Re: [cxx-mem-model] C++ atomic tests (synchronized and relaxed)

[Jason Merrill <jason at redhat dot com>]

On 04/04/2011 01:11 PM, Aldy Hernandez wrote:
> 1. Synchronized atomic load/stores:
>
> atomic_int atomi;
> long double j;
>
> Thread 1:
> j = 13.0;
> atomi.store(1);
>
> Thread 2:
> atomi.load();
>
> As I understand it, the load/stores have acquire/release semantics,
> so the store to <j> must happen before the store to <atomi>.

Yep, that does seem to be true for the default memory_order_seq_cst.

> Currently, this is not the case on x86-64, because GCC reorders the
> stores and the attached atomics-2.C test fails. We emit the
> following for thread 1:
>
> flds .LC0(%rip)
> movl $1, atomi(%rip)
> xorl %eax, %eax
> fstpt j(%rip)
> mfence
>
> Notice the store of <j> *after* the store to <atomi>. I consider
> this a bug and have put this on my laundry list.

Seems plausible, though I don't know the details of the x86_64 memory 
model well enough to be sure that it would be possible to observe this 
reordering.  But then, if your test fails I guess it is.  :)

> 2. Unsynchronized atomic load/stores.
>
> For the "memory_order_relaxed" directive, load/stores are not
> synchronized as the previous example, but stores to identical memory
> locations must be performed in modification order.
>
> For the following loop, I assume and test that the stores to x[] do
> not happen out of order:
>
> for (int i=0; i < SIZE; ++i)
> x[i].store(666, memory_order_relaxed);
>
> This is the test in the attached atomics-3.C.

I don't think that's testing what you want to test; those stores are to 
the same array, but not to the same memory location.

Jason