Re: [PATCH] selftests/mm: use __auto_type in swap() macro

[Pranav Tyagi <pranav.tyagi03@gmail.com>]

On Wed, Aug 6, 2025 at 10:16 PM Peter Xu <peterx@redhat.com> wrote:
>
> On Wed, Aug 06, 2025 at 09:15:50PM +0530, Pranav Tyagi wrote:
> > On Wed, Jul 30, 2025 at 7:53 PM Pranav Tyagi <pranav.tyagi03@gmail.com> wrote:
> > >
> > > Replace typeof() with __auto_type in the swap() macro in uffd-stress.c.
> > > __auto_type was introduced in GCC 4.9 and reduces the compile time for
> > > all compilers. No functional changes intended.
> > >
> > > Signed-off-by: Pranav Tyagi <pranav.tyagi03@gmail.com>
> > > ---
> > >  tools/testing/selftests/mm/uffd-stress.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/tools/testing/selftests/mm/uffd-stress.c b/tools/testing/selftests/mm/uffd-stress.c
> > > index 40af7f67c407..c0f64df5085c 100644
> > > --- a/tools/testing/selftests/mm/uffd-stress.c
> > > +++ b/tools/testing/selftests/mm/uffd-stress.c
> > > @@ -51,7 +51,7 @@ static char *zeropage;
> > >  pthread_attr_t attr;
> > >
> > >  #define swap(a, b) \
> > > -       do { typeof(a) __tmp = (a); (a) = (b); (b) = __tmp; } while (0)
> > > +       do { __auto_type __tmp = (a); (a) = (b); (b) = __tmp; } while (0)
> > >
> > >  const char *examples =
> > >         "# Run anonymous memory test on 100MiB region with 99999 bounces:\n"
> > > --
> > > 2.49.0
> > >
> >
> > Hi,
> >
> > Just a gentle follow-up on this cleanup patch. From what I could find,
> > this is the only use of
> > typeof() left in the mm selftests, so this should be the only instance
> > needing this change.
> >
> > Thanks for considering!
>
> Hi,
>
> Andrew should have queued this one in branch akpm/mm-nonmm-unstable (even
> though I'm not familiar with the branch).
>
> Said that, I'm also not familiar with __auto_type.  Looks like it's more
> efficiently processed by the compiler in some special use cases, however
> it's also new so maybe some tools (sparse?) may not recognize it.
>
> Is it the plan that the whole Linux kernel is moving towards __auto_type?
> I still see quite a few of typeof() usages (not "a few", but 2966 instances).
>
> Thanks,
>
> --
> Peter Xu
>

Hi Peter,

Thank you for the feedback and I sincerely apologize for the delayed
response.

You’re right — this patch was initially queued under akpm/mm-nonmm-unstable
and it now lives in the mm-unstable branch.

The motivation behind switching to __auto_type is twofold: it reduces compile
time across compilers and improves readability. Support for __auto_type has
been available in GCC since 4.9 (quite some time back) and in sparse since
v0.6.2 (2020). To the best of my knowledge, it has since been widely adopted
by toolchains.

That said, I acknowledge there are still many uses of typeof() in the kernel,
as you pointed out. My intent with this patch (and a few related ones I’ve
submitted) is to encourage more consistent use of __auto_type, starting with
selftests where it is already in use in several places.

Thanks again for your review and for pointing out the current branch status.

Regards
Pranav Tyagi