linux-kernel - Re: [PATCH 1/5] nubus: Simplify check in remove callback

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Message-ID: <194b351c-bae-27a-1875-9ad3c47292e@linux-m68k.org>
Date:   Tue, 27 Jul 2021 20:19:35 +1000 (AEST)
From:   Finn Thain <fthain@...ux-m68k.org>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
cc:     Uwe Kleine-König <u.kleine-koenig@...gutronix.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Sascha Hauer <kernel@...gutronix.de>,
        linux-m68k <linux-m68k@...ts.linux-m68k.org>
Subject: Re: [PATCH 1/5] nubus: Simplify check in remove callback

On Tue, 27 Jul 2021, Geert Uytterhoeven wrote:

> On Tue, Jul 27, 2021 at 11:50 AM Finn Thain <fthain@...ux-m68k.org> wrote:
> > On Tue, 27 Jul 2021, Uwe Kleine-König wrote:
> > > Apart from that, the compiler might already assume dev->driver being 
> > > non-NULL after to_nubus_driver(dev->driver) was called.
> >
> > I don't understand how a compiler can make that assumption. But then, 
> > I don't know why compilers do a lot of the things they do...
> 
> It is one of those recent optimizations people have been complaining 
> about.  Once you have dereferenced a pointer, compilers may remove all 
> further NULL-checks, assuming they can't happen, as the code would have 
> crashed anyway before due to the dereference.

But that's the point -- there is no dereference, just pointer arithmetic.

> Good luck running on bare metal with RAM at zero ;-)

Quite.