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Date: Wed, 4 Apr 2018 13:58:48 -0700
From: Matthias Kaehlcke <mka@chromium.org>
To: Arnd Bergmann <arnd@arndb.de>
Cc: Peter Zijlstra <peterz@infradead.org>, Ingo Molnar <mingo@kernel.org>,
	Linus Torvalds <torvalds@linux-foundation.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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El Wed, Apr 04, 2018 at 10:33:19PM +0200 Arnd Bergmann ha dit:

> On Wed, Apr 4, 2018 at 9:17 PM, Matthias Kaehlcke <mka@chromium.org> wrote:
> > El Wed, Apr 04, 2018 at 11:30:07AM +0200 Peter Zijlstra ha dit:
> >
> >> On Tue, Apr 03, 2018 at 11:06:58AM -0700, Matthias Kaehlcke wrote:
> >>
> >> > Yes, Chrome OS R67 (currently dev, soon beta) will ship a kernel built
> >> > with Clang for multiple x86 Chromebooks.
> >>
> >> But there are still _known_ miscompilations....
> >
> > Our compiler team is looking into this (missing option
> > -fno-delete-null-pointer-checks)
> 
> Do you know if anyone is looking into __builtin_constant_p()
> optimization as well? We have a lot of uses of this gcc feature
> in the kernel, and if I remember correctly, clang implements
> this by basically always returning false for the cases we
> are interested in.
> 
> In most cases, this is used to implement a fast-path for a helper
> function, so not doing it the same way as gcc just results in
> slower execution, but I assume we also have code that behaves
> differently on clang compared to gcc because of this.

I think I didn't come (knowingly) across that one yet. Could you point
me to an instance that could be used as an example in a bug report?