linux-kernel - Re: [PATCH] arm/arm64: smccc: Use xN for arm64 register constraints with clang

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Message-ID: <20180322224456.GB78232@google.com>
Date:   Thu, 22 Mar 2018 15:44:56 -0700
From:   Matthias Kaehlcke <mka@...omium.org>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     marc.zyngier@....com, Catalin Marinas <catalin.marinas@....com>,
        Robin Murphy <robin.murphy@....com>,
        linux-arm-kernel@...ts.infradead.org,
        LKML <linux-kernel@...r.kernel.org>,
        Christoffer Dall <christoffer.dall@...aro.org>,
        Dave.Martin@....com, Andrey Konovalov <andreyknvl@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Kostya Serebryany <kcc@...gle.com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Greg Hackmann <ghackmann@...gle.com>
Subject: Re: [PATCH] arm/arm64: smccc: Use xN for arm64 register constraints
 with clang

El Thu, Mar 22, 2018 at 10:26:18PM +0000 Nick Desaulniers ha dit:

> Note that a patch in this form has previously been implemented by:
> 
> Andrey Konovalov <andreyknvl@...gle.com>:
> https://gist.github.com/xairy/ee11682ea86044a45c0291c528cd936f
> 
> and another by:
> 
> Greg Hackmann <ghackmann@...gle.com>:
> https://android-review.googlesource.com/c/kernel/common/+/645181
> 
> If you used either as a reference, you may want to credit them with a
> `Suggested-by:` in the commit message.

Not really, but I think I prefer Greg's version over mine and might
use it in a respin if nobody raises objections.

> On Thu, Mar 22, 2018 at 2:28 PM Matthias Kaehlcke <mka@...omium.org> wrote:
> > +#ifndef __clang__
> > +#define __reg__ "r"
> > +#else
> > +#define __reg__ "x"
> > +#endif
> 
> Can this be flipped to #ifdef __clang__ ?  having an if...else where the
> conditional negated is kind of funny.

Sure, my thought was "common case first", but I agree that the negated
condition isn't ideal either.