The GCC Internals guide: https://gcc.gnu.org/onlinedocs/gccint/ - very useful reference for looking up, but not a good tutorial starting point
Essential abstractions in GCC course slides, exercises, notes: https://www.cse.iitb.ac.in/grc/gcc-workshop-13/ - a little dated now but still excellent starting point for understanding GCC
RISC-V LLVM patches: https://github.com/lowRISC/riscv-llvm - nice series of patches for understanding how to add a backend to LLVM
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