linux-kernel - Re: [PATCH] x86/vdso: include generic _

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Message-ID: <20190318204435.GH112750@google.com>
Date:   Mon, 18 Mar 2019 13:44:35 -0700
From:   Matthias Kaehlcke <mka@...omium.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H . Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        LKML <linux-kernel@...r.kernel.org>,
        Manoj Gupta <manojgupta@...omium.org>,
        Tiancong Wang <tcwang@...omium.org>,
        Stephen Hines <srhines@...gle.com>,
        clang-built-linux@...glegroups.com,
        Masahiro Yamada <yamada.masahiro@...ionext.com>
Subject: Re: [PATCH] x86/vdso: include generic __lshrdi3 in 32-bit vDSO

Hi Peter,

On Mon, Mar 18, 2019 at 06:16:04PM +0100, Peter Zijlstra wrote:
> On Mon, Mar 18, 2019 at 10:09:37AM -0700, Nick Desaulniers wrote:
> > On Fri, Mar 15, 2019 at 3:29 PM Matthias Kaehlcke <mka@...omium.org> wrote:
> > > On Fri, Mar 15, 2019 at 02:31:09PM -0700, 'Nick Desaulniers' via Clang Built Linux wrote:
> > > > Is there a config I can set to reproduce this, in order to help
> > > > test?
> > >
> > > I encountered it with a Chrome OS specific configuration, but a
> > > defconfig should do.
> > 
> > defconfig selects CONFIG_OPTIMIZE_FOR_PERFORMANCE=y.  I needed to
> > select CONFIG_CC_OPTIMIZE_FOR_SIZE=y to repro.  My CI is simply
> > running defconfigs, which is why I didn't catch this.
> 
> I suspect the right fix is to make clang use -Os and abolish that -Oz
> madness.

You have a point, enabling -Oz (aggressive optimization for size)
doesn't seem a good choice for the entire kernel.

I'll send a patch to change it (back) to -Os.

Matthias