clang: objtool: native_machine_emergency_restart() falls through to next function

[Matthias Kaehlcke <mka@chromium.org>]

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