[PATCH 1/2] x86_64: Use __seg_fs qualifiers in NPTL accessors

[Florian Weimer]

* Uros Bizjak:

> Use __seg_fs named address space qualifiers to cast NPTL accessors
> to %fs: prefixed addresses.  Use volatile access only where
> strictly necessary.
>
> Also remove stalled __GNUC_PREREQ (6, 0) condition.  Currenty GCC-12
> is required as the minimum supported compiler version.

Are null pointer accesses through __seg_fs permitted by GCC?

How does GCC now that these accesses my not be reordered before the
initialization of %fs?

Thanks,
Florian