Re: Portable inline asm to get address of TLS variable

[Florian Weimer]

* Stefan Hajnoczi:

> I'm basically asking whether the &tls_var input operand is treated as
> volatile and part of the inline assembly or whether it's just regular
> C code that the compiler may optimize with the surrounding function?

&tls_var is evaluated outside of the inline assembly, any compiler
barrier will come after that.  It's subject to CSE (or whatever it's
called.  Three asm statements in a row

  asm volatile("" : "=r"(dst_ptr) : "0"(&tls_var));
  asm volatile("" : "=r"(dst_ptr) : "0"(&tls_var));
  asm volatile("" : "=r"(dst_ptr) : "0"(&tls_var));

result in

        movq    tls_var@gottpoff(%rip), %rax
        addq    %fs:0, %rax
        movq    %rax, %rdx
        movq    %rax, %rdx

which is probably not what you want.

Thanks,
Florian