Re: [PATCHv14 08/17] x86/mm: Reduce untagged_addr() overhead until the first LAM user

[Nick Desaulniers]

On Tue, Jan 17, 2023 at 9:29 AM Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> On Tue, Jan 17, 2023 at 9:18 AM Linus Torvalds
> <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> >
> > The reason clang seems to generate saner code is that clang seems to
> > largely ignore the whole "__builtin_expect()", at least not to the
> > point where it tries to make the unlikely case be out-of-line.
>
> Side note: that's not something new or unusual. It's been the case
> since I started testing clang - we have several code-paths where we
> use "unlikely()" to try to get very unlikely cases to be out-of-line,
> and clang just mostly ignores it, or treats it as a very weak hint. I
> think the only way to get clang to treat it as a *strong* hint is to
> use PGO.

I'd be surprised if that were intentional or by design.

Do you guys have a bug report we could look at?

> So clang ignoring the likely() hint is probably the right thing here,
> and then the wrong thing in some other places.

-- 
Thanks,
~Nick Desaulniers