[tverbeure]

> Does this merely optimize for minimal length of the datalines?

That's how it used to be 25 years ago. It's probably still a factor in the cost function, but the biggest part is timing. Of course, timing and length are closely related.

> It seems counter-intuitive that a random placement would be superior to a more regular one.

It's not necessarily superior if you want to have optimal timing for all paths between all cells. But you don't need that: only a few percent of all paths are actually timing critical. Those determine the maximum clock speed. The others have enough slack such that it doesn't matter that they are placed tens of microns too far.

Random placement (at an ever lower level of detail) is also better to avoid crosstalk. If you'd place a bunch of driver cells in a nicely aligned stack and make those wires go a nicely aligned stack of receiving flip-flops with parallel wires in between, you'd get the mother of all crosstalk problems.