[RFC] Should we declare errno with __thread on x86?

[Torvald Riegel]

On Fri, 2016-03-18 at 16:14 -0700, Roland McGrath wrote:
> If real-world performance measurement does in fact justify it, we'll still
> need to be very circumspect about potential pitfalls (applications built
> with older compilers, etc).  We'll need to put a lot of thought into it
> collectively to achieve confidence that it's a safe and sensible change for
> all our users.

I agree that this would need a thorough investigation and assessment of
risks of doing that.  One risk I see is that it would likely make it
harder for us to support errno on threads of execution that are not OS
threads (eg, coroutines, or under work-stealing implementations).  It's
just an integer, but generally I'd prefer not to expose more TLS than
necessary.