compiling vanilla kernel with Debian clang-13

[Ryutaroh Matsumoto]

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