Re: [GIT PULL] x86/build changes for v4.17

From: James Y Knight <jyknight@google.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: mka@chromium.org, Linus Torvalds <torvalds@linux-foundation.org>,
	arnd@arndb.de, Ingo Molnar <mingo@kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	tglx@linutronix.de, Andrew Morton <akpm@linux-foundation.org>,
	Chandler Carruth <chandlerc@google.com>,
	Stephen Hines <srhines@google.com>,
	Nick Desaulniers <ndesaulniers@google.com>,
	Kees Cook <keescook@google.com>,
	groeck@chromium.org, Greg Hackmann <ghackmann@google.com>,
	gregkh@linuxfoundation.org
Subject: Re: [GIT PULL] x86/build changes for v4.17
Date: Thu, 05 Apr 2018 17:46:58 +0000	[thread overview]
Message-ID: <CAA2zVHraWtmmW1yFvVH+kUbih5wxiDLuM=wArnQanrWN9NF=Xw@mail.gmail.com> (raw)
In-Reply-To: <20180405072017.GN4043@hirez.programming.kicks-ass.net>

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On Thu, Apr 5, 2018 at 3:20 AM Peter Zijlstra <peterz@infradead.org> wrote:

> 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.
>

I think maybe you're confused; those functions do not appear to use
__builtin_constant_p, which is the issue at hand. Clang's optimizer is of
course not a complete joke...it can perfectly well optimize functions after
inlining in order to not generate "shit code gen".

GCC, however, mixes up the concept of a C "constant expression" with the
results of running optimization passes such as inlining for its
definition/implementation of __builtin_constant_p. Clang does not, and
quite likely will not ever, do that.

That said, I do believe there are ongoing discussions as to how to best
provide a useful alternative which is less semantically strange, and not
too difficult for to conditionally adopt for a gcc/clang-compatible
codebase such as the kernel.

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