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

Greg Ungerer
2023-10-03T14:14:29Z

On 3/10/23 06:07, Matthew Wilcox wrote:
> On Wed, Sep 20, 2023 at 05:22:33PM +1000, Greg Ungerer wrote:
>> On 20/9/23 01:14, Matthew Wilcox wrote:
>>> I have a 68020 book; what I don't have is a Coldfire manual.
>>
>> You can find it here: https://www.nxp.com/docs/en/reference-manual/CFPRM.pdf
>
> Thanks, Greg. This is almost good:
>
> static inline bool xor_unlock_is_negative_byte(unsigned long mask,
> volatile unsigned long *p)
> {
> #ifdef CONFIG_COLDFIRE
> __asm__ __volatile__ ("eorl %1, %0"
> : "+m" (*p)
> : "d" (mask)
> : "memory");
> return *p & (1 << 7);
> #else
> char result;
> char *cp = (char *)p + 3; /* m68k is big-endian */
>
> __asm__ __volatile__ ("eor.b %1, %2; smi %0"
> : "=d" (result)
> : "di" (mask), "o" (*cp)
> : "memory");
> return result;
> #endif
> }
>
> folio_end_read() does about as well as can be expected:
>
> 00000708 <folio_end_read>:
> 708: 206f 0004 moveal %sp@(4),%a0
> 70c: 7009 moveq #9,%d0
> 70e: 4a2f 000b tstb %sp@(11)
> 712: 6602 bnes 716 <folio_end_read+0xe>
> 714: 7001 moveq #1,%d0
> 716: b190 eorl %d0,%a0@
> 718: 2010 movel %a0@,%d0
> 71a: 4a00 tstb %d0
> 71c: 6a0c bpls 72a <folio_end_read+0x22>
> 71e: 42af 0008 clrl %sp@(8)
> 722: 2f48 0004 movel %a0,%sp@(4)
> 726: 6000 fcfe braw 426 <folio_wake_bit>
> 72a: 4e75 rts
>
> However, it seems that folio_unlock() could shave off an instruction:
>
> 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
>
> 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
>
> but gas is unhappy with everything I've tried to use eori. I'm building

I can't seem to get it to always use it either. This comes close:

__asm__ __volatile__ ("eorl %1, %0"
: "+d" (*p)
: "di" (mask)
: "memory");
return *p & (1 << 7);

Using eoril for folio_unlock, but not for folio_end_read:

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.

> with stmark2_defconfig, which I assume should work.

Yes, or any of amcore, m5208evb, m5249evb, m5272c3, m5275evb, m5307c3, m5407c3.

Regards
Greg