Segment register support for the i386

[Martin v. Loewis]

> Well, not exactly.  While the "address space attribute" approach would
> be nice to have, it isn't that easy to implement, since there seems to
> be no support for pointer attributes in GCC.  

They would not be pointer attributes. Instead, they would be type
attributes:

struct TLS{
  fill_in something;
  and something_else;
}__attribute__((segment("gs")));

struct TLS *tls;

then you do

  tls->something++;

and it knows to use a segment prefix on each reference.

Alternatively, they could be qualifiers, like const, volatile,
restrict - although this sounds like a major new extension.

> It would be nice if I could write:
> 
>    register struct thread_descr *__thread_self __asm__("%gs");
> 
> such that I could use the same C code as in the Sparc case to access
> and manipulate the thread descriptor.

No, that would not be nice. What if I do:

  int i;
  __thread_self = &i;

That does not *at all* do what it looks like it would do (if it does
anything at all).

> That's why I'm not sure if this approach is acceptable, and I'd like
> to hear what other people think of this proposal before spending too
> much time on finishing my current implementation.  I can imagine that
> people say that this extension is really an ugly hack that should
> never be allowed in GCC.

I would be such a person :(

Regards,
Martin