Re: [GIT PULL] Ceph updates for 5.20-rc1

[Linus Torvalds]

On Sun, Aug 14, 2022 at 12:08 PM Al Viro <viro@xxxxxxxxxxxxxxxxxx> 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