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));
```
Generally, fields that have a WHOLE_MESH association might be valid even
if the structure of the mesh changes. Thus, it makes sense for filters
to pass this data pretty much all the time.
Also cleaned up some code and comments to make the relationship between
`MapFieldOntoOutput` and `DoMapField` more clear.
The version of `Filter::Execute` that takes a policy as an argument is now
deprecated. Filters are now able to specify their own fields and types,
which is often why you want to customize the policy for an execution. The
other reason is that you are compiling VTK-m into some other source that
uses a particular types of storage. However, there is now a mechanism in
the CMake configuration to allow you to provide a header that customizes
the "default" types used in filters. This is a much more convenient way to
compile filters for specific types.
One thing that filters were not able to do was to customize what cell sets
they allowed using. This allows filters to self-select what types of cell
sets they support (beyond simply just structured or unstructured). To
support this, the lists `SupportedCellSets`, `SupportedStructuredCellSets`,
and `SupportedUnstructuredCellSets` have been added to `Filter`. When you
apply a policy to a cell set, you now have to also provide the filter.
VTK-m now provides the following filters with the default policy
as part of the vtkm_filter library:
- CellAverage
- CleanGrid
- ClipWithField
- ClipWithImplicitFunction
- Contour
- ExternalFaces
- ExtractStructured
- PointAverage
- Threshold
- VectorMagnitude
By building these as a library we hope to provide faster compile
times for consumers of VTK-m when using common configurations.
Previously, all the ApplyPolicy functions had the same name and used
template resolution to figure out which one to use. This was pretty
clear at first when there was just one for fields and one for cell sets.
But then it grew to several different types, particularly for fields. It
was hard to look at the code and figure out which form of ApplyPolicy
was being used, and compilers were starting to get confused.
Resolve the problem by giving all the methods unique names to make it
clear which one you expect to be called.
By removing the ability to have multiple CellSets in a DataSet
we can simplify the following things:
- Cell Fields now don't require a CellSet name when being constructed
- Filters don't need to manage what the active cellset is
Now that Threshold automatically converts its output to CellSetExplicit,
this option is no longer necessary (and the previous implementation did
not work correctly).
