It has been three months since I began this project, and it is time for a checkpoint. I just implemented the last mandatory algorithm, and Coloquinte’s algorithms are now usable to build a circuit.
It does not support standard formats, and since I still didn’t program the backend the results are still ~15% from the best state of the art. But all the core algorithms have been implemented.
The core wirelength optimizer has been the first part I implemented. I chosed to write the sparse linear algebra code from scratch rather than using a library. The whole thing is in fact closer to a true non-linear optimizer, since it never completely solves the linear system, but overall it is very similar to other placement tools.
This tool modifies a solution so that it meets cell density constraints: it drives the optimizer toward an almost “legal” solution, where there are no more cells in a region than allowed by its area. It is although used as a pre-processing step of a true legalization.
It is very simple but extremely different from other algorithms. It distributes the cells in the regions and improves the distribution between nearby regions. There are powerful algorithms to do it, but the simplest heuristics seem to work best.
The legalizer is the final step to obtain a correct placement. It places the cells so that they do not overlap, hopefully as close to the optimized solution as possible.
The naive way to do it is to sort the cells by position and chose the best position for each cell sequentially. Coloquinte’s legalizer uses a single row problem, which allows it to push already legalized cells to obtain a better solution (in Coloquinte it optimizes the displacement, but other legalizers did it for the quadratic displacement). Compared with the other solution, it is extremely stable.
Obviously, a good global placement doesn’t replace a detailed placement optimization: it is the next big step. There are still a lot of things to be done, and a lot of naive heuristics where we could do better, but detailed placement should finally close the gap with state of the art tools.