I would like to make DMDAGetISLocalToGlobalMapping() DM-level functionality since it is an essential ingredient in the MatGetLocalSubMatrix-based assembly, but it's not an essential part of how every conceivable DM functions. I would rather allocate that memory lazily so that not every user is required to pay for it (e.g. when they don't even have a matrix), but creating an ISLocalToGlobalMapping is a collective operation (somewhat strangely, because only the rarely used inversion of the map is a collective operation). This means that either DMGetLocalToGlobalMapping becomes a collective operation, or everyone needs to pay for it to exist? Thoughts?
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