Compiling the kernel with LLVM=1 fails for ARCH=arm CROSS_COMPILE=armv6l-unknown-linux-gnueabihf- kernels

[Elias Naur]

Describe the bug

The issue seems to be that https://github.com/raspberrypi/linux/blob/rpi-5.15.y/arch/arm/lib/memcmp_rpi.S (and perhaps others) are not compatible with the clang assembler.

Steps to reproduce the behaviour

On a nix system with flakes enabled,

$ nix build github:eliasnaur/rpikernel-clang-issue
error: builder for '/nix/store/3zc9nww3crck85wir5fs78zfyfv1nm5q-Raspberry-Pi-Linux-kernel.drv' failed with exit code 2;
last 10 log lines:
>         ldrmib DAT5, [S_2]
>         ^
> <instantiation>:11:9: note: while in macro instantiation
>         memcmp_trailing_15bytes unaligned
>         ^
> arch/arm/lib/memcmp_rpi.S:265:6: note: while in macro instantiation
> 140: memcmp_short_inner_loop 1
>      ^
> make[1]: *** [scripts/Makefile.build:391: arch/arm/lib/memcmp_rpi.o] Error 1
> make: *** [Makefile:1900: arch/arm/lib] Error 2
For full logs, run 'nix log /nix/store/3zc9nww3crck85wir5fs78zfyfv1nm5q-Raspberry-Pi-Linux-kernel.drv'.

The relevant configuration and building scripts are in the repository. I'm using clang 14.

Device (s)

Raspberry Pi Zero, Raspberry Pi Zero W / WH

System

The issue is during building of custom kernel, not at runtime.

Logs

No response

Additional context

I'm building a custom kernel with LLVM because I have problems producing deterministic output from a gcc build.

The issue can be worked around by disabling BCM2835_FAST_MEMCPY in the kernel config.

[Phil Elwell]

Are there no further error messages? The ones you include are showing where the problem is but not what.

[Elias Naur]

Here's the full nix log output:

log.txt

[FilthyPitDog]

for my clang kernel to compile correctly I needed to run make like

make CC=clang LD=ld.lld NM=llvm-nm AR=llvm-ar LLVM_IAS=1 -jX

[Elias Naur]

Those arguments give me the same result, and indeed LLVM=1 should effectively be equivalent. I'm guessing you're compiling your kernel for arm64? I believe BCM2835_FAST_MEMCPY is only relevant for 32-bit kernels.

[iucoen]

Try LLVM_IAS=0

clang doesn't like the fast_memcpy assembly files for some reason.