shadow variable benchmark

[slwu89]

@giovannic I have an implementation of the shadow integer variable here. The way it works is pretty straightforward, there are just 2 vectors it contains, each time step an enum is used to track which one is "active" and which is the "shadow". Queuing updates overwrites the shadow. The update sets the shadow to be the new active vector by swapping the enum and copies the old shadow to the new shadow vector.

The result of one of the benchmarks is here (the pattern is similar for the rest). As you can see, for large variables when only a few elements are being updated, the current implementation is faster. For larger variables when a high proportion of elements are being updated however, the shadow variable is faster, see limit (variable size) of 1e6 with updating 9e5 elements in the lower right, where the shadow variable is quite a bit faster.

vv_bi.pdf

So it's a tradeoff = ) Of course the double variable's shadow implementation should look almost identical. It may be less of a tradeoff for the double variable, which is probably updated much more frequently, with more of the population being updated.