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

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Message-ID: <CAHk-=wgBUvM7tc70AAvUw+HHOo6Q=jD4FVheFGDCjNaK3OCEGA@mail.gmail.com>
Date:   Fri, 15 Sep 2023 17:27:17 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     "Matthew Wilcox (Oracle)" <willy@...radead.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, 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?"

                Linus