Switching x86-64 to GNU2 TLS descriptors

[Florian Weimer]

* 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.

Unknown -z options are ignored by default:

$ ld -z gnu2-tls
ld: warning: -z gnu2-tls ignored

It would have to be a regular option.  But that's just a minor detail.

Thanks,
Florian