Coriolis’ legalizer is unexpectedly good

I spent part of the weekend trying to improve the legalization algorithm in Coloquinte – now integrated in Coriolis. It turns out that I didn’t manage to obtain any improvement: every change I tried obtained worse results than the current version.

The current algorithm doesn’t even legalize the true placement: it works on a modified placement obtained by heuristically solving a flow problem. Although this placement is supposed to be close to the best legal placement, I didn’t expect it to stand against more sophisticated heuristics that use it only as a hint.

I think that what makes it so good is that this first pass is inherently imprecise and optimistic: within the finer grained window, it doesn’t move the cells as much as it should. They are moved very close to their final position, but still a bit off in the direction of their target position. Therefore, the second pass receives a good hint placement which is still a bit biased toward the target.

Whatever the reason is, I’m finished with the legalizer. I don’t think there is much more work to do in this area, and I’m convinced that it is better than published works like Abacus and HiBinLegalizer. Sadly, benchmarking against them involves finding and installing them and MPL, which is all but trivial. Our legalizer should be good enough for us now (at least for standard cells, big macros are still a pain).

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