[Bug middle-end/96200] Implement __builtin_thread_pointer() and __builtin_set_thread_pointer() if TLS is supported

["jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96200

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
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                 CC|                            |jakub at gcc dot gnu.org

--- Comment #10 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
(In reply to Florian Weimer from comment #6)
> (In reply to H.J. Lu from comment #4)
> > On Linux/i386 and Linux/x86-64, thread pointer access is done via syscall.
> > On Linux/x86-64, __builtin_thread_pointer and __builtin_set_thread_pointer
> > may be implemented with FSGSBASE ISA.  Is it possible to implement these
> > builtins on Linux/i386 and Linux/x86-64 for all processors?
> 
> It's effectively part of the x86-64 ABI, but I think it's currently
> undocumented. On x86-64, it looks like this:
> 
> static inline void *
> thread_pointer (void)
> {
>   void *result;
>   asm ("mov %%fs:0, %0" : "=r" (result));
>   return result;
> }
> 
> i386 is similar, but with %gs, I think.
> 
> This is ABI since the early NPTL days, and GCC knows about this very
> explicitly, to implement the -mno-tls-direct-seg-refs option.

It is documented, see https://akkadia.org/drepper/tls.pdf
"The only requirement about this register is that the actual thread pointer tpt
can be loaded from the absolute address 0 via the %gs register. The following
code would load the thread pointer in the %eax register:
movl %gs:0, %eax"
for ia32.  For x86_64, it is not spelled explicitly, but it is implicit from:
"The x86-64 ABI is at its base virtually the same as the IA-32 ABI. The
difference is mainly in different size of variables containing pointers and
that it only provides one variant which closely matches the IA-32 GNU variant.
Instead of segment register %gs it uses the %fs segment register."
as well as all the explicit insn sequences that use it.  The psABI doesn't have
TLS details I believe, so Ulrich's tls.pdf is the ABI document that covers
that.