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Date: Wed, 20 Jul 2022 16:01:30 +0300 (MSK)
From: Alexander Monakov <amonakov@ispras.ru>
To: Sebastian Huber <sebastian.huber@embedded-brains.de>
cc: GCC Development <gcc@gcc.gnu.org>
Subject: Re: Use -ftls-model=local-exec for RTEMS by default?
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On Wed, 20 Jul 2022, Sebastian Huber wrote:

> On 20/07/2022 13:41, Alexander Monakov wrote:
> > On Wed, 20 Jul 2022, Sebastian Huber wrote:
> > 
> >> How does Ada get its default TLS model?
> > You shouldn't need to do anything special, GCC automatically selects
> > initial-exec or local-exec for non-PIC (including PIE).
> 
> I am not sure, for this test program:
> 
> extern _Thread_local int i;
> _Thread_local int j;
> 
> int f(void)
> {
>   return i + j;
> }
> 
> I get:
[snip]

Thanks, I missed that you are asking about promoting initial-exec to local-exec
rather than x-dynamic to y-exec. There's a pending patch that implements such
promotion based on visibility information:
https://gcc.gnu.org/pipermail/gcc-patches/2022-July/598017.html

With that patch, you'll get local-exec model for the extern variable 'i' if you
inform the compiler that its definition will end up in the current module:

__attribute__((visibility("hidden")))
extern _Thread_local int i;
_Thread_local int j;

int f(void)
{
  return i + j;
}

Thus I would try to enhance the binds_local_p target hook for RTEMS to inform
the compiler that there's no dynamic linking (although apart from TLS variables
I cannot instantly name other places where it would enhance optimization).

HTH
Alexander