Cache elimination limitations

[charleskawczynski]

This issue is meant to serve as a place where we list roadblocks when attempting to eliminate our solver cache.

• ClimaCore neighbor-wise operators require a scope where the global space is accessible. Because we're overloading Julia's broadcast machinery, and broadcast syntax limits how users express broadcast expressions (they cannot be nested, in the case of launching GPU kernels), all of our derivatives and interpolations must be written at the "top-level" inside functions. For example: we cannot compute a derivating foo(y) = 2*∇y inside foo in the outer expression @. x = foo(y) . Instead, we must write @. x = 2*∇y where ∇ is a gradient operator.