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Steps-for-compiling-the-kernel-with-Clang.md

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Dependencies

We still defer to binutils for assembling and linking (though we happily accept bug reports from trying LLVM's equivalents).

For cross-compiling, you need to have the cross-target versions of binutils in your $PATH:

$ sudo apt install binutils-aarch64-linux-gnu

Note: The minimal version of Clang required to build the kernel is hard to exactly pin down, due to the large combinations of kernel [LTS] branch, target ISA, kernel configurations. Test your combination, and report issues in our bug tracker. Clang 4 was used to ship a Clang built arm64 kernel on the Google Pixel phone.

Compiling for host

$ make CC=clang HOSTCC=clang

Cross compiling

$ ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- make CC=clang HOSTCC=clang

Note that unlike gcc/binutils, clang ships with support for all backends by default (you can configure them off, but this is considered an antipattern). ARCH= specifies the kernel's notion of target ISA. These can be found in the kernel sources under the arch/ directory. CROSS_COMPILE is a prefix for binutils tools. KBUILD will literally run $ $CROSS_COMPILE-as and $ $CROSS_COMPILE-ld, so verify that you've installed the correct cross version of binutils and they're accessible from your $PATH.

Relevant parts of Kbuild

This is the relevant part of the Kernel's top level Makefile. These define the internal variables that will be used during the build. The tools are prefixed with CROSS_COMPILE if that's set in the environment.

For example, to substitute GNU objcopy for llvm-objcopy, you might invoke your kernel build as:

$ make OBJCOPY=llvm-objcopy

where llvm-objcopy was already accessible via $PATH. Or when cross compiling:

$ ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- make OBJCOPY=llvm-objcopy

You can see the relevant variables being overridden in our CI scripts here.

So the list of relevant variables is CC=clang, AS=clang, LD=ld.lld, AR=llvm-ar, NM=llvm-nm, STRIP=llvm-strip, OBJCOPY=llvm-objcopy, OBJDUMP=llvm-objdump, HOSTCC=clang, HOSTLD=ld.lld, and HOSTAR=llvm-ar. (TODO: It's probably worthwhile to simplify this upstream).

Goals

The eventual goal is to be able to compile the kernel hermetically w/ only Clang and LLVM tools.