolchains. I used this feature to get more feedback on my code by testing it with many compilers.

I also wrote a blog where I compared GCC and LLVM build size on ARM Cortex-M: http://labapart.com/blogs/3-the-importance-of-the-toolchain-version-in-embedded-space Interesting results, Clang generated code is not much bigger than GCC on Cortex-M...

Answer 2

The best guide that I know of is here: http://wiki.osdev.org/LLVM_Cross-Compiler. It's mostly about building an LLVM cross-compiler, but it does show a "Usage" section. However, that section specifically shows an example for a Cortex-A processor, but you should be able to get the general idea.

I have created an simple clang bare metal Cortex-M3 "hello world" program, but I don't have it in front of me. IIRC, the only options I needed were -march=thumb -mcpu=cortex-m3 as long as the LLVM compiler backend was built with the ARM thumb backend support (Again, see http://wiki.osdev.org/LLVM_Cross-Compiler). I did, however, need to link with arm-none-eabi-ld from the GCC toolchain here (http://launchpad.net/gcc-arm-embedded), and I believe that is how you can get your ELF binary.

I've since moved on to the D programming language, and I have a simple example using LDC (The LLVM D compiler) here (http://wiki.dlang.org/Extremely_minimal_semihosted_%22Hello_World%22)

So, I believe compiling bare metal ARM Cortex-M3 software with LLVM can be done, but it seems not many people have tried.