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

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Date:   Tue, 27 Jul 2021 12:07:30 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Finn Thain <fthain@...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

Hi Finn,

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

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds