--emulated-tls explanation?

[IGotD-]

On Tuesday, 13 October 2020 at 10:35:57 UTC, Denis Feklushkin 
wrote:
>
> So, compiler knows what this platform is not supports 
> multithreading and does some things wrong with thread static 
> variables if "--emulated-tls" is ommited?
>

You can see the implementation yourself.

https://github.com/gcc-mirror/gcc/blob/master/libgcc/emutls.c

I have used TLS emulation myself and it just works despite the 
library has no definition of threads or mutexes so I guess these 
are just stubs or use stubs in the C library in that case. 
Basically single threaded TLS.

>
> As I understand, variables allocated by compiler, but it uses 
> internal implict call to __tls_get_addr to provide access to 
> them.

If SW call is chosen then __tls_get_addr is the function that is 
used in order to obtain the address of a TLS variable. If 
emulation is used this function is just forwarded to the 
emulation function. It is almost easier to implement 
__tls_get_addr yourself and skip the emulation. If you look at 
the emulation layer it is filled with mutexes and mallocs and on 
simple systems this can be totally avoided if you use your own 
solution. Especially in real-time systems, the emulation layer 
should not be used for obvious reasons.