On Mon, 26 Feb 2018, Szabolcs Nagy wrote:
>
> rmw load is only valid if the implementation can
> guarantee that atomic objects are never read-only.
OK, but that sounds like a matter of not emitting atomic
objects into .rodata, which shouldn't be a big problem,
if not for backwards compatibility concern?
> current implementations on linux (including clang)
> don't do that, so an rmw load can observably break
> conforming c code: a static global const object is
> placed in .rodata section and thus rmw on it is a
> crash at runtime contrary to c standard requirements.
Note that in your example GCC emits 'x' as a common symbol,
you need '... x = { 0 };' for it to appear in .rodata,
> on an aarch64 machine clang miscompiles this code:
[...]
and then with new enough libatomic on Glibc this segfaults
with GCC on x86_64 too due to IFUNC redirection mentioned
in the other subthread.
Alexander
