Switching x86-64 to GNU2 TLS descriptors

[H.J. Lu]

On Tue, Jul 15, 2025 at 8:47 AM 'Florian Weimer' via X86-64 System V
Application Binary Interface <x86-64-abi@googlegroups.com> wrote:
>
> * H. J. Lu:
>
> > Compilers will never know since the build-time glibc is independent of
> > the run-time glibc.   If compilers want to be 100% sure that the run-time
> > is GNU2 TLS bug-free, they can require linkers which generate the
> > GLIBC_ABI_GNU2_TLS dependency.
>
> (Such a linker requirement could be enforced by requiring that the
> linker recognizes a command option specific to GLIBC_ABI_GNU2_TLS, and
> that current linkers treat as an error.)
>
> Would such an unconditional requirement be acceptable to GCC 16?  I
> don't think so.  So we'd still have to design a configure option for it.
> And that requires further compiler changes.  It's also not entirely
> clear how this would interact with -fuse-ld.
>
> I just don't think option (b) is trivial from a compiler perspective.
>

It depends on what we want.  If we want 100% guarantee of glibc run-time
GNU2 TLS bug-free, GCC can pass -z gnu2-tls to linker whenever GNU2 TLS
is used with glibc.   It is an error if the linker doesn't support -z
gnu2-tls.  We
can provide a GCC option to disable -z gnu2-tls.

-- 
H.J.