linux-kernel - Re: [PATCH 08/17] alpha: Implement xor_unlock_is_negative

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Message-ID: <ZQT4/gA4vIa/7H6q@casper.infradead.org>
Date:   Sat, 16 Sep 2023 01:38:22 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-arch@...r.kernel.org, Nicholas Piggin <npiggin@...il.com>
Subject: Re: [PATCH 08/17] alpha: Implement xor_unlock_is_negative_byte

On Fri, Sep 15, 2023 at 05:27:17PM -0700, Linus Torvalds wrote:
> On Fri, 15 Sept 2023 at 11:37, Matthew Wilcox (Oracle)
> <willy@...radead.org> wrote:
> >
> > +       "1:     ldl_l %0,%4\n"
> > +       "       xor %0,%3,%0\n"
> > +       "       xor %0,%3,%2\n"
> > +       "       stl_c %0,%1\n"
> 
> What an odd thing to do.
> 
> Why don't you just save the old value? That double xor looks all kinds
> of strange, and is a data dependency for no good reason that I can
> see.
> 
> Why isn't this "ldl_l + mov %0,%2 + xor + stl_c" instead?
> 
> Not that I think alpha matters, but since I was looking through the
> series, this just made me go "Whaa?"

Well, this is my first time writing Alpha assembler ;-)  I stole this
from ATOMIC_OP_RETURN:

        "1:     ldl_l %0,%1\n"                                          \
        "       " #asm_op " %0,%3,%2\n"                                 \
        "       " #asm_op " %0,%3,%0\n"                                 \
        "       stl_c %0,%1\n"                                          \
        "       beq %0,2f\n"                                            \
        ".subsection 2\n"                                               \
        "2:     br 1b\n"                                                \
        ".previous"                                                     \

but yes, mov would do the trick here.  Is it really faster than xor?