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

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Date:   Wed, 4 Oct 2023 22:06:29 +1000
From:   Greg Ungerer <gregungerer@...tnet.com.au>
To:     Matthew Wilcox <willy@...radead.org>
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 4/10/23 06:07, Matthew Wilcox wrote:
> 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?

Yes, I reckon so.


>>> 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!

No worries. Sorry I didn't notice it earlier, but glad it is sorted now.

Regards
Greg