linux-kernel - Re: [PATCH 2/2] m68k: introduce a virtual m68k machine

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Date:   Thu, 29 Apr 2021 09:23:15 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Josh Juran <jjuran@...il.com>
Cc:     Laurent Vivier <laurent@...ier.eu>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-m68k <linux-m68k@...ts.linux-m68k.org>,
        linux-rtc@...r.kernel.org, Alessandro Zummo <a.zummo@...ertech.it>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>
Subject: Re: [PATCH 2/2] m68k: introduce a virtual m68k machine

Hi Josh,

On Thu, Apr 29, 2021 at 1:06 AM Josh Juran <jjuran@...il.com> wrote:
> On Apr 28, 2021, at 8:04 AM, Geert Uytterhoeven <geert@...ux-m68k.org> wrote:
> > This can be simplified by shifting irq_pending instead of irq_bit:
> >
> >    do {
> >            if (irq_pending & 1)
> >                    generic_handle_irq(irq_num);
> >
> >            ++irq_num;
> >            irq_pending >>= 1;
> >    } while (irq_pending);
> >
> > Unfortunately m68k doesn't have a single-instruction __ffs().
>
> The 68000 and 68010 don't, but couldn't the 68020's BFFFO do the job?

I looked at the code generated by gcc for __builtin_ffs(), and while
it did use BFFFO, it needs several other instructions.

The same can be seen in arch/m68k/include/asm/bitops.h:

    static inline int ffs(int x)
    {
            int cnt;

            __asm__ ("bfffo %1{#0:#0},%0"
                    : "=d" (cnt)
                    : "dm" (x & -x));
            return 32 - cnt;
    }


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