Nvidia, TLS and __thread keyword -- an observation

[Igor Sysoev]

On Tue, 17 Jun 2003, Marcel Moolenaar wrote:

> There's a definite advantage to supporting the __thread keyword in
> userland and we should add the support. It really isn't that hard,
> but it requires some thought and testing. In most cases you simply
> point your thread pointer between the control structure and the
> thread local segments.

If the thread implementation uses gs register to point to thread
specific data:

   gs -> [ thread specific data ]
         [ tls_array ]

   then this C code

   __thread  int a;  a = 1;

   can be translated to

   mov  tls_key, %ecx
   mov  $gs:tls_array, %eax
   mov  (%eax,%ecx,4), %eax
   mov  $1, (%eax)

In FreeBSD we use gs to point to KSE specific data

   gs -> [ KSE specific data ]
         [ current thread    ] -> [ thread specfic data ]
                                  [ tls_array ]

   and then same C code can be translate to

   mov  tls_key, %ecx
   mov  $gs:current_thread, %eax
   mov  tls_array(%eax), %eax
   mov  (%eax,%ecx,4), %eax
   mov  $1, (%eax)

And I think we should use this scheme not only in libkse but in libthr too
to be binary compatible.


Igor Sysoev
http://sysoev.ru/en/