The old atomic compare and swap operations (`vtkm::AtomicCompareAndSwap`
and `vtkm::exec::AtomicArrayExecutionObject::CompareAndSwap`) had an
order of arguments that was confusing. The order of the arguments was
shared pointer (or index), desired value, expected value. Most people
probably assume expected value comes before desired value. And this
order conflicts with the order in the `std` methods, GCC atomics, and
Kokkos.
Change the interface of atomic operations to be patterned off the
`std::atomic_compare_exchange` and `std::atomic<T>::compare_exchange`
methods. First, these methods have a more intuitive order of parameters
(shared pointer, expected, desired). Second, rather than take a value
for the expected and return the actual old value, they take a pointer to
the expected value (or reference in `AtomicArrayExecutionObject`) and
modify this value in the case that it does not match the actual value.
This makes it harder to mix up the expected and desired parameters.
Also, because the methods return a bool indicating whether the value was
changed, there is an additional benefit that compare-exchange loops are
implemented easier.
For example, consider you want to apply the function `MyOp` on a
`sharedValue` atomically. With the old interface, you would have to do
something like this.
```cpp
T oldValue;
T newValue;
do
{
oldValue = *sharedValue;
newValue = MyOp(oldValue);
} while (vtkm::AtomicCompareAndSwap(sharedValue, newValue, oldValue) != oldValue);
```
With the new interface, this is simplfied to this.
```cpp
T oldValue = *sharedValue;
while (!vtkm::AtomicCompareExchange(sharedValue, &oldValue, MyOp(oldValue));
```
Now that we have the functions in `vtkm/Atomic.h`, we can deprecate (and
eventually remove) the more cumbersome classes `AtomicInterfaceControl`
and `AtomicInterfaceExecution`.
Also reversed the order of the `expected` and `desired` parameters of
`vtkm::AtomicCompareAndSwap`. I think the former order makes more sense
and matches more other implementations (such as `std::atomic` and the
GCC `__atomic` built ins). However, there are still some non-deprecated
classes with similar methods that cannot easily be switched. Thus, it's
better to be inconsistent with most other libraries and consistent with
ourself than to be inconsitent with ourself.
Now that we have atomic free functions (e.g. `vtkm::AtomicAdd()`), we no
longer need special implementations for control and each execution
device. (Well, technically we do have special implementations for each,
but they are handled with compiler directives in the free functions.)
Convert the old atomic interface classes (`AtomicInterfaceControl` and
`AtomicInterfaceExecution`) to use the new atomic free functions. This
will allow us to test the new atomic functions everywhere that atomics
are used in VTK-m.
Once verified, we can deprecate the old atomic interface classes.
BitFields are:
- Stored in memory using a contiguous buffer of bits.
- Accessible via portals, a la ArrayHandle.
- Portals operate on individual bits or words.
- Operations may be atomic for safe use from concurrent kernels.
The new BitFieldToUnorderedSet device algorithm produces an ArrayHandle
containing the indices of all set bits, in no particular order.
The new AtomicInterface classes provide an abstraction into bitwise
atomic operations across control and execution environments and are used
to implement the BitPortals.
