Hi, this is a clang related notes on compiling vanilla Linux kernel 5.15 by Debian clang-13 (from sid/experimental).
In short, the combination of Linux 5.15 + clang 13 seems better than previous ones, e.g. Linux 5.13 + clang12, on arm64. (1) Clang-related Kconfig items seem to work fine with my use case, in particular, I have not seen any problem with the following config items: CONFIG_RELR=y CONFIG_SHADOW_CALL_STACK=y CONFIG_LTO_CLANG_FULL=y CONFIG_CFI_CLANG=y CONFIG_CFI_CLANG_SHADOW=y CONFIG_TRIM_UNUSED_KSYMS=y CONFIG_INIT_STACK_ALL_ZERO=y (or CONFIG_INIT_STACK_ALL_PATTERN=y) (2) "-mcpu=native" seems to work. With 5.13, it did not build a bootable kernel. (3) Polyhedral optimization seems to work, as proposed at https://groups.google.com/g/clang-built-linux/c/Pg7CXssRuYo/m/eHJkNgjTDgAJ (4) ld.lld-13 uses less memory than ld.lld-12. ld.lld-12 used 7GiB with CONFIG_LTO_CLANG_FULL=y, but ld.lld-13 uses "only" 4GiB. I used the following script to compile a kernel. Clang on Debian can be unstalled by apt-get install clang/exerimental llvm/experimental lld/experimental. clang in sid depends on clang-11 for now. (5) CONFIG_XFS_FS=y or CONFIG_OVERLAY_FS=y causes runtime error in clang. cd /var/tmp arch=`dpkg --print-architecture` if [ $arch = amd64 ]; then variant=amd64_none_amd64 march='-march=native' elif [ $arch = arm64 ]; then variant=arm64_none_arm64 march='-mcpu=native' elif [ $arch = armhf ]; then variant=armhf_none_armmp-lpae march='-mcpu=native' else echo "Please find a suitable config of $arch." exit 1 fi KVER=5.15.2 exec </dev/null >build-log-${KVER}.txt 2>&1 set -xe wget -T 10 -c https://cdn.kernel.org/pub/linux/kernel/v5.x/linux-${KVER}.tar.xz tar Jxf linux-${KVER}.tar.xz cd linux-${KVER} # install linux-config-5.15 from sid or experimental xzcat /usr/src/linux-config-5.15/config.${variant}.xz >.config cat >>.config <<EOF CONFIG_LTO_CLANG_FULL=y CONFIG_TRIM_UNUSED_KSYMS=y CONFIG_CFI_CLANG=y CONFIG_CFI_CLANG_SHADOW=y CONFIG_SHADOW_CALL_STACK=y CONFIG_INIT_STACK_ALL_ZERO=y CONFIG_IRQ_TIME_ACCOUNTING=y CONFIG_LD_DEAD_CODE_DATA_ELIMINATION=y CONFIG_XFS_FS=n CONFIG_DEBUG_INFO=n CONFIG_OVERLAY_FS=n EOF yes '' | chrt --batch 0 nice -19 /usr/bin/time -v make -j `nproc` V=1 LLVM=1 LLVM_IAS=1 \ KCFLAGS="-pipe ${march} -fintegrated-cc1 -mllvm -polly -mllvm -polly-ast-use-context -mllvm -polly-invariant-load-hoisting -mllvm -polly-opt-fusion=max -mllvm -polly-run-inliner -mllvm -polly-vectorizer=stripmine -mllvm -polly-run-dce" \ LOCALVERSION=-clang13ltopolly \ bindeb-pkg Best regards, Ryutaroh Matsumoto
