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

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Message-ID: <20180405072017.GN4043@hirez.programming.kicks-ass.net>
Date:   Thu, 5 Apr 2018 09:20:17 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Matthias Kaehlcke <mka@...omium.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Arnd Bergmann <arnd@...db.de>, Ingo Molnar <mingo@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        James Y Knight <jyknight@...gle.com>,
        Chandler Carruth <chandlerc@...gle.com>,
        Stephen Hines <srhines@...gle.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Kees Cook <keescook@...gle.com>,
        Guenter Roeck <groeck@...omium.org>,
        Greg Hackmann <ghackmann@...gle.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [GIT PULL] x86/build changes for v4.17

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.