linux-kernel - Re: [PATCH] m68k: One function call less in cf_tlb

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Date:   Sun, 14 Jul 2019 15:23:17 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Markus Elfring <Markus.Elfring@....de>
Cc:     Finn Thain <fthain@...egraphics.com.au>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Greg Ungerer <gerg@...ux-m68k.org>,
        Michal Hocko <mhocko@...e.com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        linux-m68k <linux-m68k@...ts.linux-m68k.org>,
        LKML <linux-kernel@...r.kernel.org>,
        kernel-janitors@...r.kernel.org
Subject: Re: [PATCH] m68k: One function call less in cf_tlb_miss()

Hi Markus,

On Sat, Jul 6, 2019 at 1:57 AM Finn Thain <fthain@...egraphics.com.au> wrote:
> On Fri, 5 Jul 2019, Markus Elfring wrote:
> > From: Markus Elfring <elfring@...rs.sourceforge.net>
> > Date: Fri, 5 Jul 2019 17:11:37 +0200
> >
> > Avoid an extra function call
> > by using a ternary operator instead of a conditional statement for a
> > setting selection.

Have you looked at the actual assembler output generated by the compiler?

> > This issue was detected by using the Coccinelle software.
> >
> > Signed-off-by: Markus Elfring <elfring@...rs.sourceforge.net>
> > ---
> >  arch/m68k/mm/mcfmmu.c | 10 ++++------
> >  1 file changed, 4 insertions(+), 6 deletions(-)
> >
> > diff --git a/arch/m68k/mm/mcfmmu.c b/arch/m68k/mm/mcfmmu.c
> > index 6cb1e41d58d0..02fc0778028e 100644
> > --- a/arch/m68k/mm/mcfmmu.c
> > +++ b/arch/m68k/mm/mcfmmu.c
> > @@ -146,12 +146,10 @@ int cf_tlb_miss(struct pt_regs *regs, int write, int dtlb, int extension_word)
> >
> >       mmu_write(MMUDR, (pte_val(*pte) & PAGE_MASK) |
> >               ((pte->pte) & CF_PAGE_MMUDR_MASK) | MMUDR_SZ_8KB | MMUDR_X);
> > -
> > -     if (dtlb)
> > -             mmu_write(MMUOR, MMUOR_ACC | MMUOR_UAA);
> > -     else
> > -             mmu_write(MMUOR, MMUOR_ITLB | MMUOR_ACC | MMUOR_UAA);
> > -
> > +     mmu_write(MMUOR,
> > +               dtlb
> > +               ? MMUOR_ACC | MMUOR_UAA
> > +               : MMUOR_ITLB | MMUOR_ACC | MMUOR_UAA);

While the ternary operator can be useful for short expressions, it can
also lead to hard-to-read code.  IMHO the latter is the case here
(hint: the expression needs 3 lines).

> If you are trying to avoid redundancy, why not finish the job?
>
> +     mmu_write(MMUOR, (dtlb ? 0 : MMUOR_ITLB) | MMUOR_ACC | MMUOR_UAA);

Thanks Finn, much better!

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