Looks like we have to match the attributes in the header with the declarations in the source file. Since this will also affect v4.14 we should find a solution that is acceptable by upstream.

BTW, how do I use r316199? I tried 'USE="llvm-next" sudo emerge llvm clang', but that leaves me with version 5.0_pre305632_p20170806-r10.

We still need to land the llvm-next CL 724372. 
Once it lands, the problem can be reproed as:

1. sudo emerge cross-x86_64-cros-linux-gnu/gcc cross-armv7a-cros-linux-gnueabi/gcc cross-aarch64-cros-linux-gnu/gcc
2. USE="llvm-next" sudo emerge llvm clang cross-armv7a-cros-linux-gnueabi/compiler-rt cross-aarch64-cros-linux-gnu/compiler-rt
3. Modify (Need sudo)  /usr/x86_64-pc-linux-gnu/*/gcc-bin/4.9.x/sysroot_wrapper.hardened files and comment out the line with '-Wno-section'.
4. Build 4.4 kernel.

I sent a patch upstream:

https://patchwork.kernel.org/patch/10033003/

I would prefer to wait a few days to see if there are further comments, but if you want to proceed with the compiler migration before that let me now and we can integrate the patch.

We can proceed with the compiler migration before that if necessary since we have temporarily added -Wno-section to the compiler wrapper. 

It would be cleaner if we just add that workaround for the kernel but it depends on how easy it is to add for you.

Upstream is reluctant about the change as is, it might still take a few days to get to a final solution.

My patch was accepted upstream after all:

https://chromium-review.googlesource.com/#/c/chromiumos/third_party/kernel/+/748688/

cool. thanks

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/third_party/kernel/+/81d77aa0f392efdc582e7decb670357a114d620e

commit 81d77aa0f392efdc582e7decb670357a114d620e
Author: Matthias Kaehlcke <mka@chromium.org>
Date: Wed Nov 01 23:47:18 2017

FROMGIT: sched/sysctl: Fix attributes of some extern declarations

The definition of sysctl_sched_migration_cost, sysctl_sched_nr_migrate
and sysctl_sched_time_avg includes the attribute const_debug. This
attribute is not part of the extern declaration of these variables in
include/linux/sched/sysctl.h, while it is in kernel/sched/sched.h,
and as a result Clang generates warnings like this:

  kernel/sched/sched.h:1618:33: warning: section attribute is specified on redeclared variable [-Wsection]
  extern const_debug unsigned int sysctl_sched_time_avg;
                                ^
  ./include/linux/sched/sysctl.h:42:21: note: previous declaration is here
  extern unsigned int sysctl_sched_time_avg;

The header only declares the variables when CONFIG_SCHED_DEBUG is defined,
therefore it is not necessary to duplicate the definition of const_debug.
Instead we can use the attribute __read_mostly, which is the expansion of
const_debug when CONFIG_SCHED_DEBUG=y is set.

Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
Reviewed-by: Nick Desaulniers <nick.desaulniers@gmail.com>
Cc: Douglas Anderson <dianders@chromium.org>
Cc: Guenter Roeck <groeck@chromium.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Shile Zhang <shile.zhang@nokia.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20171030180816.170850-1-mka@chromium.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

(cherry picked from git.kernel.org tip/tip sched/core
commit a9903f04e0a4ea522d959c2f287cdf0ab029e324)

Conflicts:
	include/linux/sched/sysctl.h

BUG= 778867 
TEST=build for pyro with llvm-next
  no warning of section attribute specified on redeclared variable

Change-Id: I5cb3572036d826feba8133dc0f74d10f3b04c985
Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/748688
Reviewed-by: Manoj Gupta <manojgupta@chromium.org>
Reviewed-by: Guenter Roeck <groeck@chromium.org>

[modify] https://crrev.com/81d77aa0f392efdc582e7decb670357a114d620e/include/linux/sched/sysctl.h

great! thanks for the quick fix.