Re: Use -ftls-model=local-exec for RTEMS by default?

[Iain Sandoe <idsandoe@googlemail.com>]

> On 21 Jul 2022, at 06:25, Sebastian Huber <sebastian.huber@embedded-brains.de> wrote:
> 
> On 20.07.22 15:01, Alexander Monakov wrote:
>> 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).
> 
> This sounds like an interesting approach in the long run, however, I need a short term solution which I can back port to GCC 10, 11, and 12. I guess I will add a
> 
> MULTILIB_EXTRA_OPTS = ftls-model=local-exec
> 
> to all RTEMS multilib configurations.
> 
> In general I think the target hooks are hard to customize for operating systems.

(IMO) It can be not too tricky -  Darwin customises several - you just have to override the default definition in your target-specific header and provide the replacement e.g ( override in config/darwin.h, replacement in config/darwin.cc):

#undef TARGET_ENCODE_SECTION_INFO
#define TARGET_ENCODE_SECTION_INFO  darwin_encode_section_info

0.02GBP only, as always ;)
Iain

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