Re: Portable inline asm to get address of TLS variable

[Florian Weimer]

* Stefan Hajnoczi:

> I've been trying to make the inline asm that gets the address of a TLS
> variable for QEMU coroutines pass QEMU's GitLab CI.
> https://gitlab.com/stefanha/qemu/-/blob/coroutine-tls-fix/include/qemu/coroutine-tls.h#L89
>
> The code isn't -fPIC-friendly (R_X86_64_TPOFF32 relocations aren't
> allowed in -fPIC shared libraries) so builds fail with ./configure
> --enable-modules. While I was tackling this I stumbled on this:
>
>   void *dst_ptr;
>   asm volatile("" : "=r"(dst_ptr) : "0"(&tls_var))
>
> What's nice about it:
> - It's portable, there are no arch-specific assembly instructions.
> - It works for both -fPIC and non-PIC.
>
> However, I wonder if the compiler might reuse a register that already
> contains the address. Then we'd have the coroutine problem again when
> qemu_coroutine_yield() is called between the earlier address calculation
> and the asm volatile statement.
>
> Thoughts?

Sorry, I don't see why this isn't equivalent to a plain &tls_var.
What exactly are you trying to achieve?

Thanks,
Florian