Re: [PATCH 09/17] m68k: Implement xor_unlock_is_negative_byte

Greg Ungerer
2023-10-04T12:36:45Z

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 # ,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