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Document how to use clang in README

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Fixes: https://github.com/littlekernel/lk/issues/426
This commit is contained in:
Alexander Richardson
2025-01-08 10:33:34 -08:00
committed by Travis Geiselbrecht
parent f9a5a16db5
commit e7b71623c4
1 changed files with 17 additions and 0 deletions
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README.md
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@@ -43,3 +43,20 @@ Note: for ubuntu x86-64
sudo apt-get install gcc-aarch64-linux-gnu
or fetch a prebuilt toolchain from
https://newos.org/toolchains/aarch64-elf-14.1.0-Linux-x86_64.tar.xz
### Building with LLVM-based toolchains
To build LK with a LLVM-based toolchain you will have to manually specify the compiler and linker in the environemnt.
Unlike GCC clang is a cross-compiler by default, so the target needs to be specified as part of the CC/CXX/CPP variables.
For example, assuming LLVM is in `/opt/llvm/bin/`, the following command will work to build for 64-bit RISC-V:
```
gmake qemu-virt-riscv64-test 'CC=/opt/llvm/bin/clang --target=riscv64-unknown-elf' 'CPP=/opt/llvm/bin/clang-cpp --target=riscv64-unknown-elf' 'CXX=/opt/llvm/bin/clang++ --target=riscv64-unknown-elf' 'LD=/opt/llvm/bin/ld.lld' TOOLCHAIN_PREFIX=/opt/llvm/bin/llvm- CPPFILT=/opt/llvm/bin/llvm-cxxfilt
```
TOOLCHAIN_PREFIX can be set to use the LLVM binutils, but due to the different naming of `llvm-cxxfilt` vs `c++filt` it needs to be set explicitly.
To build for AArch64 the command looks similar, just with a different `--target=` flag.
```
gmake qemu-virt-arm64-test 'CC=/opt/llvm/bin/clang --target=aarch64-unknown-elf' 'CPP=/opt/llvm/bin/clang-cpp --target=aarch64-unknown-elf' 'CXX=/opt/llvm/bin/clang++ --target=aarch64-unknown-elf' 'LD=/opt/llvm/bin/ld.lld' TOOLCHAIN_PREFIX=/opt/llvm/bin/llvm- CPPFILT=/opt/llvm/bin/llvm-cxxfilt
```