[Issue 15523] extern(C++) support TLS (C++ 'thread_local') linkage?

[via Digitalmars-d-bugs]

https://issues.dlang.org/show_bug.cgi?id=15523

--- Comment #6 from Jacob Carlborg <doob at me.com> ---
(In reply to Manu from comment #4)
> I would suggest that since 'thread_local' is C++11 standard, then that
> should be the one.

That would be a breaking change.

> Since '__thread' is not standard, I imagine it would be best supported by
> GDC(/Clang?) via the @attribute("__thread") mechanism that they support for
> their compiler-specific attributes.

thread_local is not supported by the version of Clang supplied by Apple. The
one most likely installed.

> extern(C++):
> 
> // Standard:
> extern int i; // thread_local
> 
> // GCC(/Clang?):
> @attribute("__thread") extern int i; // __thread (this is probably very easy
> for those toolchains to implement)

DMD need to be able to handle it as well. Perhaps through a pragma?

This would only affect extern(C++)? Would it be confusing to need to specify
@attribute("__thread") only when the linkage is C++?

I found this stackoverflow thread explaning the different sematnics of __thread
and thread_local [1]. It looks like if you use thread_local for any use case
that __thread is valid, it will behave the same, except when it's declared
"extern".

[1]
http://stackoverflow.com/questions/13106049/c11-gcc-4-8-thread-local-performance-penalty

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