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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Sun, 14 Aug 2022 13:03:28 -0700
Message-ID: <CAHk-=wjvKtkqF9AXx8GoA80h_RNUV=Ld8qhi8ZEPmDXC0VUDUA@mail.gmail.com>
Subject: Re: [GIT PULL] Ceph updates for 5.20-rc1
To:     Al Viro <viro@zeniv.linux.org.uk>
Cc:     Nathan Chancellor <nathan@kernel.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Jeff Layton <jlayton@kernel.org>,
        Ilya Dryomov <idryomov@gmail.com>, ceph-devel@vger.kernel.org,
        linux-kernel@vger.kernel.org, Matthew Wilcox <willy@infradead.org>,
        clang-built-linux <llvm@lists.linux.dev>
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On Sun, Aug 14, 2022 at 12:08 PM Al Viro <viro@zeniv.linux.org.uk> wrote:
>
>
> There's a cheap way to reduce the register pressure:
>                 seq = raw_seqcount_begin(&dentry->d_seq);
>                 if (dentry->d_parent != parent)
>                         continue;
>                 if (d_unhashed(dentry))
>                         continue;
>                 if (dentry->d_name.hash_len != hashlen)
>                         continue;
>                 if (dentry_cmp(dentry, str, hashlen_len(hashlen)) != 0)
>                         continue;
>                 *seqp = seq;
> could move the last store to before dentry_cmp().

I actually tried that, it doesn't really end up helping.

Gcc does well regardless, and clang ends up really wanting to move so
much out of the dentry_cmp() loop that it runs out of registers and
always ends up doing a couple of spills.

I think it reduced the spills by one, but not enough to generate the
nice non-frame code that gcc does.

It's a bit ugly, but the code probably performs quite well - the
spills aren't in the inner loop.

             Linus