RV32GC = RV32IMAFDC is implemented
RV64GC = RV64IMAFDC is implemented
RV32E is not supported
Some non-ratified extensions have been merged mainline (they need the flag -menable-experimental-extensions to enable them):
- Bitmanip v0.93
  - b, zba, zbb, zbc, zbe, zbf, zbm, zbp, zbr, zbs, zbt, zbproposedc
- Vector v0.10
  - z, zvamo, zvlsseg
  - Example command for building: clang --target=riscv64-unknown-elf -march=rv64gcv0p10 -menable-experimental-extensions
- Floating-point v0.1
  - zfh
LLVM supports -msave-restore
RISC-V specific command-line options: https://clang.llvm.org/docs/ClangCommandLineReference.html#riscv
LLVM's extension parsing code: https://github.com/llvm/llvm-project/blob/main/clang/lib/Driver/ToolChains/Arch/RISCV.cpp

Newlib

Newlib is a C standard library implementation intended for use on embedded systems.

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