clang: objtool: native_machine_emergency_restart() falls through to next function
From: Matthias Kaehlcke
Date: Fri Jun 15 2018 - 17:14:49 EST
Hi Josh,
with your work on objtool and the upcoming implementation of
-fno-delete-null-pointer-checks (https://reviews.llvm.org/D47894,
https://reviews.llvm.org/D47895) in clang most objtool warnings
for clang builds will be fixed.
However even with -fno-delete-null-pointer-checks we currently still
get a warning about native_machine_emergency_restart():
arch/x86/kernel/reboot.o: warning: objtool:
native_machine_emergency_restart()
falls through to next function machine_power_off()
This only occurs when building with -Oz.
One of our compiler engineers looked into this, LLVM optimizes the
return statement away since the function has an endless loop, and
actually never returns.
Is there a way to tell objtool that the function is not expected to
return (I tried the attribute __noreturn, but it doesn't make a
difference), or do we have to tell clang to restrain from optimizing
returns out?
To repro:
git checkout v4.16
make CC=clang defconfig
echo "CONFIG_CC_OPTIMIZE_FOR_SIZE=y" >> .config
make CC=clang arch/x86/kernel/reboot.o
Thanks
Matthias