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

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Message-ID: <ZRsi7smLotWDwoNP@casper.infradead.org>
Date:   Mon, 2 Oct 2023 21:07:10 +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, 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 #<data>,Dx L Immediate Data ^ Destination → Destination ISA_A

but gas is unhappy with everything I've tried to use eori.  I'm building
with stmark2_defconfig, which I assume should work.