linux-kernel - Re: [PATCH 09/17] m68k: Implement xor_unlock_is_negative

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Date:   Tue, 3 Oct 2023 21:07:48 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Greg Ungerer <gregungerer@...tnet.com.au>
Cc:     David Laight <David.Laight@...lab.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        "torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
        Nicholas Piggin <npiggin@...il.com>
Subject: Re: [PATCH 09/17] m68k: Implement xor_unlock_is_negative_byte

On Wed, Oct 04, 2023 at 12:14:10AM +1000, Greg Ungerer wrote:
> On 3/10/23 06:07, Matthew Wilcox wrote:
> > 00000918 <folio_unlock>:
> >       918:       206f 0004       moveal %sp@(4),%a0
> >       91c:       7001            moveq #1,%d0
> >       91e:       b190            eorl %d0,%a0@
> >       920:       2010            movel %a0@,%d0
> >       922:       4a00            tstb %d0
> >       924:       6a0a            bpls 930 <folio_unlock+0x18>
> >       926:       42a7            clrl %sp@-
> >       928:       2f08            movel %a0,%sp@-
> >       92a:       4eba fafa       jsr %pc@(426 <folio_wake_bit>)
> >       92e:       508f            addql #8,%sp
> >       930:       4e75            rts

fwiw, here's what folio_unlock looks like today without any of my
patches:

00000746 <folio_unlock>:
     746:       206f 0004       moveal %sp@(4),%a0
     74a:       43e8 0003       lea %a0@(3),%a1
     74e:       0891 0000       bclr #0,%a1@
     752:       2010            movel %a0@,%d0
     754:       4a00            tstb %d0
     756:       6a0a            bpls 762 <folio_unlock+0x1c>
     758:       42a7            clrl %sp@-
     75a:       2f08            movel %a0,%sp@-
     75c:       4eba fcc8       jsr %pc@(426 <folio_wake_bit>)
     760:       508f            addql #8,%sp
     762:       4e75            rts

Same number of instructions, but today's code has slightly longer insns,
so I'm tempted to take the win?

> > We could use eori instead of eorl, at least according to table 3-9 on
> > page 3-8:
> > 
> > EOR Dy,<ea>x L Source ^ Destination → Destination ISA_A
> > EORI #<data>,Dx L Immediate Data ^ Destination → Destination ISA_A

Oh.  I misread.  It only does EORI to a data register; it can't do EORI
to an address.

> 400413e6 <folio_unlock>:
> 400413e6:       206f 0004       moveal %sp@(4),%a0
> 400413ea:       2010            movel %a0@,%d0
> 400413ec:       0a80 0000 0001  eoril #1,%d0
> 400413f2:       2080            movel %d0,%a0@
> 400413f4:       2010            movel %a0@,%d0
> 400413f6:       4a00            tstb %d0
> 400413f8:       6c0a            bges 40041404 <folio_unlock+0x1e>
> 400413fa:       42a7            clrl %sp@-
> 400413fc:       2f08            movel %a0,%sp@-
> 400413fe:       4eba ff30       jsr %pc@(40041330 <folio_wake_bit>)
> 40041402:       508f            addql #8,%sp
> 40041404:       4e75            rts
> 
> But that is still worse anyway.

Yup.  Looks like the version I posted actually does the best!  I'll
munge that into the patch series and repost.  Thanks for your help!