linux-kernel - Re: [PATCH] sched: Work around undefined behavior in sched class checking

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Date:   Wed, 5 May 2021 10:08:03 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Andi Kleen <ak@...ux.intel.com>
Cc:     Florian Weimer <fweimer@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Andi Kleen <andi@...stfloor.org>,
        linux-toolchains@...r.kernel.org
Subject: Re: [PATCH] sched: Work around undefined behavior in sched class checking

On Wed, May 5, 2021 at 9:49 AM Andi Kleen <ak@...ux.intel.com> wrote:
>
>
> On 5/5/2021 9:41 AM, Nick Desaulniers wrote:
> > On Wed, May 5, 2021 at 7:39 AM Andi Kleen <ak@...ux.intel.com> wrote:
> >>>   Would you please provide a
> >>> minimal test case?
> >> You can only reproduce it with a LTO build because it needs knowledge
> >> between different translation units for this specific case.
> >>
> >> But gcc will totally do the optimization even without LTO if it can
> >> prove the same inside a single TU.
> > It would be helpful to isolate a test case that doesn't rely on LTO,
> > if possible.
>
> Like I wrote earlier we used to see it all the time in __pa_symbol
> before it used RELOC_HIDE. I bet if you make RELOC_HIDE a nop you'll see
> multiple instances.
>
> But not sure why you want a test case?

In general,
when making a feature request to a compiler vendor, having a
digestible snippet of code that demonstrates the problem goes a long
way, much further than "clone this branch of my fork of this project
and do a build and something goes wrong somewhere."  We're too busy to
do that, please take the time to isolate it before making such
requests.

-- 
Thanks,
~Nick Desaulniers