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Date: Thu, 5 Apr 2018 09:20:17 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Matthias Kaehlcke <mka@chromium.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
	Arnd Bergmann <arnd@arndb.de>, Ingo Molnar <mingo@kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Andrew Morton <akpm@linux-foundation.org>,
	James Y Knight <jyknight@google.com>,
	Chandler Carruth <chandlerc@google.com>,
	Stephen Hines <srhines@google.com>,
	Nick Desaulniers <ndesaulniers@google.com>,
	Kees Cook <keescook@google.com>,
	Guenter Roeck <groeck@chromium.org>,
	Greg Hackmann <ghackmann@google.com>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Subject: Re: [GIT PULL] x86/build changes for v4.17
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On Wed, Apr 04, 2018 at 04:31:11PM -0700, Matthias Kaehlcke wrote:

> From some experiments it looks like clang, in difference to gcc, does
> not treat constant values passed as parameters to inline function as
> constants.

Then you're also missing a heap of optimizations in code like
rb_erase_augmented() which is specifically constructed to take advantage
of constant propagation like that.

Other sites where we expect that to happen is __mutex_lock_common(),
__update_load_sum() and a bunch of others. There isn't strictly a bug
here, but not doing that constant propagation will still result in shit
code gen.