> On Thu, Jul 27, 2023 at 07:06:03PM +0000, Joseph Myers wrote:
> > I think there should be tests for _Atomic _BitInt types. Hopefully atomic
> > compound assignment just works via the logic for compare-and-exchange
> > loops, but does e.g. atomic_fetch_add work with _Atomic _BitInt types?
>
> So, there are 2 issues.
>
> One is something I haven't seen being handled for C at all so far, but
> handled for C++ - padding bits.
>
> Already e.g. x86 long double has some padding bits - 16 bits on ia32,
> 48 bits on x86_64, when one does
> _Atomic long double l;
> ...
> l += 2.0;
> it will sometimes work and sometimes hang forever.
> Similarly atomic_compare_exchange with structs which contain padding
> (unions with padding bits are lost case, there is nothing that can be
> reliably done for that, because we don't know at runtime what is the active
> union member if any). And _BitInt if it doesn't use all bits in
> all containing limbs has padding as well (and psABI doesn't say it is sign
> or zero extended).
What is the problem here? In C, atomic_compare_exchange is defined in terms
of the memory content which includes padding. So it may fail spuriously
due to padding differences (but it may fail anyway for arbitrary reasons
even without padding differences), but then should work in the second
iterations.
Martin
