Switching x86_64-linux-gnu to GNU2 TLS descriptors by default

Switching x86_64-linux-gnu to GNU2 TLS descriptors by default

[Florian Weimer]

I feel like I have asked this before.  Currently, GCC uses calls to
__tls_get_addr to obtain the address of global-dynamic TLS variables.
On other architectures with support for GNU2 TLS descriptors, those are
used by default.

Should we flip the default to GNU2 descriptors?  Support has been
available in glibc for a long, long time.  Is there any other reason for
not doing this?  On the glibc side, the behavior regarding lazy
initialization and symbol binding does not change whether the old or new
interface is used.

Thanks,
Florian

Re: Switching x86_64-linux-gnu to GNU2 TLS descriptors by default

[H.J. Lu]

On Wed, Dec 13, 2023 at 6:19 AM Florian Weimer via Gcc <gcc@gcc.gnu.org> wrote:
>
> I feel like I have asked this before.  Currently, GCC uses calls to
> __tls_get_addr to obtain the address of global-dynamic TLS variables.
> On other architectures with support for GNU2 TLS descriptors, those are
> used by default.
>
> Should we flip the default to GNU2 descriptors?  Support has been
> available in glibc for a long, long time.  Is there any other reason for
> not doing this?  On the glibc side, the behavior regarding lazy
> initialization and symbol binding does not change whether the old or new
> interface is used.
>
> Thanks,
> Florian
>

It sounds good to me.

Thanks.

-- 
H.J.

Re: Switching x86_64-linux-gnu to GNU2 TLS descriptors by default

[Sam James]

Florian Weimer via Gcc <gcc@gcc.gnu.org> writes:

> I feel like I have asked this before.  Currently, GCC uses calls to
> __tls_get_addr to obtain the address of global-dynamic TLS variables.
> On other architectures with support for GNU2 TLS descriptors, those are
> used by default.
>
> Should we flip the default to GNU2 descriptors?  Support has been
> available in glibc for a long, long time.  Is there any other reason for
> not doing this?  On the glibc side, the behavior regarding lazy
> initialization and symbol binding does not change whether the old or new
> interface is used.
>

I was planning on bringing this up but I was worried there was some
reason we hadn't done it given it's been so long. Thank you!

Re: Switching x86_64-linux-gnu to GNU2 TLS descriptors by default

[Andrew Pinski]

On Wed, Dec 13, 2023 at 6:19 AM Florian Weimer via Gcc <gcc@gcc.gnu.org> wrote:
>
> I feel like I have asked this before.  Currently, GCC uses calls to
> __tls_get_addr to obtain the address of global-dynamic TLS variables.
> On other architectures with support for GNU2 TLS descriptors, those are
> used by default.
>
> Should we flip the default to GNU2 descriptors?  Support has been
> available in glibc for a long, long time.  Is there any other reason for
> not doing this?  On the glibc side, the behavior regarding lazy
> initialization and symbol binding does not change whether the old or new
> interface is used.

Just FYI, the last time this was asked was 6 years ago but maybe
things has changed since:
https://inbox.sourceware.org/gcc-patches/CAMe9rOp_68qpdLz25PoHa1ewB6pGQuVv_+H5bXGFhU05MH9+Vg@mail.gmail.com/

Thanks,
Andrew

>
> Thanks,
> Florian
>

Re: Switching x86_64-linux-gnu to GNU2 TLS descriptors by default

[Andrew Pinski]

On Wed, Dec 13, 2023 at 1:08 PM Andrew Pinski <pinskia@gmail.com> wrote:
>
> On Wed, Dec 13, 2023 at 6:19 AM Florian Weimer via Gcc <gcc@gcc.gnu.org> wrote:
> >
> > I feel like I have asked this before.  Currently, GCC uses calls to
> > __tls_get_addr to obtain the address of global-dynamic TLS variables.
> > On other architectures with support for GNU2 TLS descriptors, those are
> > used by default.
> >
> > Should we flip the default to GNU2 descriptors?  Support has been
> > available in glibc for a long, long time.  Is there any other reason for
> > not doing this?  On the glibc side, the behavior regarding lazy
> > initialization and symbol binding does not change whether the old or new
> > interface is used.
>
> Just FYI, the last time this was asked was 6 years ago but maybe
> things has changed since:
> https://inbox.sourceware.org/gcc-patches/CAMe9rOp_68qpdLz25PoHa1ewB6pGQuVv_+H5bXGFhU05MH9+Vg@mail.gmail.com/

Oh I noticed that was a bug filed before that asking for testcases to
be added for it on x86_64 but it looks like it was not implemented:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=48868

So it might even be broken.

Thanks,
Andrew


>
> Thanks,
> Andrew
>
> >
> > Thanks,
> > Florian
> >

Re: Switching x86_64-linux-gnu to GNU2 TLS descriptors by default

[Sam James]

Florian Weimer via Gcc <gcc@gcc.gnu.org> writes:

> [...]
> On other architectures with support for GNU2 TLS descriptors, those are
> used by default.
>

It looks like arm32 defaults to gnu, not gnu2. andrew mentioned fdpic
will be an issue there but maybe we could carve that out.

> Should we flip the default to GNU2 descriptors?
> [...]

thanks,
sam