llvm dev - Mar 2020 - __builtin_thread_pointer for RISC-V

Hi Devs,
since risc-v has a register $tp which is thread pointer.
is it possible to have __builtin_thread_pointer for RISC-V?
I am not sure what could be corresponding instructions?

./kamlesh

Hi kamlesh,

Does GCC provide such a builtin?

At the moment, you can write inline assembly to access the `tp` register if you
need to read it directly, like so (you can directly inline this if you need to):

void* read_thread_pointer(void) {
  void* thread_pointer;
  asm("mov %0, tp" : "=r"(thread_pointer) : );
  return thread_pointer;
}

I am not entirely sure how where `tp` points to in the different thread local
models. There is more information on these models in the RISC-V ELF psABI
document:
https://github.com/riscv/riscv-elf-psabi-doc/blob/master/riscv-elf.md#thread-local-storage

Sam
> On 25 Mar 2020, at 12:34 pm, kamlesh kumar via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
> 
> Hi Devs,
> since risc-v has a register $tp which is thread pointer.
> is it possible to have __builtin_thread_pointer for RISC-V?
> I am not sure what could be corresponding instructions?
> 
> ./kamlesh
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
--
Sam Elliott
Software Developer - LLVM and OpenTitan
lowRISC CIC

Thanks for your reply Sam,
GCC rejects the builtin but clang crashes.



On Wed, Mar 25, 2020, 9:06 PM Sam Elliott <selliott at lowrisc.org> wrote:
> Hi kamlesh,
>
> Does GCC provide such a builtin?
>
> At the moment, you can write inline assembly to access the `tp` register
> if you need to read it directly, like so (you can directly inline this if
> you need to):
>
> void* read_thread_pointer(void) {
>   void* thread_pointer;
>   asm("mov %0, tp" : "=r"(thread_pointer) : );
>   return thread_pointer;
> }
>
> I am not entirely sure how where `tp` points to in the different thread
> local models. There is more information on these models in the RISC-V ELF
> psABI document:
>
https://github.com/riscv/riscv-elf-psabi-doc/blob/master/riscv-elf.md#thread-local-storage
>
> Sam
>
> > On 25 Mar 2020, at 12:34 pm, kamlesh kumar via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
> >
> > Hi Devs,
> > since risc-v has a register $tp which is thread pointer.
> > is it possible to have __builtin_thread_pointer for RISC-V?
> > I am not sure what could be corresponding instructions?
> >
> > ./kamlesh
> > _______________________________________________
> > LLVM Developers mailing list
> > llvm-dev at lists.llvm.org
> > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
> --
> Sam Elliott
> Software Developer - LLVM and OpenTitan
> lowRISC CIC
>
>
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