eliminate unused memoisation

[rade]

Rendering makes use of memoisation, by wrapping filters, maps, reducers and conditionalRenderers in render.Memoise structs. The objective here is to

• evaluate common sub-expressions of a rendering function only once
• evaluate the top-level rendering functions only once when they are invoked multiple times on the same report (e.g. when the browser requests the same rendering for the same timespan and no new reports have been received for that timespan)

The memoisation is accomplished in the renderer constructors, i.e. MakeFilter, MakeFilterPseudo, MakeMap, MakeReduce, ConditionalRenderer, by wrapping with a render.Memoise struct.

That actually leads to a lot of unnecessary memoisation, since many of these constructors are in places that are neither common sub-expressions nor the top-level.

We should remove the memoisation from the constructors and instead insert it at the call sites that are in these places.

[bboreham]

Are there good examples when memoisation is expected to help common subexpressions?

[rade]

It definitely happens - a while ago I instrumented the memoisation code and it showed plenty of cache hits even in a single render of a report.

[bboreham]

Here's one:

HostRenderer (called by the hosts api) calls ColorConnectedProcessRenderer, and calls ContainerRenderer which also calls ColorConnectedProcessRenderer.

Ironically ColorConnectedProcessRenderer is not memoised.

[rade]

I fixed that in the #2924 :)